Distribution of zooxanthellate zoantharians in the Canary Islands: Potential indicators of ocean warming

Journal Pre-proof Distribution of zooxanthellate zoantharians in the Canary Islands: Potential indicators of ocean warming C. López, S. Moreno, A. Brito, S. Clemente PII:

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https://doi.org/10.1016/j.ecss.2019.106519

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YECSS 106519

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Estuarine, Coastal and Shelf Science

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Please cite this article as: López, C., Moreno, S., Brito, A., Clemente, S., Distribution of zooxanthellate zoantharians in the Canary Islands: Potential indicators of ocean warming, Estuarine, Coastal and Shelf Science (2020), doi: https://doi.org/10.1016/j.ecss.2019.106519. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

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Distribution of zooxanthellate zoantharians in the Canary Islands: Potential indicators of ocean

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warming

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Short title: Zoantharians as bioindicators of ocean warming

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C. López1*, S. Moreno1, A. Brito1 & S. Clemente1

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Departamento de Biología Animal, Edafología y Geología. Facultad de Ciencias, Universidad de La Laguna, 38206

La Laguna, Canary Islands, Spain

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*[email protected] +34628020536 (corresponding author)

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Abstract

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Global warming is driving changes in the distribution patterns of many species, leading to a general

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tropicalization and meridionalization of biota. In this context, populations of some marine species are in

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regression while others are expanding their populations. Such is the case of benthic cnidarians belonging

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to the order Zoantharia and suborder Brachycnemina, whose populations are able to cause phase-shifts in

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coral reef ecosystems. Marine assemblages in the subtropical Canary Islands region consist of a

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combination of both temperate and tropical species, mainly due to the east-to-west seawater temperature

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gradient that naturally exists throughout the archipelago. This can reach a 2ºC difference (≈23-25ºC east

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to west in summer months). These biogeographical features make the archipelago a unique location to

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research into biota reorganisation processes. The aim of this study was to establish a baseline of the

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distribution and abundance data of zoantharian Brachycnemina populations in the Canary Islands. To

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elucidate whether these species are potential bioindicators of ocean warming processes, patterns of

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species distribution and their relationships with the temperature gradient across the archipelago were also

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evaluated. Results demonstrated that intertidal and subtidal populations of Palythoa aff. clavata and P.

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caribaeorum, respectively, followed distribution patterns related to the temperature ranges recorded in

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situ by data loggers. Extensive populations were found in the western islands where seawater

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temperatures are warmer than the eastern islands. Since biota reorganisation usually produces loss of

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ecosystem functions, it is essential to establish baseline datasets of climate change indicators and also

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effective monitoring programmes. These will allow early detection of phase-shifts before they lead to

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significant changes in ecosystem dynamics.

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Keywords: Climate change, phase-shift, distribution patterns, bioindicators, Palythoa spp., SST.

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1. Introduction

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The Canary archipelago consists of seven main islands distributed around latitude 28°N, where the

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Saharan upwelling, cold Canary Current and trade winds give rise to a variety of mesoscale phenomena

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(Barton et al., 1998, 2004). These oceanographic and climatic characteristics generate a longitudinal sea

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water surface temperature (SST) gradient that can exceed 2°C between the coldest easternmost island

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(Lanzarote) and the warmest westernmost (El Hierro and La Palma) (Braun and Molina 1984; Barton et

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al., 1998) (Fig. 1a). In this context, the marine biota of the eastern islands of Lanzarote and Fuerteventura

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show many species with temperate affinities that they share with the Mediterranean region and higher

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latitudes, while on the western islands of El Hierro and La Palma there are more elements with tropical

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affinities (Sangil et al., 2011; Falcón et al., 2015). Moreover, the disturbance of the oceanic flow by an

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island, called “island mass effect” (Doty and Oguri 1956; Hernández-León et al., 2007), leaves wakes of

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warm seawater on the leeward sides of the islands, especially during summer when the trade winds are

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more intense (Van Camp and Nykjaer 1988; Arístegui el al. 1994). Owing to this effect, eddies or vortices

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are also formed on the lee side depending on the shape of the island and the fluid speed passing by it

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(Simpson and Tett 1986; Arístegui et al., 1994). Combined with the influence of upwelling filaments

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from the African coast, this mesoscale variability results in a complex pattern of regional oceanographic

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variability (Barton et al., 1998; Arístegui et al., 2009), with lower SST on the northern and north-eastern

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coasts than the western coasts of each island (Fig. 1a). This spatial heterogeneity is more evident off the

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highest islands (Tenerife, La Palma and Gran Canaria) which show clearly differentiated coastal

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orientations in terms of temperature regimes related to the main current and wind directions (Fig. 1a). As

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a result of these complex environmental conditions, the marine biota of the Canary Islands is

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characterised by a mixture of tropical, subtropical and temperate species, distributed over a relative small

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area (Falcón et al., 1996; Brito and Ocaña 2004; Brito 2010; Sangil et al., 2011; González et al., 2016).

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In recent decades the biota of the Canary Islands has been modified by the effects of ocean warming,

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especially in the warmer westernmost islands, where several tropical species are now frequently found

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(Brito et al., 2005; Falcón et al., 2015; González et al., 2017). Most of them appear sporadically, but some

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already have stable populations within the archipelago, e.g. the ocean triggerfish Canthidermis sufflamen,

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the goldspot goby Gnatholepis thompsoni and the blenny Parablennius goreensis (Brito et al., 2005;

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Falcón et al., 2015; Brito et al., 2017). Not only vagile but also sessile organisms have been able to settle

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down in this new environment, such as the fire coral Millepora alcicornis (Clemente et al., 2011) or the

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algae Penicillus capitatus (Sangil et al., 2010). These recent colonisations are closely associated with the

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increase in ocean and air temperatures (+0.28 °C and +0.09 °C per decade, respectively) measured in this

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region during the last few decades (Martín-Esquivel et al., 2012; Vélez et al., 2015) (Fig. 1b). These

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changing conditions enable tropical species to establish new populations towards higher latitudes

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(tropicalization process) (Perry et al., 2005; Brito et al., 2005; Bañón and Mucientes 2009; Costa et al.,

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2014), while native species with warm-water affinities are able to expand their populations to warmer

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sites within their native biogeographic region (meridionalization process) (Sangil et al., 2012; Brito et al.,

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2014; Yapici et al., 2016). The impact of this biota reorganisation has been widely recognised, resulting

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in modified distribution patterns of many species around the world. It causes changes in biodiversity

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and/or in ecological processes (Done 1999), modifying local ecosystems (Bellwood et al., 2004; Caralt

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and Cebrián 2013; Vergés et al., 2014; Piazzi et al., 2016). Reef building coral species are among the

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organisms most affected by the increase in ocean temperature, mainly because of their delicate

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association with symbiotic dinoflagellates of the family Symbiodiniaceae, which are greatly affected by

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changes in temperature (Muscatine 1990). This association can break under several stressful conditions.

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For instance, when sea water warms, corals expel their zooxanthellae (bleaching), losing their main

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source of food (Baker et al., 2008; Davy et al., 2012).

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Zoantharia of the order Brachycnemina (Anthozoa, Hexacorallia) are a group of benthic cnidarians found

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in most marine environments in tropical and subtropical areas around the world (Ryland and Lancaster

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2003). Currently, five Brachycnemina zooxanthellate species are known in the Canary Islands: Palythoa

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aff. clavata, P. caribaeorum and P. grandiflora belonging to the family Sphenopidae, and Zoanthus

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pulchellus and Isaurus tuberculatus to the family Zoanthidae (López et al., 2019). All of them are amphi-

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atlantic tropical and subtropical species that have their northern limit of distribution within the eastern

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Atlantic Ocean in the Canary Islands, with the exception of two isolated colonies of P. aff. clavata

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recorded further north at one site in the south of Madeira island (Araújo and Freitas 2003; López et al.,

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2019). Palythoa aff. clavata and P. caribaeorum are currently catalogued as species of special interest in

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Canarian ecosystems (BOE-A-2010-9772), as they were previously only known to show discrete

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populations around the intertidal and subtidal zones of the archipelago (Arechavaleta 2009). Although

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zoantharians are common sessile organisms worldwide, few studies have paid attention to the group in the

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Canaries, and some of them focused only on their toxins (Villar et al., 2003; Cen-Pacheco et al., 2014;

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Fraga et al., 2017) or taxonomy (López et al., 2019), but ecological studies are still scarce.

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Recent studies have shown how some zoantharian species can cover extensive areas and cause changes in

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local biota composition due to their substrate dominance (Cruz et al., 2015; González et al., 2018), even

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replacing hard coral ecosystems (Cruz et al., 2016a, b). Due to their warm water affinities, it would be

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expected that the present increase in SST favours the expansion of zooxanthellate Brachycnemina

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species, especially at the periphery of their distribution ranges (Durante et al., 2018; Reimer et al., 2018).

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In this context, the purpose of this study was to establish a baseline of the distribution and abundance of

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zooxanthellate zoantharian populations at their northern distribution limit, and to assess their relationship

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with the temperature gradient that exists within the Canary archipelago. Our aim was to determine

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whether populations of these species are appropriate bioindicators of ocean warming, and therefore

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monitoring programmes should be considered. If these species respond to changes in ocean temperature,

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we hypothesised that zoantharian populations should be more abundant at the western island of El Hierro,

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where SST is the warmest within the archipelago, than Lanzarote, which is closer to the cool Saharan

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upwelling (Fig. 1a). We also evaluated populations of P. aff. clavata and P. caribaeorum, since by law

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they are considered species of special interest in Canarian ecosystems and therefore periodical monitoring

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of population status and progression is highly recommended.

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Fig. 1. (a) Image generated by satellites AVHRR/NOAA showing sea water temperature (SST) around

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the Canary Islands region in summer time, highlighting the east-west gradient due to the influence of the

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Saharan upwelling: Differences in SST originating in the Canary Current and trade winds are also

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noticed, especially in the western islands which have the highest altitude in the archipelago. (b) Long-

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term trend of mean SST in summer months from 1960 to 2018 at El Hierro (red line) and La Graciosa

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(blue line) (Kalnay et al., 1996). La Palma (LP), El Hierro (EH), La Gomera (GO), Tenerife (TF), Gran

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Canaria (GC), Fuerteventura (FV), Lanzarote (LZ) and La Graciosa (LG).

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2. Material and Methods 2.1 Environmental data

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Temperature data loggers (HOBO Water Temp Pro v1) were deployed in September 2016 at two

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intertidal and subtidal sites at the three surveyed islands (Lanzarote, Tenerife and El Hierro) (Fig. 2).

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Subtidal loggers were placed between 1 to 7 m in depth (Fig. 2), in order to register in situ in coastal

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waters the east-to-west temperature gradient that prevails within the archipelago. In addition, intertidal

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HOBO loggers were deployed to record data at intertidal sites where large zoantharian populations were

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found. These provided accurate information about temperature oscillations experienced by the species

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during tidal cycles. All rock pools chosen to deploy loggers were similar in size and depth and avoided

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exposing loggers to air during low tide. In all cases temperature data were recorded at hourly intervals for

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12 months.

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2.2 Distribution of zooxanthellate zoantharians in the intertidal zone

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In order to detect any regional variability in zooxanthellate zoantharian populations, sampling surveys

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were conducted during 2015 and 2016 in the Canary Islands (Fig. 2), covering intertidal rocky platforms

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inhabitable by zoantharian species. Given that their populations are tentatively not very abundant in the

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archipelago, these surveys consisted of exhaustive searches through the entire study area during monthly

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spring low tides by at least two people, so as to detect every single colony that could indicate population

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expansions. Each location was visited as many times as needed to cover the whole intertidal platform,

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recording the number of colonies of all the zooxanthellate species found. Due to the complexity of the

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substrate, which makes it difficult to delimit clones in the field, we considered all clustered polyps

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connected by the same coenenchyma to be colonies, including polyps within a 5 cm radius (Irei et al.,

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2011). Species were identified by means of morphological characteristics and genetic analyses were

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performed when necessary (López et al., 2019).

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The effect of the east-to-west ocean temperature gradient across the archipelago on zoantharian

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populations was evaluated by comparing the number of colonies found at the westernmost (El Hierro) and

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easternmost islands (Lanzarote and La Graciosa) (Fig. 2). In order to do so, six intertidal rocky platforms

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at El Hierro were sampled as locations with warm conditions, and six on Lanzarote and one on La

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Graciosa as more temperate conditions (Fig. 2). Given that the complex oceanographic conditions of the

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archipelago lead to differences in SST between coastal orientations of the highest islands -northern,

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south-western and south-eastern-, exhaustive surveys of 20 sites around Tenerife were conducted.

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2.3 Distribution of zooxanthellate zoantharians in the subtidal zone

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Subtidal surveys were conducted at six sites, up to 20 m depth, by means of scuba diving at El Hierro,

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Tenerife and Lanzarote. Whenever zoantharian species were found, their depth range was also recorded.

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Size frequency distributions of the species were also recorded, as some zoantharians are able to cover

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enormous areas in the subtidal zone. To do so, photographs taken with a metric scale were used to count

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the number of polyps by means of Image J-software, assigning each colony to one of the following five

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size categories: < 50 polyps (XS), 50-100 polyps (S), 100-500 polyps (M), 500-1000 polyps (L) and >

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1000 polyps (XL). Due to the inherent characteristics of P. caribaeorum growth, with polyps embedded

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in a well-developed coenenchyma, size classes were defined according to the colony coverage area,

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which was estimated in situ with the aid of a tape measure in the following categories: < 0.5 m2 (XS),

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0.5-1 m2 (S), 1-2 m2 (M), 2-4 m2 (L) and > 4 m2 (XL).

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Fig. 2. Sites across the Canary Islands sampled during the course of the study: 2 sites on La Graciosa

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(LG), 6 on Lanzarote (LZ), 20 on Tenerife (TF) and 6 sites on El Hierro (EH). *Locations where

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intertidal temperature data loggers (HOBO Water Temp Pro v1) were deployed. Green circles show

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locations where subtidal temperature HOBO (HS) loggers were deployed; their depth is also included.

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2.4 Statistical Analyses

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Geographic Information System QGIS open source software (QGIS Development Team 2018) was used

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to grid the coastline of the islands into 500 x 500 m. The biological data recorded during surveys were

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referred to the corresponding grid and mapped as points. Likewise, all available information about

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zoantharian populations around the Canary Islands, including references as well as presence sites where

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species have been sighted, was also mapped using the same software but not incorporated into further

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statistical analyses.

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2.4.1 Environmental data

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In order to assess whether there were differences in SST among the studied islands, data collected by the

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HOBO loggers in the subtidal habitat were analysed by one-way permutational ANOVAs (Anderson

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2001) with the factor ‘Island’ treated as fixed factor with 2 levels (El Hierro and Lanzarote). Data from

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Tenerife was excluded from analyses given that the two HOBO located in the subtidal zone were lost and

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only those data from a 7-month period located on the south western coast were retrieved from the logger

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there. The daily mean SST during the year, summer and winter SST average, and higher and lower

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records of summer (from July to October), and winter months (January to April) were specifically

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compared. Because the SST in the intertidal zone greatly depends on air and rocky platform conditions,

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only the data collected by subtidal loggers were used to perform the statistical analyses. We used

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Euclidean distances and all p-values were obtained using 9999 permutations of the appropriate

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exchangeable units, considering all p-values <0.05 significant.

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2.4.2 Distribution of zooxanthellate zoantharians in the intertidal zone

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Differences in zoantharian assemblages recorded in each 500 x 500 m grid sampled from the different

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surveyed islands were assessed by means of a distance-based permutational multivariate analysis of

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variance (PERMANOVA), in relation to the east-to-west ocean temperature gradient of the archipelago

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(Anderson 2001). Moreover, species-specific analyses were performed separately, to contrast species

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abundances with distance-based permutational ANOVAs (Anderson 2001). Analyses were conducted

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using Euclidean distances for both PERMANOVA and ANOVAs, with log (X+1) transformed and raw

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data respectively. We used one-way designs in which ‘Island’ was treated as a fixed factor (4 levels: El

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Hierro, Tenerife, Lanzarote and La Graciosa).

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Patterns of distribution of zoantharian species at the different coastal orientations -northern, south-western

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and south-eastern coasts- were assessed using comparisons of the number of colonies of all species

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recorded for Tenerife (Fig. 2) by means of PERMANOVA and ANOVA procedures, as previously

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mentioned above. In this case we used one-way designs in which ‘Orientation’ was treated as a fixed

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factor with 3 levels (north, south-west and south-east).

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In all previous analyses, the size of intertidal rocky platforms surveyed was included as a covariable. The

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area of each intertidal platform (m2) was calculated from aerial ortho-photographs of study sites during

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low tide using image processing software (Image J). All p-values were obtained using 9999 permutations

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of the appropriate exchangeable units and significant terms were examined individually using appropriate

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a posteriori pairwise comparisons, also conducted by permutations (Anderson 2001). Hypothesis-driven

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tests were accepted as significant with p-values ≤0.05 and as marginally significant with p-values

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between 0.05-0.09. Principal coordinates analyses (PCO) were performed to visualise the basic

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community structure and groups obtained for the zoantharian distribution along the archipelago, as well

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as at different coastal orientations around Tenerife. The statistical software PRIMER 7 &

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PERMANOVA+ (www.primer-e.com) was used to conduct all analyses of variance.

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2.4.3 Distribution of zooxanthellate zoantharians in the subtidal zone

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Species-specific analyses for the most abundant species found in the subtidal zone, i.e. Palythoa aff.

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clavata, P. caribaeorum and Zoanthus pulchellus, were conducted to compare the number of colonies of

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each species found at each island surveyed, within the temperature gradient of the archipelago. One-way

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distance-based permutational ANOVAs (Anderson 2001) were conducted with ‘Island’ treated as a fixed

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factor (3 levels: El Hierro, Tenerife and Lanzarote). We used Euclidean distances and all p-values were

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obtained using 9999 permutations of the appropriate exchangeable units, considering significant all p-

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values <0.05. Significant terms were further examined using a posteriori pairwise comparisons, also

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conducted by permutations (Anderson 2001).

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Size frequency distribution data of P. caribaeorum colonies was compared among islands by means of a

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Kolmogorov-Smirnov test, using SPSS software (version 25). Numbers of recorded colonies of P. aff.

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clavata and Zoanthus pulchellus were not enough to enable size distribution analyses.

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3. Results 3.1 Environmental data

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Data collected by the three temperature data loggers, recovered out of the six initially deployed on

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intertidal platforms, were very heterogeneous. On Lanzarote, the data logger was deployed at Órzola (site

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3 in Fig. 2) (Fig. 3a), in a tidal rock pool completely covered by Palythoa caribaeorum with few discrete

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colonies of P. aff. clavata. The recorded temperature range varied between 14.10 - 30.72°C, constituting

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the lowest mean temperature (20.36 ±2.28°C) recorded in the studied intertidal habitats of the Canary

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Islands (Fig. 3a). On Tenerife, the HOBO logger was placed at Punta del Hidalgo in the north (site 28 in

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Fig. 2) in an area where Z. pulchellus has one of its largest populations known for the archipelago,

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cohabiting with several colonies of P. aff. clavata. This location underwent the most extreme temperature

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variations, showing the minimum and maximum values for the whole archipelago (11.832 - 31.077°C).

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However, the mean temperature during the whole study period (21.15 ±2.24°C) was slightly higher than

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that recorded for Lanzarote (Fig. 3a). On El Hierro, the HOBO data logger was deployed on the south-

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eastern coast at Timijiraque (site 29 in Fig. 2), where a large population of P. aff. clavata was found. At

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this site, the temperature variation range during the study period was the shortest (15.557 - 29.715°C)

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(Fig. 3a), but the highest mean temperatures were recorded (21.23 ±1.77°C) in comparison to the other

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two islands.

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Fig. 3. Sea water temperature oscillations (ºC) during an annual cycle recorded by HOBO data loggers

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deployed in a) intertidal rock pools inhabited by zoantharians and b) subtidal locations on El Hierro,

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Tenerife and Lanzarote.

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The loggers deployed on each studied island clearly showed the east-to-west temperature gradient in the

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subtidal habitat of the archipelago. The warmest mean SST was recorded on El Hierro (21.79°C ±0.06

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°C), with an annual range of variation from 18.937 to 24.388°C at La Caleta (northern site, El Hierro, Fig.

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2) and from 18.771–25.501°C at Los Mozos (south-eastern site, El Hierro, Fig. 2) (Fig. 3b). In contrast,

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the coldest mean SST was recorded on Lanzarote (20.80°C ±0.09 °C), with a temperature range from

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17.534 – 24.436°C and from 17.201- 25.065 °C at Playa Quemada and Órzola, respectively (north and

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south sites on Lanzarote in Fig. 2, respectively) (Fig. 3b). In fact, there were statistical differences in the

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daily average throughout the year (F= 119.58; p=0.001), the average between summer and winter months

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(F= 210.1; p=0.015 and F= 177.93; p=0.001, respectively), and the minimum SST recorded during the

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winter (F= 177.93; p=0.001), between the two ends of the archipelago, El Hierro and Lanzarote. The

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highest temperature recordings during summer months showed non-significant differences between these

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two islands (F= 3.17; p(MC)=0.101). Despite the lack of year-around SST data for Tenerife, the

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information gathered was enough to highlight the importance of the island mass effect on the south-

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western coast of Tenerife, as temperature at this location started to increase even above the levels

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recorded for El Hierro (Fig. 3b), oscillating from 23.136°C in November 2016 to 18.628 °C in June 2017

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(Fig. 3b).

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3.2 Distribution of zooxanthellate zoantharians in the intertidal zone

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A total of 1534 colonies of Zoantharia, Suborder Brachycnemina, were recorded within the sixty-five 500

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x 500 m grids visited in 34 localities across the Canary Islands, comprising a surveyed area of ≈616,000

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m2. Palythoa aff. clavata was the dominant species with 1109 colonies observed within the archipelago

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(ESM).

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3.2.1 Zoantharia populations across the Canary Islands

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Results of the PERMANOVA analysing the abundance and composition of zoantharian populations

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within the archipelago showed a significant interaction between the factor ‘Island’ and covariable

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‘Platform size’ (Table 1). A posteriori pairwise analyses only revealed significant differences in species

277

distribution between Lanzarote and the other surveyed islands (Table 1). La Graciosa, Tenerife and El

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Hierro showed similar species abundances and compositions, with no significant differences detected

279

among these islands (Table 1). The PCO, explaining 82% of total data variability, revealed greater data

280

dispersion for Tenerife and the highest densities of Z. pulchellus recorded at sites around this Island (Fig.

281

4). Data from Lanzarote and El Hierro were less dispersed, with populations of P. aff. clavata more

282

frequently found in surveys conducted on the latter island (Fig. 4). La Graciosa showed the highest

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abundances of P. aff. clavata, along with a few sites on Tenerife (Fig. 4). PERMANOVA

Pairwise test

Source

df

SS

MS

Pseudo-F

p (perm)

Groups

Platform size

t

p (perm)

1

7.80

7.18

2.40

0.098

LG vs. LZ

3.19

0.001**

Island

3

40.97

13.66

4.57

0.002**

LG vs. TF

0.79

0.468

Ps x Island

3

27.35

9.12

3.05

0.021*

Res

58

173.47

299

Total

65

248.98

LG vs. EH

0.58

0.563

LZ vs. TF

3.56

0.001**

LZ vs. EH

2.61

0.006**

TF vs. EH

0.27

0.983

284

Table 1. Results of the one-way PERMANOVA analysing the variation in the number of colonies of

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zooxanthellate zoantharians on different islands of the Canary archipelago (LG: La Graciosa, LZ:

286

Lanzarote, TF: Tenerife and EH: El Hierro). Intertidal platform size (m2) was included as a covariable in

287

the analysis. Results of pairwise tests examining the significant factor ‘Island’ are shown. *p<0.05 and

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**p<0.01.

289 290

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Fig. 4: Principal coordinate analysis (PCO) showing the first two axes (82% of variation), based on

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Euclidean distances of log(x + 1) transformed data for intertidal zooxanthellate zoantharian population

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densities at the islands studied in the archipelago.

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Palythoa aff. clavata, P. caribaeorum and Z. pulchellus were the most abundant species recorded across

297

the archipelago with densities that varied between islands (Fig. 5). Species-specific analyses showed a

298

significant effect of the interaction between factor ‘Island’ and covariable ‘Platform size’ for P. aff.

299

clavata (Table 2). Significantly higher densities of this species were found in one location on La Graciosa

300

in comparison with the other studied islands, while abundances recorded for Lanzarote were significantly

301

lower than the rest of the islands (Table 2, Fig. 5). Differences in the number of colonies of P. aff. clavata

302

were not significant between Tenerife and El Hierro, with intermediate species abundances (Table 2, Fig.

303

5). On comparing abundances of P. caribaeorum, there was a significant interaction between the size of

304

the intertidal platform and the factor ‘Island’ (Table 2). Large population abundances of the species were

305

recorded on the largest intertidal rocky platform on Lanzarote, at Órzola (site 3 in Fig. 2) (Fig. 5, ESM).

306

No significant differences in the abundance of Z. pulchellus among islands and no significant effect of the

307

area of the intertidal platform were found (Table 2). However, despite the lack of statistical significance,

308

data showed that intertidal populations of Z. pulchellus only occurred on Tenerife (Fig. 5, ESM).

309 Palythoa. aff. clavata Source

df

SS

Platform size

1

201.28

Island

3

25713.00

Ps x Island

3

Res

58

Total

65

91066.00

MS

P. caribaeorum

Pseudo-F

P (perm)

SS

MS

201.28

0.34

8570.90

14.39

0.520

2345.00

0.001**

59.96

30609.00

10203.00

17.13

31544.00

595.58

0.001**

Zoanthus pulchellus Pseudo-F

P (perm)

SS

2345.00

36.09

0.001**

987.81

19.98

0.30

0.612

1499.70

944.48

314.83

4.85

0.053*

3768.30

64.97

7117.80

Pseudo-F

P (perm)

987.81

2.52

0.108

499.90

1.28

0.224

4295.90

1432.00

3.65

0.103

22735.00

391.99

29519.00

11

MS

Pairwise

t

P (perm)

LG vs. TF

112.86

0.001**

LG vs. LZ

5.49

0.001**

LG vs. EH

6.75

0.001**

LZ vs. TF

2.17

0.040*

LZ vs. EH

2.10

0.037*

TF vs. EH

0.14

0.904

310 311

Table 2. Results of one-way permutational analyses of variance (ANOVAs) in the density of the most

312

abundant zoantharian species (Palythoa aff. clavata, P. caribaeorum and Zoanthus pulchellus) on

313

different Canary Islands (LG: La Graciosa, LZ: Lanzarote, TF: Tenerife and EH: El Hierro). Intertidal

314

platform size (m2) was included as a covariable in the analysis. Results of pairwise tests examining the

315

effect of the factor ‘Island’ are shown whenever significant. *p<0.05 and **p<0.01.

316 317

Fig. 5. Number of colonies (mean ± SD) of the three most abundant zooxanthellate zoantharian species,

318

Palythoa aff. clavata, P. caribaeorum and Zoanthus pulchellus, on four of the Canary Islands.

319 320

3.2.2 Zoantharia populations on Tenerife

321

The PERMANOVA analysing densities and composition of zoantharian populations recorded for

322

Tenerife revealed significant differences among orientations and the covariable ‘Platform Size’ (Table 3).

323

A posteriori pairwise analyses showed that differences in species distribution were found between the

324

south-western and northern sides of the island. Population differences between the south-western and

325

south-eastern coasts were marginally significant, and non-significant when comparing its northern and

326

south-eastern coasts (Table 3). The PCO analysis, explaining ≈85% of total data variability, showed that

327

surveys conducted on Tenerife displayed high data variability overall, especially along the northern coast

328

where populations of Z. pulchellus were more frequent and reached higher abundances (Fig. 6). Data

329

from its south-eastern and south-western coasts were less dispersed and varied mainly according to the

12

330

abundance of P. aff. clavata and P. grandiflora, with the south-eastern coast frequently showing less

331

abundance of colonies of the studied species (Fig. 6, ESM).

332 PERMANOVA

333 334 335

Pairwise tests

Source

df

SS

Platform size

1

11.15

Orientation

2

Ps x Or

MS

Pseudo-F

p(perm)

Groups

t

p(perm)

11.15

3.45

0.029*

N vs. SE

1.26

0.204

17.33

8.67

2.68

0.027*

N vs. SW

2.11

0.009**

2

6.83

3.42

1.06

0.380

SE vs. SW

1.57

0.081

Res

39

126.00

3.23

Total

44

161.30

336

Table 3. Results of the one-way PERMANOVA analysing the variation in the number of colonies of

337

zooxanthellate zoantharians around sites located in different orientations of Tenerife. Intertidal platform

338

size (m2) was included as a covariable in the analysis. Results of pairwise tests examining the significant

339

factor ‘Orientation’ obtained in the analysis are shown. *p<0.05 and **p<0.01. Underlined p-values were

340

accepted as marginally significant.

341 342

Fig. 6: Principal coordinate analysis (PCO) showing the first two axes (84.6% of variability), based on

343

Euclidean distances of log (x + 1) transformed data of abundances (number of colonies) of zooxanthellate

344

zoantharian populations around Tenerife.

345 346

Species-specific analyses only showed significant differences among orientations on Tenerife in the case

347

of P. caribaeorum (F= 5.53; p=0.015), which was more abundant on the south-western than the south-

348

eastern and northern coasts, with no occurrence recorded in the latter orientation (Fig. 7). In the case of P.

349

aff. clavata, the most abundant zoantharian species across Tenerife, there were no significant differences

350

between island orientations in the number of colonies recorded (F= 1.06; p= 0.334). However, the highest

351

mean number of colonies of the species was also found on the south-western coast, followed by the

13

352

northern and south-eastern (Fig. 7). Palythoa grandiflora showed marginally significant differences

353

among orientations (F= 3.04; p= 0.084) since this species was located at only one site on the western

354

coast of Tenerife (Alcalá, site number 18 in Fig. 2, ESM), in high population densities (72 colonies in 250

355

m²) (Fig. 7).

356

Species belonging to the family Zoanthidae were significantly influenced by platform size (Z. pulchellus:

357

F= 4.32, p=0.039; I. tuberculatus: F=4.21, p=0.049), but not by coastal orientation or the interaction

358

between the factor and covariable. The greatest populations of both species were found on the two largest

359

rocky platforms of northern Tenerife, at Los Silos (site 22, Fig. 2) and Punta del Hidalgo (site 28, Fig. 2)

360

(93 and 157 colonies of Z. pulchellus, respectively; 5 and 6 colonies of I. tuberculatus, respectively,

361

ESM).

362 363

Fig. 7: Number of colonies (mean ± SD) of all zooxanthellate zoantharians recorded at different sites

364

located on the northern (N), south-eastern (SE) and south-western (SW) coasts of Tenerife.

365 366

3.3 Distribution of zooxanthellate zoantharians in the subtidal zone

367

Only three zoantharian species were found in the subtidal zone of the Canary Islands –P. aff. clavata, P.

368

caribaeorum and Z. pulchellus- with only two colonies of the latter species recorded for Tenerife and El

369

Hierro (Table 4). Species-specific analyses of the most abundant species, including P. aff. clavata and P.

370

caribaeorum, showed contrasting effects of the factor ‘Island’. Marginally non-significant differences in

371

P. aff. clavata abundance were recorded among the studied islands (F= 3.21; p=0.067), with higher mean

372

numbers of colonies on Tenerife than El Hierro (t= 2.04; p= 0.064), while there were no differences

373

between the lowest abundances of Lanzarote and El Hierro (t= 0.15; p= 0.888) (Table 4). Discrete

374

colonies of this species were found in some subtidal locations on Tenerife and El Hierro, within a depth

375

range of 1 to 10 m (Table 4). The two colonies recorded on Lanzarote in the shallower subtidal, at a

376

maximum depth of 3 m, were the largest found in the whole study, considering both the subtidal and

377

intertidal zones (Table 4).

14

378 Total number of colonies P. aff. clavata Lanzarote Tenerife Hierro Z. pulchellus Lanzarote Tenerife Hierro

Mean abundance (± SD)

Depth range (m)

2 7 2

0.29 ± 0.49 1.4 ± 1.14 0.25 ± 0.46

1.5 - 3 1-6 10 - 11

0 1 1

0.2 ± 0.44 0.125 ± 0.35

3 4

Frequency distribution of colony size (%)

XS

S

71 50

29 50

M

L

XL 100

100 100

379 380

Table 4: Summary of population features recorded for zoantharian species Palythoa aff. clavata and

381

Zoanthus pulchellus at the different islands surveyed across the Canary archipelago. Total number, mean

382

abundance, depth range, and size frequency distribution of colonies (measured by number of polyps) are

383

reported for each island.

384 385

Palythoa caribaeorum was only found in the subtidal zone of the western islands, being especially

386

abundant on both El Hierro and Tenerife (19.75 ± 24.11 and 26.00 ± 50.75 colonies, respectively).

387

However, the analyses of variance did not detect statistical differences in population abundances between

388

islands (F= 0.37, p= 0.488). On the south-eastern coast of Tenerife, the species locally covered wide areas

389

(> 100 m²), constituting the largest subtidal coverage by the species found to date in the archipelago,

390

between 2-7 m depth. Although size frequency distribution analysis showed no significant differences

391

between islands (Z=0.63; p= 0.819), a trend towards higher occurrence of the largest colony-size

392

categories (2-4 m2 and > 4 m2) on Tenerife than El Hierro was observed, accounting for >26% of the

393

colonies surveyed for the former, while on El Hierro only ≈14% of the colonies reached large sizes (Fig.

394

8). Smaller size categories of this species were more frequent on El Hierro (74% of occurrence) than

395

Tenerife (≈58%) (Fig. 8). Despite the larger areas occupied by P. caribaeorum in Tenerife, the species

396

was more widespread on El Hierro, where it reached up to 17 m, the deepest record for the species.

397 15

398

Fig. 8. Size frequency distribution (%) of subtidal populations of Palythoa caribaeorum found on El

399

Hierro and Tenerife. Size categories of the colonies are given, considering colony coverage area. XS: <

400

0.5 m2, S: 0.5-1 m2, M: 1-2 m2, L: 2-4 m2, and XL: > 4 m2.

401 402 403

4. Discussion

404

The results of this study highlight a wide variability in distribution patterns of zooxanthellate zoantharians

405

across the Canary Islands. The respective distributions of intertidal and subtidal populations of Palythoa

406

aff. clavata and P. caribaeorum followed the contrasting sea water temperature regimes recorded in the

407

benthic habitats of these islands. In contrast, species such as Z. pulchellus were more influenced by

408

platform size. Intertidal P. aff. clavata was most abundant on the western coast of Tenerife, while its

409

smallest populations were seen on Lanzarote. In the subtidal zone, P. caribaeorum was only recorded on

410

the western islands where SST are warmer throughout the year. Despite results showing that P. aff.

411

clavata is more influenced by extreme environmental conditions in the intertidal zone, we consider both

412

Palythoa species populations to be potential indicators of tropicalization processes in the Canaries,

413

showing their value for monitoring programmes. Furthermore, since P. aff. clavata and P. caribaeorum

414

are in fact common species found throughout the whole archipelago, they should no longer be catalogued

415

as species of special interest in Canarian ecosystems, according to species conservation criteria of

416

endemicity, level of endangerment, etc.

417 418

4.1 Distribution of zooxanthellate zoantharians in the intertidal zone

419

The two recorded species of the family Zoanthidae, Isaurus tuberculatus and Zoanthus pulchellus, were

420

only found in the intertidal zone of Tenerife, showing preference for large rocky platforms on the north

421

coast (ESM). Few discrete colonies of I. tuberculatus were found in the low intertidal of Los Silos and

422

Punta del Hidalgo (sites 22 and 28 in Fig. 2, respectively), locations that are characterised by high

423

hydrodynamic levels (Yanes et al., 2006) and more constant sea water parameters, given their proximity

424

to the subtidal zone and shorter exposure times during low tides. Although I. tuberculatus has a pan-

425

tropical distribution, it has been claimed to be uncommon, due mainly to its cryptic appearance and

426

habitat preferences, which usually hinder the detection of colonies (Reimer et al., 2008a). In contrast, Z.

427

pulchellus showed high population densities covering extensive areas in the rock pools at the same

428

northern locations on Tenerife (ESM). The species showed great tolerance to the low SST recorded in

429

rock pools at Punta del Hidalgo, where winter temperatures reached minimum values as low as 11.8°C.

430

Such a surprisingly low temperature range for the subtropical location of the islands is probably due to the

431

cool moist trade winds that blow mainly against the northern sides of the islands (Font 1956; García-

432

Herrera et al., 2001). These conditions affect Z. pulchellus the most, given the location of the species in

433

rock pools that remain disconnected from the sea for long periods. However, despite this great tolerance

434

to low temperatures, Z. pulchellus has not been recorded for the easternmost island Lanzarote,

16

435

characterised by the coldest waters of the archipelago, not even on the wide rocky intertidal platform at

436

Órzola (site 3, Fig. 2). In general, we did not find a clear longitudinal distribution pattern related to SST

437

for the species in members of the family Zoanthidae, so other factors such as specific habitat features or

438

interspecific interactions may be responsible for the abundance of the species.

439

Regarding the family Sphenopidae, P. grandiflora showed high densities at only one location on the

440

warmest coast of south-western Tenerife (ESM), where cropland irrigation water flows directly into the

441

sea. It is known that P. grandiflora can become locally dominant by taking advantage of degraded coral

442

reefs, being able to cause community phase shifts (Cruz et al., 2016a). Future studies focusing on both

443

biotic and abiotic parameters are needed, to clarify the environmental and ecological requirements of this

444

species and facilitate early detection of any potential ecosystem effects in the Canaries.

445

Palythoa aff. clavata is an amphi-Atlantic tropical species that inhabits a range from Cape Verde to

446

Madeira in the East Atlantic (Reimer et al., 2012; Araújo and Freitas 2003; López et al., 2019). Previous

447

studies have only described a few dispersed colonies of the species in the Canary Islands (López-Pérez

448

2014; Riera et al., 2014). Nevertheless, our results showed that P. aff. clavata has become a very common

449

element in this archipelago, with colonies recorded for the surveyed islands (ESM). The tropical affinities

450

of the species are highlighted by the fact that the largest population densities were found on the western

451

islands El Hierro and Tenerife, especially in their warmest locations (Barton et al., 1998). What is more,

452

despite reduced habitat availability due to the narrower intertidal rocky platforms of the more tropical

453

island of El Hierro compared to Tenerife, densities of P. aff. clavata were similar on the two islands.

454

Unexpectedly, one of the largest populations of P. aff. clavata was recorded on the eastern side of the

455

archipelago, at La Graciosa. This appears to be a recent population expansion, as only one colony was

456

previously recorded in the year 2000 at the same location (pers. comment.). Colonies were found

457

clustered at a specific location and species abundance may be related to the topographic conditions of a

458

very flat intertidal rocky platform with shallow rock pools (5-10 cm depth), in which SST easily warms

459

up and reaches high values during low tide. However, futures studies should consider other parameters

460

that might determine the high abundances of P. aff. clavata at this specific location, such as nutrient

461

availability or type of substrate.

462

Palythoa caribaeorum is known to show great tolerance to changes in temperature and water conditions

463

in tropical rock pool environments (Bastidas and Bone 1996; Bouzon et al., 2012). However, few

464

populations of the species were found on intertidal coasts in the Canary Islands subtropical region (ESM).

465

One of the most extensive populations was on Lanzarote (~50 m²), in an intertidal habitat very similar to

466

that of the other large population of the species already known on the nearby island of Fuerteventura (~90

467

m², previous unpublished data). In both cases, there are extensive marine abrasion platforms (Carracedo

468

and Rodrıguez Badiola 1993; Meco et al., 1997) on the eastern side of both islands. The large size and

469

height of these platforms, combined with the widest tidal amplitude within the archipelago (maximum

470

low tide of 1.8 m in comparison to 1.1 m maximum in the western islands; source:

471

https://puertoscanarios.es/), leave many rock pools separated from the open ocean for several hours. This

472

explains the huge annual variation in temperature ranges recorded in situ by the temperature data logger

473

(14.098 - 30.722°C). Indeed, the warming during low tide might be the reason why the thermophilic P.

17

474

caribaeorum is able to inhabit the intertidal zone of the cooler eastern islands. Moreover, a distribution

475

pattern related to SST variability within a single island was detected for this species. Colonies of P.

476

caribaeorum have been known to inhabit the western intertidal coast of Tenerife since 1980

477

(Arechavaleta 2009), where air and sea water temperatures are warmer than in other orientations of the

478

island (Barton et al., 1998). This study constitutes the first intertidal record of P. caribaeorum on the

479

eastern coast of Tenerife, which suggests a recent expansion at the island scale. However, the species has

480

not yet been able to settle on its northern coast, nor on Madeira island, despite the high dispersal abilities

481

of its larvae (Ryland et al., 2000). This is probably related to the lower temperatures reached during

482

winter and the sensitivity of Sphenopidae larvae to cold SST (Ryland et al., 2000), or to spatial

483

competition processes with other components of intertidal habitats at northern locations, where algal

484

stands are more developed that on southern coasts (Sangil et al., 2011).

485

4.2 Distribution of zooxanthellate zoantharians in the subtidal zone

486

Palythoa caribaeorum, P. aff. clavata and Zoanthus pulchellus were more common in subtidal locations

487

of the warmer western Canaries (ESM). On the other hand, P. grandiflora and I. tuberculatus were not

488

found in the subtidal sites surveyed within this study, probably as a result of the low colony abundance of

489

the latter species. In fact, some observations of disperse colonies of I. tuberculatus are known for several

490

islands of the archipelago (Tenerife, El Hierro and Fuerteventura), up to 30 m depth (L. Martín and R.

491

Herrera pers. comment).

492

Members of the family Zoanthidae have shown to be more tolerant than other zoantharians to low

493

temperatures in the Pacific Ocean (Reimer et al., 2008b) and species such as Z. sansibaricus can be found

494

up to 36°N (Reimer et al., 2018). On the other hand, distribution of their planktonic larvae around the

495

Atlantic also confirms that Zoanthidae species present higher tolerance than Sphenopidae species (up to

496

18°C and 22°C respectively) (Ryland et al., 2000). However, the results of this study showed that

497

Zoanthus pulchellus was absent in eastern subtidal locations of the archipelago, while P. aff. clavata was

498

the most widespread zoantharian. In fact, the latter species was the only zoantharian able to inhabit

499

subtidal locations of Lanzarote, where SST reached the minimum values recorded (17.53°C) due to the

500

influence of the Saharan up-welling (Barton et al., 1998; Vélez et al., 2015). Moreover, only Palythoa

501

spp. have so far been recorded at 32°N on Madeira island, constituting the northernmost distribution limit

502

for Brachycnemina zoantharians in the East Atlantic (Araújo and Freitas 2003; López et al., 2019).

503

Therefore, our data showed species-specific temperature affinities along the temperature gradient of the

504

archipelago, P. aff. clavata being more tolerant to cold water than Z. pulchellus. Nevertheless, other

505

factors such as habitat availability, incidence of currents or simply stochastic features might determine the

506

distribution of these species throughout the Archipelago.

507

Results of our extensive surveys and previous literature revised showed that all the subtidal populations of

508

P. caribaeorum were found in the western islands of La Palma, El Hierro and Tenerife. In fact, a large

509

population of >100 m2 has also recently been recorded on the south eastern coast of La Gomera (ESM).

510

Palythoa caribaeorum distribution clearly showed a relationship with warmer environments, but also in

511

association with high densities of the common echinoid Diadema africanum, which leaves unvegetated

18

512

spaces available for P. caribaeorum growth (pers. comment). These wide extensions of bare rocky

513

substrate were completely covered by this species, leaving little free space available for other important

514

benthic organisms such as macroalgae and other sessile invertebrates, therefore greatly modifying the

515

underwater landscapes. Palythoa caribaeorum has been reported as a widespread tropical species with

516

great tolerance to environmental conditions (Sebens 1982), being locally dominant in hard bottom

517

substrates of tropical and subtropical reefs (Sebens 1982; Acosta 2001; Silva et al., 2015). Given the

518

potential of P. caribaeorum to completely modify benthic ecosystems, further ecological studies are

519

needed to better understand the processes of habitat and community transformation mediated by the

520

proliferation of this zoantharian in the Canary Islands.

521

The subtidal distribution of P. caribaeorum restricted to the western islands shows that the temperature

522

gradient across the archipelago is an important factor influencing species distribution within the region.

523

This gradient has been proven to cause significant differences in temperature regimes in nearshore

524

environments. Although on Lanzarote coastal SST reached similar maximum values recorded in subtidal

525

benthic habitats to those of El Hierro, minimum temperature records during winter months greatly

526

contrasted between the two ends of the Archipelago. These differences could be hindering the settlement

527

of this species in the eastern islands. Indeed, its strong competitive ability (Sebens 1982) and antipredator

528

mechanisms (Bastidas and Bone 1996; Tubaro et al., 2011; Cen-Pacheco et al., 2014) limit the factors that

529

can determine its distribution. In an ocean warming scenario, it is expected that these Palythoa species

530

will become dominant in subtidal zones, expanding their populations to the eastern islands as temperature

531

rises. Following this regional expansion of the species’ distribution range, important changes in subtidal

532

landscapes and potential alterations in natural ecosystems can be expected for the archipelago.

533 534 535

4.3 Brachycnemina species as indicators of ocean warming

536 537

Zoantharians have shown to be more resistant than hermatypic corals to current habitat degradation,

538

pollution and the increase in ocean temperature related to climate change (Dubinsky and Stambler 1996;

539

Hoegh-Guldberg et al., 2007; Vermeij et al., 2011). In fact, some species are presently expanding their

540

populations, becoming locally abundant under these conditions (Karlson 1981; Sebens 1982; Acosta

541

2001; Silva et al., 2015; López et al., 2018) and even causing phase-shifts in coral reef ecosystems (Cruz

542

et al., 2016a, b). Intrinsic characteristics of Zoantharia, such as colony plasticity (Karlson 1983; Costa et

543

al., 2011) and fast growth combined with effective asexual reproduction (Suchanek and Green 1981;

544

Rabelo et al., 2013; Fadlallah et al., 1984; Acosta and Asbahr 2000; Acosta et al., 2001) and their potent

545

palytoxin content (Tubaro et al., 2011) are probably involved in such colonisation success. This study

546

constitutes an example of such population spread outside tropical regions and, to our knowledge, one of

547

the few cases occurring in macroalgae-dominated systems.

548 549

Biological indicators that react to a given impact can facilitate evaluating the status of the whole

550

ecosystem. However, the selection of an appropriate indicator is a difficult task, since the selected

551

organisms or their populations depend on the final goal and the region under study (Gerhardt 2002). In

19

552

general, they should specifically respond to a single pressure, be easy to identify and sample, as well as

553

show high abundances and widespread distributions (Gerhardt 2002; Rice and Rochet 2005; Dulvy et al.,

554

2008). Considering these characteristics, Brachycnemina populations appear to be ideal indicators of

555

ocean warming in the Canaries, especially P. aff. clavata and P. caribaeorum, which showed a clearer

556

pattern of distribution related to SST. In this field, programmes to monitor subtidal populations of P.

557

caribaeorum should especially be considered, as its tropical affinity has led to a current outbreak and

558

expansion of the species with a great potential to alter local ecosystems. As their populations are easy to

559

sample, and individuals are long-lived and without many predators, these species are ideal to develop

560

long-term monitoring actions and compare changes in ecosystems. They can be useful not only in the

561

Canary Islands but also at other locations, since they are globally widespread species in both coral reefs

562

and macroalgae-dominated systems. The present study would be crucial in providing a baseline reference

563

of the status of these zoantharian populations, of great use in future assessments interpreting trajectories

564

of alterations in local ecosystems faced with the current level of global change.

565 566 567

5. Acknowledgements

568

We are grateful to Adriana Rodriguez, Omar Álvarez and Alejandro Escánez for helping during

569

fieldwork. We thank Laura Martín and Jacinto Barquín for providing maps. CL was co-funded by the

570

Canarian Agency for Research, Innovation and Information Society of the Ministry of Economy,

571

Industry, Trade and Knowledge and by the European Social Fund (ESF) integrated operational program

572

of the Canary Islands 2014–2020. This research was conducted within the framework of the

573

‘INDICATROP’ project conducted with the financial support of ‘CajaCanarias’ Foundation. The research

574

was supported by Programa Mecenazgo Alumni of the University of La Laguna (2017).

575 576

6. References

577 578

1.

Acosta, A., 2001. Disease in Zoanthids: dynamics in space and time. Hydrobiologia 460,113130. http://dx.doi.org/10.1590/S0001-37652008000300010

579 580

2.

Acosta, A., Asbahr, M., 2000. Reproductive effort in Palythoa caribaeorum. 9th International Coral Reef Symposium. Bali, Indonesia, pp. 295.

581 582 583

3.

Anderson, M.J., 2001. Permutation tests for univariate or multivariate analysis of variance and regression. Canadian Journal of Fisheries and Aquatic Science 58, 626639. https://doi.org/10.1139/f01-004

584 585

4.

Araújo, R., Freitas, M., 2003. A new crab record Platypodiella picta (A. Milne-Edwards, 1869) (Crustacea: Decapoda: Xanthidae) from Madeira Islands waters. Bocagiana 212, 1-6.

586 587 588 589

5.

Arechavaleta, M., 2009. Palythoa caribaea (Expte Palcar 08/2009). In: Suárez-Sanchez, C.L. (Eds.), Evaluación de especies catalogadas de Canarias. Dirección General del Medio Natural. Consejeria de Medio Ambiente y Ordenación Territorial, Gobierno de Canarias, Canarias, Spain, pp. 1-9.

590 591 592

6.

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Supplementary material

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Fig. Electronic supplementary material (ESM). Distribution and abundances of Zoantharia

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Brachycnemina species around intertidal habitats (above) and subtidal habitats (below) throughout the

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Canary Islands, including previous data from the literature (Arechavaleta M 2009, Martín-García 2013).

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26

Highlights

Data loggers showed the east-to-west SST gradient in coastal habitats of the Canaries

Palythoa spp. distributions followed patterns related to oscillations of SST

Palythoa spp. populations are potential indicators of tropicalization processes

First Zoantharia population spread occurring in macroalgae-dominated systems

Author statement First author (CL) performed all surveys, data analyses and wrote the manuscript that was revised extensively by SC who also contributed to data analyses and surveys. SM participated in data collection, data analyses and preparation of the revised draft. AB was in charge of the project coordination, responsible for the research activity planning and acquisition of the financial support, as well as critically revising data analyses and drafts. All authors contributed to subsequent revisions of the manuscript and edited the final version of the manuscript.

Changes to authorship We also want to inform you that the author Omar Alvaréz has decided to resign the authorship of the manuscript given that despite he participated in filed work, due to personal incompatibilities he has not been able to contributed to data analyses, preparation of the draft neither the revised version of the manuscript. For these reasons, and due to his personal and professional interest have changed, he has decided to quit from authorship. If it is necessary, he can send you a formal renounce resignation letter signed by himself or any other information that you request. All authors agree with removing OA as he asked but if you need any further information you can contact him ([email protected]) and the rest of authors.

In behalf of all authors Cataixa López Batista

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: