- Email: [email protected]
Growth anomalies on Acropora cytherea corals
Marine Pollution Bulletin 62 (2011) 1702–1707
Contents lists available at ScienceDirect
Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Growth anomalies on Acropora cytherea corals Akiyuki Irikawa a,⇑, Beatriz E. Casareto a, Yoshimi Suzuki a, Sylvain Agostini a, Michio Hidaka b, Robert van Woesik c a
Graduate School of Science and Technology, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan Department of Marine and Environmental Science, Graduate School of Engineering and Science, University of the Ryukyus, 1-1 Senbaru, Nishihara-city, Okinawa 902-0129, Japan c Department of Biological Sciences, Florida Institute of Technology, 150 West University Boulevard, Melbourne, FL 32901, USA b
a r t i c l e Keywords: Corals Growth anomalies Acropora cytherea Disease Aging
i n f o
a b s t r a c t This ten-year study examined the morphological, physiological, and ecological characteristics of coral growth anomalies on Acropora cytherea on Amuro Island, Okinawa, Japan. The objectives of the study were to assess whether the growth anomalies, identified as diffuse disruptions on the skeleton: (i) were more prevalent on large colonies than on small colonies, (ii) were more common near the center of the colonies than peripherally, (iii) affected colony growth and mortality, and (iv) affected coral-colony fecundity and photosynthetic capacity. We hypothesized that the growth anomalies were signs of the onset of aging. The growth anomalies were more prevalent on colonies >2 m diameter, and were concentrated near the central (older) portions of the colonies. The growth anomalies were also associated with reduced productivity and dysfunctional gametogenesis. Still, the growth anomalies did not appear to affect colony survival. The contact experiments showed that the growth anomalies were not contagious, and were most likely a sign of aging that was exacerbated by thermal stress. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Although the number of reports on coral diseases is increasing, our general understanding of coral diseases is still rudimentary (Harvell et al., 2002; Sutherland et al., 2004; Reed et al., 2010). While some diseases are clearly transmissible (Zvuloni et al., 2009), and others are exacerbated by thermal stress (Muller et al., 2008), some purported diseases may be merely a consequence of abnormal growth, the likelihood of which increases with aging (Bak, 1983; Loya et al., 1984). In most clonal organisms, such as corals, it is thought that the environment, rather than programmed aging (i.e., senescence), controls the death of a genet. Although coral senescence has been suggested in some studies, most evidence has been circumstantial (Bak, 1983; Loya et al., 1984) or exists in theory (Orive, 1995), and the paradigm remains – coral growth is indeterminate. Still, in the Indo-Pacific Acropora nasuta are rarely found >40 cm diameter, and the tabulate coral Acropora cytherea are rarely found to be >3.5 m diameter (pers. obs.). Moreover, the growth of many corymbose Acropora follows a logistic function (van Woesik et al., 2011), which means that the colonies slow their growth as they age. But evidence for coral-colony aging remains unclear. It is unknown whether growth anomalies are potentially indicative of aging, and whether environmental stress exacerbates the formation of anomalies, and hastens the aging process. ⇑ Corresponding author. Tel.: +81 3 3748 5900; fax: +81 3 3748 5939. E-mail address: [email protected] (A. Irikawa). 0025-326X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2011.05.033
This study tracks growth anomalies in A. cytherea colonies. Growth anomalies are defined as focal or multi-focal annular to diffuse lesions, consisting of abnormally arranged skeletal elements (Coral Disease Handbook, 2008, p. 32). When scleractinian corals suffer mechanical damage, for example by wave impact or predation, they often swell through the recovery process (Sparks, 1972; Bruno and Edmunds, 1997). However, growth anomalies are different from the swelling that is apparent through the recovery process. Instead, growth anomalies are characterized by abnormal polyps and abnormal coenosarc (Gateno et al., 2003; Stimson, 2010). Since 1965, growth anomalies have been given a variety of names, including neoplasia (White, 1965; Squires, 1965; Bak, 1983), hyperplasia (Willis et al., 2004), calicoblastic epitheliomas (Coles and Seapy, 1998), and tumors (Cheney, 1975; Loya et al., 1984; Yamashiro et al., 2001). Growth anomalies have been described on 10 families of scleractinian corals (Peters et al., 1986), and have been found at various locations including the Hawaiian Islands (Squires, 1965), Guam, Saipan, the Marianas, Palau, Pohnpei, Enewetak in the Marshall Islands (Cheney, 1975), the Netherlands Antilles (Bak, 1983), Magnetic Island on the Great Barrier Reef (Loya et al., 1984), the Florida Keys (Peters et al., 1986), French Polynesia (Le Campion-Alsumard et al., 1995), the Pacific coast of Costa Rica (Gateno et al., 2003), and Okinawa (Yamashiro et al., 2001). Growth anomalies are often displayed as wart-like, spherical protuberances, characterized as a chaotic array of low-density corallites (Domart-Coulon et al., 2006; McClanahan et al., 2009;
A. Irikawa et al. / Marine Pollution Bulletin 62 (2011) 1702–1707
Stimson, 2010). In several cases, tissues covering the growth anomalies appear pale, generally because the tissue supports low densities of zooxanthellae (Cheney, 1975; Bak, 1983; Peters et al., 1986; Yamashiro et al., 2001; Gateno et al., 2003). Previous studies have also reported some necrosis of the growth anomalies (Coles and Seapy, 1998; Work et al., 2008), and others have reported negative growth effects on the adjacent tissue (Cheney, 1975; Bak, 1983; Stimson, 2010), or inhibition of reproduction (Domart-Coulon et al., 2006; Yamashiro et al., 2000). Still, the etiology of growth anomalies remains largely unknown (Peters et al., 1986; Domart-Coulon et al., 2006). Grygier and Cairns (1996) reported that parasitic (endoparasitic, petrarcid ascothoracidan) crustaceans were involved in the generation of growth anomalies in deep-sea corals (Grygier and Cairns, 1996). In other cases, growth anomalies were reported to be the corals’ response to either bacterial infection (Domart-Coulon et al., 2006), or parasitic endolithic algae (Le Campion-Alsumard et al., 1995; Coles and Seapy, 1998; Domart-Coulon et al., 2004; Breitbart et al., 2005; Work et al., 2008). Other researchers have suggested that high intensity ultra-violet light (Loya et al., 1984; Peters et al., 1986; Coles and Seapy, 1998), and high temperatures (Stimson, 2010) cause growth anomalies. McClanahan et al. (2009) suggested that environmental stresses (i.e., high temperature) allowed fungi to invade corals and influence coral calcification. Generally, the prevalence of growth anomalies is considered to be localized (Cheney, 1975; Bak, 1983; Coles and Seapy, 1998). Yet, long-term studies are few, and we do not know whether growth anomalies spread through populations. This study examined whether the growth anomalies on A. cytherea colonies, defined as diffuse disruptions of the skeleton: (i) were more prevalent on large colonies than on small colonies, (ii) were more common near the center of the colonies than on their periphery, (iii) affected colony growth and mortality, and (iv) affected coral-colony fecundity and photosynthetic capacity. This study tested the hypothesis that the growth anomalies on A. cytherea were signs of the onset of aging. 2. Materials and methods 2.1. Study site This study was undertaken on the fringing reef on the east side of Amuro Island (26°140 4000 N, 127°140 3100 E), in the Kerama Islands, 40 km west of Okinawa, Japan. The study area supported diverse coral assemblages that were dominated by tabulate acroporid species. Corals were assessed annually at three sampling stations, from May 2001 to May 2010. Each station was approximately 100 m 100 m, and located between 9.5 and 11.4 m, below low water spring tide. The study area had consistently strong tidal currents, reaching up to 60 cm/s on spring tides, and low turbidity. The ten-year monthly means of sea surface temperature varied from 19.5 °C in March to 31.5 °C in August (data provided by the Akajima Marine Science Laboratory). Nutrient levels were generally low (for example, on June 5, 2004 nutrients were: 0.1 lmol, of PO4, 0.3 lmol of NO3, <0.1 lmol of NO2, and <0.4 lmol of NH4). There is no human habitation on Amuro Island. 2.2. Epidemiology of growth anomalies The relationship between the occurrence of growth anomalies and coral specificity was assessed at the three stations in August 2004, using 3, 50 m by 2 m belt transects at each station. All A. cytherea colonies supporting growth anomalies were photographed with a digital camera, OLYMPUS-5060, with a 30 cm metric stainless steel scale. The photographs were used to calculate: (i)
1703
the growth rates of each coral colony, (ii) the growth rates of each growth anomaly, and (iii) the rates of spread of the dead area on the growth anomalies. Each growth anomaly was allocated to one of four concentric zones on each colony. Each zone had approximately the same projected surface area. The zone near the center of the colonies was classified as Zone 1, and the zone near the colony periphery was classified as Zone 4. A chi-square analysis tested the hypothesis that there were more growth anomalies near the center of the colonies, in Zone 1, than elsewhere. We biannually measured the density, size, and position of the growth anomalies on each colony of A. cytherea between May 2001 and May 2010. We also tracked colonies without growth anomalies in order to estimate: (i) anomaly prevalence, to determine how many colonies in the population had growth anomalies, (ii) anomaly incidence (or the number of new colonies acquiring anomalies), (iii) anomaly growth rate, (iv) the spread of the anomalies within and among colonies, (v) the change in tissue mortality within each growth anomaly, and (vi) the mortality of colonies. The epidemiological analysis was conducted using field surveys and NIH Image Analysis. Field surveys were initiated in 2001, but some inference was made using the size of the anomalies. For example, a growth anomaly that was 2 cm diameter in October 2001 was estimated to have started growing in 2000 (based on subsequent growth rates). Growth rates of anomalies were compared using linear measurements and using the proportion of the projected surface area. The latter was also used for survival comparisons. Differences in growth rates were compared among colonies with and without growth anomalies using a Tukey-multiple comparison test. 2.3. Polyps, calices, and gametes The morphology of the skeleton of tissue-free anomalies was examined at three different spatial scales: (i) cm, (ii) mm, and (iii) lm. At the centimeter scale, the density of polyps on growth anomalies was compared with the density of polyps on normal tissue. At the millimeter scale, the diameter of calices was recorded using Vernier calipers and compared. At the micrometer scale, a Scanning Electron Microscope (SEM) JEOL, JSM-6060LV, was used to compare skeletal structures. In order to examine oocyte formation, healthy tissue and growth-anomaly tissue were sampled (on June 7, 2004) two days before coral spawning. The skeletons were fixed with 5% formaldehyde, and decalcified in a solution of 5% acetic acid and 5% formaldehyde. The remaining tissue was embedded in paraffin, and sectioned at 7 lm intervals. All sections were stained with hematoxylin eosin and examined under a Nikon OPTIPHOT-2 microscope. 2.4. Physiology of growth anomalies Net photosynthesis, chlorophyll fluorescence, and the density of symbiotic zooxanthellae were compared between growth anomalies and normal coral tissue. In situ observations and experiments were conducted in flow-through tanks at the Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus. Twelve fragments of circular or elliptical growth anomalies, ranging from 2.1 to 2.9 cm in mean diameter, were collected in April 2004. The fragments were kept in constant running seawater, under 100–500 lmol photons m2 s1, with temperatures ranging from 24 to 26 °C. Dissolved oxygen within the culture tank was measured using a dissolved oxygen meter (YSI Biological Oxygen Monitor, Model 5300). Photosynthetic activity of Photosystem II was estimated using a Pulse Amplitude Modulation (PAM) Fluorometer (Mini-PAM, Waltz, Germany) using the following procedure. After 15 min of
1704
A. Irikawa et al. / Marine Pollution Bulletin 62 (2011) 1702–1707
dark-adaptation, the Fv/Fm value was measured, where Fv = Fm – Fo, and Fo is the initial fluorescence in the dark adapted state, and Fm is the maximal fluorescence in the dark adapted state. The distance between the coral specimens and the optic-fiber head of the PAM was kept constant. Fluorescence data were recorded six times from different parts of each coral fragment. The densities of symbiotic algae were also examined. Coral tissue was removed from the skeleton using a water-jet (Ricohelemex Dentrex T-8311). The samples were homogenized and centrifuged at 3000 rpm for 10 min using a Hitachi, Himac CR21, centrifuge. The pellets were re-suspended, and the aliquot was placed on a hemocytometer, where the density of zooxanthellae was counted within a 1 mm3 volume, under a Nikon OPTIPHOT-2 microscope at 400 magnification. 2.5. Field experiments In 2004, manipulative transplant experiments were conducted to examine whether the growth anomalies were contagious. Six growth anomalies were cut off from three, 2 m diameter A. cytherea source colonies (n = 18). Using cable ties, three of the growth anomalies from the source colony were reattached to the donor colony, whereas the other three growth anomalies were attached to another (2 m diameter) colony that had no growth anomalies. This process was repeated using two other A. cytherea colonies.
and A. secale) at Amuro Island, Okinawa, southern Japan. The frequency of growth anomalies was highest on A. cytherea, and less common on the other tabulate Acropora species, such as A. latistella, A. hyacinthus, A. microclados, A. subulata, and A. paniculata (Table 1). There was a considerable range in total surface area occupied by the growth anomalies on individual A. cytherea colonies (from <1 cm2 to >2500 cm2). Compared with small colonies, the prevalence of growth anomalies was higher in colonies >120 cm in diameter (Fig. 1a and b). The proportional area occupied by growth anomalies was also significantly greater on colonies >2 m in diameter compared with colonies <2 m in diameter (1.1 ± 2.5 and 0.17 ± 0.22, in May of 2006, Mann–Whitney U test; p < 0.01, n = 47). Colonies <1 m in diameter showed no signs of growth anomalies. For A. cytherea colonies >1 m, the growth anomalies were centrally concentrated in Zone 1. In fact, almost 80% of the growth anomalies were recorded in Zones 1 and 2 (significant Chi-square test, p < 0.05, Fig. 2). In a total of 273 A. cytherea colonies examined, 70 colonies had growth anomalies. The incidence of growth anomalies varied through time, and was greatest in 2002, but only on large colonies (Fig. 3). In general, as the size of the growth anomalies increased their expansion rate decreased (Fig. 4). There was no significant difference in the increase of colony diameter per year for colonies with and without growth anomalies. Growth anomalies >10 cm in diameter showed progressive necrosis (Fig. 5), and partial mortality in their centers.
3. Results 3.1. Epidemiology of growth anomalies
3.2. Polyps, calices, and gametes
Growth anomalies occurred on 11 species of Acropora (A. cytherea, A. latistella, A. hyacinthus, A. clathrata, A. florida, A. valenciennesi, A. palifera, A. abrotanoides, A. samoensis, A. digitifera,
The density of polyps in the tissue of growth anomalies (8.9 ± 5.4 per cm2) was significantly lower (Kruskal Wallis, p < 0.05) than the polyp densities in tissue without growth
Table 1 Prevalence of growth anomalies in Acropora cytherea and other Acropora species within the three survey areas at Amuro Island, Kerama Islands, southern Japan.
A. A. A. A. A. A. A. A. A. A. A. A.
cytherea hyacinthus latistella clathrata microclados paniculata subulata florida valenciennesi abrotanoides samoensis secale
Area 1
Area 2
Area 3
Mean prevalence
S. dev.
0.13 (n = 84) 0.13 (n = 8) 0.07 (n = 15) 0 (n = 3) 0 (n = 16) 0 (n = 1) 0 (n = 17) 0.08 (n = 12) 0.33 (n = 3) 0 (n = 0) 0.04 (n = 26) 0 (n = 10)
0.29 (n = 78) 0.09 (n = 11) 0 (n = 9) 0.20 (n = 5) 0 (n = 6) 0 (n = 1) 0 (n = 20) 0 (n = 7) 0.50 (n = 2) 0 (n = 1) 0 (n = 12) 0 (n = 4)
0.13 (n = 67) 0 (n = 3) 0 (n = 22) 0 (n = 2) 0 (n = 23) 0 (n = 3) 0 (n = 4) 0.22 (n = 9) 0 (n = 0) 0.33 (n = 3) 0.2 (n = 5) 0.07 (n = 14)
0.19 0.07 0.04 0.07 0.00 0.00 0.00 0.10 0.28 0.11 0.08 0.02
0.09 0.06 0.03 0.12 0.00 0.00 0.00 0.11 0.25 0.19 0.11 0.04
Fig. 1. The prevalence of Acropora cytherea growth anomalies (which is equivalent to the proportion of colonies with growth anomalies relative to the total number of colonies), where white bars indicate the frequency of colonies without growth anomalies, and the black bars indicate the frequency of colonies with growth anomalies for (a) 2003 and (b) 2004.
A. Irikawa et al. / Marine Pollution Bulletin 62 (2011) 1702–1707
1705
anomalies (12.3 ± 2.3). The alignment of axial and radial polyps was chaotic in growth anomalies, the calices were wider, the corallites were fewer, and many polyps were buried under a proliferating coenosteum. Growth anomalies were also covered with a dense array of irregular, short spines (Fig. 6). There was no gonad development observed in the tissue of the growth anomalies (Fig. 6). Yet, normal tissue near the growth anomalies contained well developed oocytes and sperm in all polyps. 3.3. Physiology of growth anomalies
Fig. 2. The proportion of the Acropora cytherea colony’s surface area supporting growth anomalies in relation to zones (in 2004); with Zone 1 located in the center of the colony and Zone 4 on the colony periphery (the short, horizontal lines are the means, the open circles are the standard errors, and the filled circles represent the standard deviations.
The ratio of production to respiration (P/R ratio) on coral tissue with growth anomalies was low compared with the P/R ratios of tissue without growth anomalies. For example, the P/R ratio of healthy looking tissue at 100 lmol photons m2 s1 was 0.71 ± 0.54 (n = 4), and at 250 lmol photons m2 s1 the P/R ratio was 1.63 ± 0.09 (n = 4). The net productivity of the growth anomalies was almost zero. The photosynthetic capacity of the coral symbionts, measured as Fv/Fm values, in the growth anomaly (0.48 ± 0.03, n = 3), was significantly lower (Mann Whitney U test, p < 0.05) than the photosynthetic capacity of the normal tissue (0.68 ± 0.04, n = 3) (Fig. 7). The densities of symbiotic zooxanthellae in the coenosarc tissue of the growth anomalies (1.21 ± 0.25 106 cells/cm2, n = 3) was significantly lower (Mann Whitney U test, p < 0.05) than in healthy looking tissue (2.36 ± 0.23 106 cells/cm2, n = 3). The transplant experiment showed that the transfer of the growth anomalies had no effect on the maternal colony, nor was there any noticeable effect of the transplanted growth anomalies on the colonies without growth anomalies. The growth anomalies, therefore, did not appear to be contagious. Two weeks after the transplant, the growth anomalies were covered in turf algae, and were assumed to be dead. 4. Discussion
Fig. 3. Incidence of Acropora cytherea growth anomalies (or the number of new cases of colonies with growth anomalies per year) within the 3 hectare study area, from 1999 to 2010. Incidences from 1999 to 2002 were estimated using backcalculations of the 2001 data, and by using anomaly growth rates derived from photographic analysis (from 2003 to 2010).
Growth anomalies on A. cytherea were caused by the swelling of the corals’ coenosarc and skeletal coenosteum. The skeleton of the growth anomalies consisted of irregular short spines, appearing reticulated or porous, supporting considerably fewer and widerthan-normal polyps. The growth anomalies were clearly the result of an interruption, or deterioration, of the bio-calcification process. Similar characteristics have been previously reported in other
Fig. 4. Mean annual growth rates (and 1 standard deviation) of Acropora cytherea growth anomalies in relation to their initial size.
1706
A. Irikawa et al. / Marine Pollution Bulletin 62 (2011) 1702–1707
Fig. 5. Metastasis and lesions generated by Acropora cytherea growth anomalies. (a) July 2003, (b) January 2004, and (c) linear regression of diameter of growth anomaly in relation to the percentage dead area on each growth anomaly (R2 = 0.5688, p = 0.003).
Fig. 7. Photosynthetic activity of zooxanthellae in Acropora cytherea colonies with and without growth anomalies (July 2005), where thick lines are the means, the boxes are the standard deviations, and the lines are the range of measured values.
Fig. 6. Morphological and histological characteristics of growth anomalies. (a) a Scanning Electron Microscope (SEM) image of a growth anomaly (right) on Acropora cytherea overgrowing the normal skeleton (left), (b) a cross section of healthy polyp with a large oocyte (o), (c) a vertical section of healthy polyp with oocytes and testes (t), (d) a cross section of growth anomaly tissue displaying no oocytes, and (e) a vertical section of a polyp in growth anomaly tissue displaying no gamete development.
acroporid species, including Montipora informis (Yamashiro et al., 2000). A reduction in zooxanthellae density and a low activity of the photosynthetic activity of Photosystem II in the remaining symbionts within the growth anomalies, suggest that the symbio-
sis between the coral host and the symbionts was dysfunctional. Moreover, without the support of normal-adjoining coral tissue the growth anomalies seemingly lost their autotrophic capacity, and the isolated growth-anomaly fragments died within 2 weeks after reattachment. A progressive increase in the size of the growth anomalies resulted in a loss of reproductive functionality, progressive necrosis of the growth anomalies, and an increase in partial-colony mortality. Yet, there was no evidence that the growth anomalies had changed the growth capacity of the host colonies, nor did they increase the likelihood of total colony mortality. Therefore, the question remains: Are these wart-like growth anomalies simply a sign of aging? Weismann (1891) first advanced a theory of senescence, which he considered as an aging process that was subjected to natural selection. These ideas were expanded by Williams (1957), who considered that senescence should be regarded as the general deterioration of biological systems within organisms. Williams reasoned that senescence is a consequence of a decline in repro-
A. Irikawa et al. / Marine Pollution Bulletin 62 (2011) 1702–1707
ductive capacity with increasing age, which is solely directed by the process of natural selection. Indeed, senescence in animals and plants is generally regarded as genetically programmed degeneration. After a specific number of cell divisions, somatic cells stop growing and discontinue their normal function. In some plants, senescence is associated with membrane deterioration and an increase in membrane permeability (Dhindas et al., 1981). Such replicative senescence, as it is generally called, is thought to suppress tumor growth but cause aging (Dimri et al., 1995). Therefore, by escaping the tumor-suppressive mechanisms of senescence, somatic cells acquire indeterminate, often malignant growth (Dimri et al., 1995). Nevertheless, A. cytherea growth anomalies were benign, and their growth slowed as they expanded. There was no significant difference in growth and mortality between corals with and without growth anomalies. Therefore, anomalies are not likely to be an acute disease. Moreover, the transplant experiment showed that the growth anomalies were not contiguous. The growth anomalies were, however, concentrated centrally on A. cytherea colonies >1.0 m in diameter, where the oldest cells were located. We suggest that the growth anomalies we tracked through time on A. cytherea colonies were a consequence of colony aging, which had detrimental effects on the biocalcification process. The present study showed that the density of growth anomalies increased immediately after thermal stress events. Okinawa and surrounding reefs experienced anomalous sea surface temperatures in 1998 and in 2001 (Loya et al., 2001; Bena and van Woesik, 2004; Roth et al., 2010), and the greatest densities of growth anomalies occurred in 1999 and 2002 (Fig. 2). Such environmental stress may disrupt the calcification process and cause the formation of growth anomalies. We conclude that A. cytherea colonies have determinate growth, that cell maintenance is disrupted in aged cells, in the center of large colonies, and that the formation of growth anomalies is potentially exacerbated by environmental stress. Acknowledgments We would like to thank the Tropical Biosphere Research Center (TBRC), University of the Ryukyus, for providing a visiting professor fellowship to RvW in the summer of 2010, and to the Mitsubishi Foundation for funding A.I., B.E.C., Y.S., and S.A. We also thank N. Yasuda, Dr. M. Hirose, Prof. E. Hirose, Prof. H. Yamasaki, and Dr. N. Isomura for their valuable research suggestions, and H. Chiku, Y. Anno, K. Miyagi the boat captains and master divers who enabled us to carry out the field survey. A special thanks goes to Sandra van Woesik for editorial comments. References Bak, R.P.M., 1983. Neoplasia, regeneration and growth in the reef building coral Acropora palmata. Mar. Biol. 77, 221–227. Bena, C., van Woesik, R., 2004. The impact of two bleaching events on the survival of small coral colonies (Okinawa, Japan). Bull. Mar. Sci. 75, 115–126. Bruno, J., Edmunds, P., 1997. Clonal variation for phenotypic plasticity in the coral Madracis mirablilis. Ecology 78, 21–177. Breitbart, M., Bhagooli, R., Griffin, S., Johnston, I., Rohwer, F., 2005. Microbial communities associated with skeletal tumors on Porites compressa. FEMS Microbiol. Lett. 243, 431–436. Cheney, D., 1975. Hard tissue tumors of scleractinian corals. In: Hildemann, W.H., Benedict, A.A. (Eds.), Immunologic Phylogeny. Plenum, New York, pp. 77–87.
1707
Coles, S.L., Seapy, D.G., 1998. Ultra-violet absorbing compounds and tumorous growths on acroporid corals from Bandar Khayran, Gulf of Oman, Indian Ocean. Coral Reefs 17, 195–198. Dhindas, R.S., Plumb-Dhindsa, P., Thorpe, T.A., 1981. Leaf senescence: correlated with increased levels of membrane permeability and lipid peroxidation, and decreased levels of superoxide dismutase and catalase. J Exp Bot 32, 93–101. Dimri, G., Lee, X., Basile, G., Acosta, M., Scorrt, G., Roskelley, C., Medranos, E.E., Linskensi, M., Rubeljii, I., Pereira-Smithii, O., Peacocke, M., Campisi, J., 1995. A biomarker that identifies senescent human cells in culture and in aging skin in vivo. Proc. Natl. Acad. Sci. 92, 363–9367. Domart-Coulon, I.J., Sinclair, C., Hill, R., Tambutté, S., Puverel, S., Ostrander, G.K., 2004. A basidiomycete isolated from the skeleton of Pocillopora damicornis (Scleractinia) selectively stimulates short-term survival of coral skeletogenic cells. Mar. Biol. 144, 83–592. Domart-Coulon, I.J., Traylor-Knowles, N., Peters, E., Elbert, D., Downs, C.A., Price, K., Stubbs, J., McLaughlin, S., Cox, E., Aeby, G., Brown, P.R., Ostrander, G.K., 2006. Comprehensive characterization of skeletal tissue growth anomalies of the finger coral Porites compressa. Coral Reefs 25, 531–543. Gateno, D., Leon, A., Barki, Y., Cortes, J., Rinkevich, B., 2003. Skeletal tumor formations in the massive coral Pavona clavus. Mar. Ecol. Prog. Ser. 258, 97–108. Grygier, M.J., Cairns, S.D., 1996. Suspected neoplasms in deep-sea corals (Scleractinia: Oculinidae: Madrepora spp.) reinterpreted as galls caused by Petrarca madreporaen. sp. (Crustacea:Ascothoracida: Petrarcidae). Dis. Aquat. Org. 24, 61–69. Harvell, C.D., Mitchell, C.E., Ward, J.R., Altizer, S., Dobson, A.P., Ostfeld, R.S., Samuel, M.D., 2002. Climate warming and disease risks for terrestrial and marine biota. Science 296, 2158–2162. Le Campion-Alsumard, T., Golubic, S., Priess, K., 1995. Fungi in corals: symbiosis or disease? Interaction between polyps and fungi causes pearl-like skeleton biomineralization. Mar. Ecol. Prog. Ser. 117, 137–147. Loya, Y., Bull, G., Pichon, M., 1984. Tumor formations in scleractinian corals. Helgol. Meeresunters. 37, 99–112. Loya, Y., Sakai, K., Yamazato, K., Nakano, Y., Sambali, H., van Woesik, R., 2001. Coral bleaching: the winners and the losers. Ecol. Lett. 4, 122–131. McClanahan, T.R., Weil, E., Maina, J., 2009. Strong relationship between coral bleaching and growth anomalies in massive Porites. Global Change Biol. 15, 1804–1816. Muller, E.M., Rogers, C.S., Spitzack, A.S., van Woesik, R., 2008. Bleaching increases likelihood of disease on Acropora palmata (Lamarck) at Hawksnest Bay, St. John, US Virgin Islands. Coral Reefs 27, 191–195. Orive, M.E., 1995. Senescence in organisms with clonal reproduction and complex life histories. Am. Nat. 145, 90–108. Peters, E.C., Halas, J.C., McCarty, H.B., 1986. Calicoblastic neoplasms in Acropora palmata, with a review of reports on anomalies of growth and form in corals. J. Natl Cancer Inst. 76, 895–912. Reed, K.C., Muller, E.M., van Woesik, R., 2010. Coral immunology and resistance to disease. Dis. Aquat. Org. 90, 85–92. Roth, L., Koksal, S., van Woesik, R., 2010. Effects of thermal stress on key processes driving coral population dynamics. Mar. Ecol. Prog. Ser. 411, 73–87. Sparks, A.K., 1972. Invertebrate Pathology, Noncommunicable Diseases. Acad. Press, New York, 387pp.. Squires, D.F., 1965. Neoplasia in a coral? Science 148, 503–505. Stimson, J., 2010. Ecological characterization of coral growth anomalies on Porites compressa in Hawai‘i. Coral Reefs 29. doi:10.1007/s00338-010-0672-8. Sutherland, K.P., Porter, J.W., Torres, C., 2004. Disease and immunity in Caribbean and Indo-Pacific zooxanthellate corals. Mar. Ecol. Prog. Ser. 266, 273–302. van Woesik, R., Sakai, K., Ganase, A., Loya, Y., 2011. Revisiting the winners and the losers a decade after coral bleaching. Marine Ecology Progress Series. doi:10.3354/meps09203. Willis, B.L., Page, C.A., Dinsdale, E.A., 2004. Coral disease on the Great Barrier Reef. In: Rosenberg, E., Loya, Y. (Eds.), Coral Health and Disease. Springer, Tel Aviv, pp. 69–104. Yamashiro, H., Yamamoto, M., van Woesik, R., 2000. Tumor formation on the coral Montipora informis. Dis. Aquat. Org. 41, 211–217. Yamashiro, H., Oku, H., Onaga, K., Iwasaki, H., Takara, K., 2001. Coral tumors store reduced level of lipids. J. Exp. Mar. Biol. Ecol. 265, 171–179. Weismann, A., 1891. Essays on Heredity. Clarendon Press, Oxford, 471pp. Williams, G.C., 1957. Pleiotrophy, natural selection, and the evolution of senescence. Evolution 11, 398–411. White, P., 1965. Abnormal corallites. Science 150, 77–78. Work, T., Aeby, G., Coles, S., 2008. Distribution and morphology of growth anomalies in Acropora from across the Indo-Pacific. Dis. Aquat. Org. 78, 255–264. Zvuloni, A., Artzy-Randrup, Y., Stone, L., Kramarsky-Winter, E., Barkan, R., Loya, Y., 2009. Spatio-temporal transmission patterns of black-band disease (BBD) in a coral community. PLoS ONE 4 (4), e4993.
Contents lists available at ScienceDirect
Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Growth anomalies on Acropora cytherea corals Akiyuki Irikawa a,⇑, Beatriz E. Casareto a, Yoshimi Suzuki a, Sylvain Agostini a, Michio Hidaka b, Robert van Woesik c a
Graduate School of Science and Technology, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan Department of Marine and Environmental Science, Graduate School of Engineering and Science, University of the Ryukyus, 1-1 Senbaru, Nishihara-city, Okinawa 902-0129, Japan c Department of Biological Sciences, Florida Institute of Technology, 150 West University Boulevard, Melbourne, FL 32901, USA b
a r t i c l e Keywords: Corals Growth anomalies Acropora cytherea Disease Aging
i n f o
a b s t r a c t This ten-year study examined the morphological, physiological, and ecological characteristics of coral growth anomalies on Acropora cytherea on Amuro Island, Okinawa, Japan. The objectives of the study were to assess whether the growth anomalies, identified as diffuse disruptions on the skeleton: (i) were more prevalent on large colonies than on small colonies, (ii) were more common near the center of the colonies than peripherally, (iii) affected colony growth and mortality, and (iv) affected coral-colony fecundity and photosynthetic capacity. We hypothesized that the growth anomalies were signs of the onset of aging. The growth anomalies were more prevalent on colonies >2 m diameter, and were concentrated near the central (older) portions of the colonies. The growth anomalies were also associated with reduced productivity and dysfunctional gametogenesis. Still, the growth anomalies did not appear to affect colony survival. The contact experiments showed that the growth anomalies were not contagious, and were most likely a sign of aging that was exacerbated by thermal stress. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Although the number of reports on coral diseases is increasing, our general understanding of coral diseases is still rudimentary (Harvell et al., 2002; Sutherland et al., 2004; Reed et al., 2010). While some diseases are clearly transmissible (Zvuloni et al., 2009), and others are exacerbated by thermal stress (Muller et al., 2008), some purported diseases may be merely a consequence of abnormal growth, the likelihood of which increases with aging (Bak, 1983; Loya et al., 1984). In most clonal organisms, such as corals, it is thought that the environment, rather than programmed aging (i.e., senescence), controls the death of a genet. Although coral senescence has been suggested in some studies, most evidence has been circumstantial (Bak, 1983; Loya et al., 1984) or exists in theory (Orive, 1995), and the paradigm remains – coral growth is indeterminate. Still, in the Indo-Pacific Acropora nasuta are rarely found >40 cm diameter, and the tabulate coral Acropora cytherea are rarely found to be >3.5 m diameter (pers. obs.). Moreover, the growth of many corymbose Acropora follows a logistic function (van Woesik et al., 2011), which means that the colonies slow their growth as they age. But evidence for coral-colony aging remains unclear. It is unknown whether growth anomalies are potentially indicative of aging, and whether environmental stress exacerbates the formation of anomalies, and hastens the aging process. ⇑ Corresponding author. Tel.: +81 3 3748 5900; fax: +81 3 3748 5939. E-mail address: [email protected] (A. Irikawa). 0025-326X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2011.05.033
This study tracks growth anomalies in A. cytherea colonies. Growth anomalies are defined as focal or multi-focal annular to diffuse lesions, consisting of abnormally arranged skeletal elements (Coral Disease Handbook, 2008, p. 32). When scleractinian corals suffer mechanical damage, for example by wave impact or predation, they often swell through the recovery process (Sparks, 1972; Bruno and Edmunds, 1997). However, growth anomalies are different from the swelling that is apparent through the recovery process. Instead, growth anomalies are characterized by abnormal polyps and abnormal coenosarc (Gateno et al., 2003; Stimson, 2010). Since 1965, growth anomalies have been given a variety of names, including neoplasia (White, 1965; Squires, 1965; Bak, 1983), hyperplasia (Willis et al., 2004), calicoblastic epitheliomas (Coles and Seapy, 1998), and tumors (Cheney, 1975; Loya et al., 1984; Yamashiro et al., 2001). Growth anomalies have been described on 10 families of scleractinian corals (Peters et al., 1986), and have been found at various locations including the Hawaiian Islands (Squires, 1965), Guam, Saipan, the Marianas, Palau, Pohnpei, Enewetak in the Marshall Islands (Cheney, 1975), the Netherlands Antilles (Bak, 1983), Magnetic Island on the Great Barrier Reef (Loya et al., 1984), the Florida Keys (Peters et al., 1986), French Polynesia (Le Campion-Alsumard et al., 1995), the Pacific coast of Costa Rica (Gateno et al., 2003), and Okinawa (Yamashiro et al., 2001). Growth anomalies are often displayed as wart-like, spherical protuberances, characterized as a chaotic array of low-density corallites (Domart-Coulon et al., 2006; McClanahan et al., 2009;
A. Irikawa et al. / Marine Pollution Bulletin 62 (2011) 1702–1707
Stimson, 2010). In several cases, tissues covering the growth anomalies appear pale, generally because the tissue supports low densities of zooxanthellae (Cheney, 1975; Bak, 1983; Peters et al., 1986; Yamashiro et al., 2001; Gateno et al., 2003). Previous studies have also reported some necrosis of the growth anomalies (Coles and Seapy, 1998; Work et al., 2008), and others have reported negative growth effects on the adjacent tissue (Cheney, 1975; Bak, 1983; Stimson, 2010), or inhibition of reproduction (Domart-Coulon et al., 2006; Yamashiro et al., 2000). Still, the etiology of growth anomalies remains largely unknown (Peters et al., 1986; Domart-Coulon et al., 2006). Grygier and Cairns (1996) reported that parasitic (endoparasitic, petrarcid ascothoracidan) crustaceans were involved in the generation of growth anomalies in deep-sea corals (Grygier and Cairns, 1996). In other cases, growth anomalies were reported to be the corals’ response to either bacterial infection (Domart-Coulon et al., 2006), or parasitic endolithic algae (Le Campion-Alsumard et al., 1995; Coles and Seapy, 1998; Domart-Coulon et al., 2004; Breitbart et al., 2005; Work et al., 2008). Other researchers have suggested that high intensity ultra-violet light (Loya et al., 1984; Peters et al., 1986; Coles and Seapy, 1998), and high temperatures (Stimson, 2010) cause growth anomalies. McClanahan et al. (2009) suggested that environmental stresses (i.e., high temperature) allowed fungi to invade corals and influence coral calcification. Generally, the prevalence of growth anomalies is considered to be localized (Cheney, 1975; Bak, 1983; Coles and Seapy, 1998). Yet, long-term studies are few, and we do not know whether growth anomalies spread through populations. This study examined whether the growth anomalies on A. cytherea colonies, defined as diffuse disruptions of the skeleton: (i) were more prevalent on large colonies than on small colonies, (ii) were more common near the center of the colonies than on their periphery, (iii) affected colony growth and mortality, and (iv) affected coral-colony fecundity and photosynthetic capacity. This study tested the hypothesis that the growth anomalies on A. cytherea were signs of the onset of aging. 2. Materials and methods 2.1. Study site This study was undertaken on the fringing reef on the east side of Amuro Island (26°140 4000 N, 127°140 3100 E), in the Kerama Islands, 40 km west of Okinawa, Japan. The study area supported diverse coral assemblages that were dominated by tabulate acroporid species. Corals were assessed annually at three sampling stations, from May 2001 to May 2010. Each station was approximately 100 m 100 m, and located between 9.5 and 11.4 m, below low water spring tide. The study area had consistently strong tidal currents, reaching up to 60 cm/s on spring tides, and low turbidity. The ten-year monthly means of sea surface temperature varied from 19.5 °C in March to 31.5 °C in August (data provided by the Akajima Marine Science Laboratory). Nutrient levels were generally low (for example, on June 5, 2004 nutrients were: 0.1 lmol, of PO4, 0.3 lmol of NO3, <0.1 lmol of NO2, and <0.4 lmol of NH4). There is no human habitation on Amuro Island. 2.2. Epidemiology of growth anomalies The relationship between the occurrence of growth anomalies and coral specificity was assessed at the three stations in August 2004, using 3, 50 m by 2 m belt transects at each station. All A. cytherea colonies supporting growth anomalies were photographed with a digital camera, OLYMPUS-5060, with a 30 cm metric stainless steel scale. The photographs were used to calculate: (i)
1703
the growth rates of each coral colony, (ii) the growth rates of each growth anomaly, and (iii) the rates of spread of the dead area on the growth anomalies. Each growth anomaly was allocated to one of four concentric zones on each colony. Each zone had approximately the same projected surface area. The zone near the center of the colonies was classified as Zone 1, and the zone near the colony periphery was classified as Zone 4. A chi-square analysis tested the hypothesis that there were more growth anomalies near the center of the colonies, in Zone 1, than elsewhere. We biannually measured the density, size, and position of the growth anomalies on each colony of A. cytherea between May 2001 and May 2010. We also tracked colonies without growth anomalies in order to estimate: (i) anomaly prevalence, to determine how many colonies in the population had growth anomalies, (ii) anomaly incidence (or the number of new colonies acquiring anomalies), (iii) anomaly growth rate, (iv) the spread of the anomalies within and among colonies, (v) the change in tissue mortality within each growth anomaly, and (vi) the mortality of colonies. The epidemiological analysis was conducted using field surveys and NIH Image Analysis. Field surveys were initiated in 2001, but some inference was made using the size of the anomalies. For example, a growth anomaly that was 2 cm diameter in October 2001 was estimated to have started growing in 2000 (based on subsequent growth rates). Growth rates of anomalies were compared using linear measurements and using the proportion of the projected surface area. The latter was also used for survival comparisons. Differences in growth rates were compared among colonies with and without growth anomalies using a Tukey-multiple comparison test. 2.3. Polyps, calices, and gametes The morphology of the skeleton of tissue-free anomalies was examined at three different spatial scales: (i) cm, (ii) mm, and (iii) lm. At the centimeter scale, the density of polyps on growth anomalies was compared with the density of polyps on normal tissue. At the millimeter scale, the diameter of calices was recorded using Vernier calipers and compared. At the micrometer scale, a Scanning Electron Microscope (SEM) JEOL, JSM-6060LV, was used to compare skeletal structures. In order to examine oocyte formation, healthy tissue and growth-anomaly tissue were sampled (on June 7, 2004) two days before coral spawning. The skeletons were fixed with 5% formaldehyde, and decalcified in a solution of 5% acetic acid and 5% formaldehyde. The remaining tissue was embedded in paraffin, and sectioned at 7 lm intervals. All sections were stained with hematoxylin eosin and examined under a Nikon OPTIPHOT-2 microscope. 2.4. Physiology of growth anomalies Net photosynthesis, chlorophyll fluorescence, and the density of symbiotic zooxanthellae were compared between growth anomalies and normal coral tissue. In situ observations and experiments were conducted in flow-through tanks at the Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus. Twelve fragments of circular or elliptical growth anomalies, ranging from 2.1 to 2.9 cm in mean diameter, were collected in April 2004. The fragments were kept in constant running seawater, under 100–500 lmol photons m2 s1, with temperatures ranging from 24 to 26 °C. Dissolved oxygen within the culture tank was measured using a dissolved oxygen meter (YSI Biological Oxygen Monitor, Model 5300). Photosynthetic activity of Photosystem II was estimated using a Pulse Amplitude Modulation (PAM) Fluorometer (Mini-PAM, Waltz, Germany) using the following procedure. After 15 min of
1704
A. Irikawa et al. / Marine Pollution Bulletin 62 (2011) 1702–1707
dark-adaptation, the Fv/Fm value was measured, where Fv = Fm – Fo, and Fo is the initial fluorescence in the dark adapted state, and Fm is the maximal fluorescence in the dark adapted state. The distance between the coral specimens and the optic-fiber head of the PAM was kept constant. Fluorescence data were recorded six times from different parts of each coral fragment. The densities of symbiotic algae were also examined. Coral tissue was removed from the skeleton using a water-jet (Ricohelemex Dentrex T-8311). The samples were homogenized and centrifuged at 3000 rpm for 10 min using a Hitachi, Himac CR21, centrifuge. The pellets were re-suspended, and the aliquot was placed on a hemocytometer, where the density of zooxanthellae was counted within a 1 mm3 volume, under a Nikon OPTIPHOT-2 microscope at 400 magnification. 2.5. Field experiments In 2004, manipulative transplant experiments were conducted to examine whether the growth anomalies were contagious. Six growth anomalies were cut off from three, 2 m diameter A. cytherea source colonies (n = 18). Using cable ties, three of the growth anomalies from the source colony were reattached to the donor colony, whereas the other three growth anomalies were attached to another (2 m diameter) colony that had no growth anomalies. This process was repeated using two other A. cytherea colonies.
and A. secale) at Amuro Island, Okinawa, southern Japan. The frequency of growth anomalies was highest on A. cytherea, and less common on the other tabulate Acropora species, such as A. latistella, A. hyacinthus, A. microclados, A. subulata, and A. paniculata (Table 1). There was a considerable range in total surface area occupied by the growth anomalies on individual A. cytherea colonies (from <1 cm2 to >2500 cm2). Compared with small colonies, the prevalence of growth anomalies was higher in colonies >120 cm in diameter (Fig. 1a and b). The proportional area occupied by growth anomalies was also significantly greater on colonies >2 m in diameter compared with colonies <2 m in diameter (1.1 ± 2.5 and 0.17 ± 0.22, in May of 2006, Mann–Whitney U test; p < 0.01, n = 47). Colonies <1 m in diameter showed no signs of growth anomalies. For A. cytherea colonies >1 m, the growth anomalies were centrally concentrated in Zone 1. In fact, almost 80% of the growth anomalies were recorded in Zones 1 and 2 (significant Chi-square test, p < 0.05, Fig. 2). In a total of 273 A. cytherea colonies examined, 70 colonies had growth anomalies. The incidence of growth anomalies varied through time, and was greatest in 2002, but only on large colonies (Fig. 3). In general, as the size of the growth anomalies increased their expansion rate decreased (Fig. 4). There was no significant difference in the increase of colony diameter per year for colonies with and without growth anomalies. Growth anomalies >10 cm in diameter showed progressive necrosis (Fig. 5), and partial mortality in their centers.
3. Results 3.1. Epidemiology of growth anomalies
3.2. Polyps, calices, and gametes
Growth anomalies occurred on 11 species of Acropora (A. cytherea, A. latistella, A. hyacinthus, A. clathrata, A. florida, A. valenciennesi, A. palifera, A. abrotanoides, A. samoensis, A. digitifera,
The density of polyps in the tissue of growth anomalies (8.9 ± 5.4 per cm2) was significantly lower (Kruskal Wallis, p < 0.05) than the polyp densities in tissue without growth
Table 1 Prevalence of growth anomalies in Acropora cytherea and other Acropora species within the three survey areas at Amuro Island, Kerama Islands, southern Japan.
A. A. A. A. A. A. A. A. A. A. A. A.
cytherea hyacinthus latistella clathrata microclados paniculata subulata florida valenciennesi abrotanoides samoensis secale
Area 1
Area 2
Area 3
Mean prevalence
S. dev.
0.13 (n = 84) 0.13 (n = 8) 0.07 (n = 15) 0 (n = 3) 0 (n = 16) 0 (n = 1) 0 (n = 17) 0.08 (n = 12) 0.33 (n = 3) 0 (n = 0) 0.04 (n = 26) 0 (n = 10)
0.29 (n = 78) 0.09 (n = 11) 0 (n = 9) 0.20 (n = 5) 0 (n = 6) 0 (n = 1) 0 (n = 20) 0 (n = 7) 0.50 (n = 2) 0 (n = 1) 0 (n = 12) 0 (n = 4)
0.13 (n = 67) 0 (n = 3) 0 (n = 22) 0 (n = 2) 0 (n = 23) 0 (n = 3) 0 (n = 4) 0.22 (n = 9) 0 (n = 0) 0.33 (n = 3) 0.2 (n = 5) 0.07 (n = 14)
0.19 0.07 0.04 0.07 0.00 0.00 0.00 0.10 0.28 0.11 0.08 0.02
0.09 0.06 0.03 0.12 0.00 0.00 0.00 0.11 0.25 0.19 0.11 0.04
Fig. 1. The prevalence of Acropora cytherea growth anomalies (which is equivalent to the proportion of colonies with growth anomalies relative to the total number of colonies), where white bars indicate the frequency of colonies without growth anomalies, and the black bars indicate the frequency of colonies with growth anomalies for (a) 2003 and (b) 2004.
A. Irikawa et al. / Marine Pollution Bulletin 62 (2011) 1702–1707
1705
anomalies (12.3 ± 2.3). The alignment of axial and radial polyps was chaotic in growth anomalies, the calices were wider, the corallites were fewer, and many polyps were buried under a proliferating coenosteum. Growth anomalies were also covered with a dense array of irregular, short spines (Fig. 6). There was no gonad development observed in the tissue of the growth anomalies (Fig. 6). Yet, normal tissue near the growth anomalies contained well developed oocytes and sperm in all polyps. 3.3. Physiology of growth anomalies
Fig. 2. The proportion of the Acropora cytherea colony’s surface area supporting growth anomalies in relation to zones (in 2004); with Zone 1 located in the center of the colony and Zone 4 on the colony periphery (the short, horizontal lines are the means, the open circles are the standard errors, and the filled circles represent the standard deviations.
The ratio of production to respiration (P/R ratio) on coral tissue with growth anomalies was low compared with the P/R ratios of tissue without growth anomalies. For example, the P/R ratio of healthy looking tissue at 100 lmol photons m2 s1 was 0.71 ± 0.54 (n = 4), and at 250 lmol photons m2 s1 the P/R ratio was 1.63 ± 0.09 (n = 4). The net productivity of the growth anomalies was almost zero. The photosynthetic capacity of the coral symbionts, measured as Fv/Fm values, in the growth anomaly (0.48 ± 0.03, n = 3), was significantly lower (Mann Whitney U test, p < 0.05) than the photosynthetic capacity of the normal tissue (0.68 ± 0.04, n = 3) (Fig. 7). The densities of symbiotic zooxanthellae in the coenosarc tissue of the growth anomalies (1.21 ± 0.25 106 cells/cm2, n = 3) was significantly lower (Mann Whitney U test, p < 0.05) than in healthy looking tissue (2.36 ± 0.23 106 cells/cm2, n = 3). The transplant experiment showed that the transfer of the growth anomalies had no effect on the maternal colony, nor was there any noticeable effect of the transplanted growth anomalies on the colonies without growth anomalies. The growth anomalies, therefore, did not appear to be contagious. Two weeks after the transplant, the growth anomalies were covered in turf algae, and were assumed to be dead. 4. Discussion
Fig. 3. Incidence of Acropora cytherea growth anomalies (or the number of new cases of colonies with growth anomalies per year) within the 3 hectare study area, from 1999 to 2010. Incidences from 1999 to 2002 were estimated using backcalculations of the 2001 data, and by using anomaly growth rates derived from photographic analysis (from 2003 to 2010).
Growth anomalies on A. cytherea were caused by the swelling of the corals’ coenosarc and skeletal coenosteum. The skeleton of the growth anomalies consisted of irregular short spines, appearing reticulated or porous, supporting considerably fewer and widerthan-normal polyps. The growth anomalies were clearly the result of an interruption, or deterioration, of the bio-calcification process. Similar characteristics have been previously reported in other
Fig. 4. Mean annual growth rates (and 1 standard deviation) of Acropora cytherea growth anomalies in relation to their initial size.
1706
A. Irikawa et al. / Marine Pollution Bulletin 62 (2011) 1702–1707
Fig. 5. Metastasis and lesions generated by Acropora cytherea growth anomalies. (a) July 2003, (b) January 2004, and (c) linear regression of diameter of growth anomaly in relation to the percentage dead area on each growth anomaly (R2 = 0.5688, p = 0.003).
Fig. 7. Photosynthetic activity of zooxanthellae in Acropora cytherea colonies with and without growth anomalies (July 2005), where thick lines are the means, the boxes are the standard deviations, and the lines are the range of measured values.
Fig. 6. Morphological and histological characteristics of growth anomalies. (a) a Scanning Electron Microscope (SEM) image of a growth anomaly (right) on Acropora cytherea overgrowing the normal skeleton (left), (b) a cross section of healthy polyp with a large oocyte (o), (c) a vertical section of healthy polyp with oocytes and testes (t), (d) a cross section of growth anomaly tissue displaying no oocytes, and (e) a vertical section of a polyp in growth anomaly tissue displaying no gamete development.
acroporid species, including Montipora informis (Yamashiro et al., 2000). A reduction in zooxanthellae density and a low activity of the photosynthetic activity of Photosystem II in the remaining symbionts within the growth anomalies, suggest that the symbio-
sis between the coral host and the symbionts was dysfunctional. Moreover, without the support of normal-adjoining coral tissue the growth anomalies seemingly lost their autotrophic capacity, and the isolated growth-anomaly fragments died within 2 weeks after reattachment. A progressive increase in the size of the growth anomalies resulted in a loss of reproductive functionality, progressive necrosis of the growth anomalies, and an increase in partial-colony mortality. Yet, there was no evidence that the growth anomalies had changed the growth capacity of the host colonies, nor did they increase the likelihood of total colony mortality. Therefore, the question remains: Are these wart-like growth anomalies simply a sign of aging? Weismann (1891) first advanced a theory of senescence, which he considered as an aging process that was subjected to natural selection. These ideas were expanded by Williams (1957), who considered that senescence should be regarded as the general deterioration of biological systems within organisms. Williams reasoned that senescence is a consequence of a decline in repro-
A. Irikawa et al. / Marine Pollution Bulletin 62 (2011) 1702–1707
ductive capacity with increasing age, which is solely directed by the process of natural selection. Indeed, senescence in animals and plants is generally regarded as genetically programmed degeneration. After a specific number of cell divisions, somatic cells stop growing and discontinue their normal function. In some plants, senescence is associated with membrane deterioration and an increase in membrane permeability (Dhindas et al., 1981). Such replicative senescence, as it is generally called, is thought to suppress tumor growth but cause aging (Dimri et al., 1995). Therefore, by escaping the tumor-suppressive mechanisms of senescence, somatic cells acquire indeterminate, often malignant growth (Dimri et al., 1995). Nevertheless, A. cytherea growth anomalies were benign, and their growth slowed as they expanded. There was no significant difference in growth and mortality between corals with and without growth anomalies. Therefore, anomalies are not likely to be an acute disease. Moreover, the transplant experiment showed that the growth anomalies were not contiguous. The growth anomalies were, however, concentrated centrally on A. cytherea colonies >1.0 m in diameter, where the oldest cells were located. We suggest that the growth anomalies we tracked through time on A. cytherea colonies were a consequence of colony aging, which had detrimental effects on the biocalcification process. The present study showed that the density of growth anomalies increased immediately after thermal stress events. Okinawa and surrounding reefs experienced anomalous sea surface temperatures in 1998 and in 2001 (Loya et al., 2001; Bena and van Woesik, 2004; Roth et al., 2010), and the greatest densities of growth anomalies occurred in 1999 and 2002 (Fig. 2). Such environmental stress may disrupt the calcification process and cause the formation of growth anomalies. We conclude that A. cytherea colonies have determinate growth, that cell maintenance is disrupted in aged cells, in the center of large colonies, and that the formation of growth anomalies is potentially exacerbated by environmental stress. Acknowledgments We would like to thank the Tropical Biosphere Research Center (TBRC), University of the Ryukyus, for providing a visiting professor fellowship to RvW in the summer of 2010, and to the Mitsubishi Foundation for funding A.I., B.E.C., Y.S., and S.A. We also thank N. Yasuda, Dr. M. Hirose, Prof. E. Hirose, Prof. H. Yamasaki, and Dr. N. Isomura for their valuable research suggestions, and H. Chiku, Y. Anno, K. Miyagi the boat captains and master divers who enabled us to carry out the field survey. A special thanks goes to Sandra van Woesik for editorial comments. References Bak, R.P.M., 1983. Neoplasia, regeneration and growth in the reef building coral Acropora palmata. Mar. Biol. 77, 221–227. Bena, C., van Woesik, R., 2004. The impact of two bleaching events on the survival of small coral colonies (Okinawa, Japan). Bull. Mar. Sci. 75, 115–126. Bruno, J., Edmunds, P., 1997. Clonal variation for phenotypic plasticity in the coral Madracis mirablilis. Ecology 78, 21–177. Breitbart, M., Bhagooli, R., Griffin, S., Johnston, I., Rohwer, F., 2005. Microbial communities associated with skeletal tumors on Porites compressa. FEMS Microbiol. Lett. 243, 431–436. Cheney, D., 1975. Hard tissue tumors of scleractinian corals. In: Hildemann, W.H., Benedict, A.A. (Eds.), Immunologic Phylogeny. Plenum, New York, pp. 77–87.
1707
Coles, S.L., Seapy, D.G., 1998. Ultra-violet absorbing compounds and tumorous growths on acroporid corals from Bandar Khayran, Gulf of Oman, Indian Ocean. Coral Reefs 17, 195–198. Dhindas, R.S., Plumb-Dhindsa, P., Thorpe, T.A., 1981. Leaf senescence: correlated with increased levels of membrane permeability and lipid peroxidation, and decreased levels of superoxide dismutase and catalase. J Exp Bot 32, 93–101. Dimri, G., Lee, X., Basile, G., Acosta, M., Scorrt, G., Roskelley, C., Medranos, E.E., Linskensi, M., Rubeljii, I., Pereira-Smithii, O., Peacocke, M., Campisi, J., 1995. A biomarker that identifies senescent human cells in culture and in aging skin in vivo. Proc. Natl. Acad. Sci. 92, 363–9367. Domart-Coulon, I.J., Sinclair, C., Hill, R., Tambutté, S., Puverel, S., Ostrander, G.K., 2004. A basidiomycete isolated from the skeleton of Pocillopora damicornis (Scleractinia) selectively stimulates short-term survival of coral skeletogenic cells. Mar. Biol. 144, 83–592. Domart-Coulon, I.J., Traylor-Knowles, N., Peters, E., Elbert, D., Downs, C.A., Price, K., Stubbs, J., McLaughlin, S., Cox, E., Aeby, G., Brown, P.R., Ostrander, G.K., 2006. Comprehensive characterization of skeletal tissue growth anomalies of the finger coral Porites compressa. Coral Reefs 25, 531–543. Gateno, D., Leon, A., Barki, Y., Cortes, J., Rinkevich, B., 2003. Skeletal tumor formations in the massive coral Pavona clavus. Mar. Ecol. Prog. Ser. 258, 97–108. Grygier, M.J., Cairns, S.D., 1996. Suspected neoplasms in deep-sea corals (Scleractinia: Oculinidae: Madrepora spp.) reinterpreted as galls caused by Petrarca madreporaen. sp. (Crustacea:Ascothoracida: Petrarcidae). Dis. Aquat. Org. 24, 61–69. Harvell, C.D., Mitchell, C.E., Ward, J.R., Altizer, S., Dobson, A.P., Ostfeld, R.S., Samuel, M.D., 2002. Climate warming and disease risks for terrestrial and marine biota. Science 296, 2158–2162. Le Campion-Alsumard, T., Golubic, S., Priess, K., 1995. Fungi in corals: symbiosis or disease? Interaction between polyps and fungi causes pearl-like skeleton biomineralization. Mar. Ecol. Prog. Ser. 117, 137–147. Loya, Y., Bull, G., Pichon, M., 1984. Tumor formations in scleractinian corals. Helgol. Meeresunters. 37, 99–112. Loya, Y., Sakai, K., Yamazato, K., Nakano, Y., Sambali, H., van Woesik, R., 2001. Coral bleaching: the winners and the losers. Ecol. Lett. 4, 122–131. McClanahan, T.R., Weil, E., Maina, J., 2009. Strong relationship between coral bleaching and growth anomalies in massive Porites. Global Change Biol. 15, 1804–1816. Muller, E.M., Rogers, C.S., Spitzack, A.S., van Woesik, R., 2008. Bleaching increases likelihood of disease on Acropora palmata (Lamarck) at Hawksnest Bay, St. John, US Virgin Islands. Coral Reefs 27, 191–195. Orive, M.E., 1995. Senescence in organisms with clonal reproduction and complex life histories. Am. Nat. 145, 90–108. Peters, E.C., Halas, J.C., McCarty, H.B., 1986. Calicoblastic neoplasms in Acropora palmata, with a review of reports on anomalies of growth and form in corals. J. Natl Cancer Inst. 76, 895–912. Reed, K.C., Muller, E.M., van Woesik, R., 2010. Coral immunology and resistance to disease. Dis. Aquat. Org. 90, 85–92. Roth, L., Koksal, S., van Woesik, R., 2010. Effects of thermal stress on key processes driving coral population dynamics. Mar. Ecol. Prog. Ser. 411, 73–87. Sparks, A.K., 1972. Invertebrate Pathology, Noncommunicable Diseases. Acad. Press, New York, 387pp.. Squires, D.F., 1965. Neoplasia in a coral? Science 148, 503–505. Stimson, J., 2010. Ecological characterization of coral growth anomalies on Porites compressa in Hawai‘i. Coral Reefs 29. doi:10.1007/s00338-010-0672-8. Sutherland, K.P., Porter, J.W., Torres, C., 2004. Disease and immunity in Caribbean and Indo-Pacific zooxanthellate corals. Mar. Ecol. Prog. Ser. 266, 273–302. van Woesik, R., Sakai, K., Ganase, A., Loya, Y., 2011. Revisiting the winners and the losers a decade after coral bleaching. Marine Ecology Progress Series. doi:10.3354/meps09203. Willis, B.L., Page, C.A., Dinsdale, E.A., 2004. Coral disease on the Great Barrier Reef. In: Rosenberg, E., Loya, Y. (Eds.), Coral Health and Disease. Springer, Tel Aviv, pp. 69–104. Yamashiro, H., Yamamoto, M., van Woesik, R., 2000. Tumor formation on the coral Montipora informis. Dis. Aquat. Org. 41, 211–217. Yamashiro, H., Oku, H., Onaga, K., Iwasaki, H., Takara, K., 2001. Coral tumors store reduced level of lipids. J. Exp. Mar. Biol. Ecol. 265, 171–179. Weismann, A., 1891. Essays on Heredity. Clarendon Press, Oxford, 471pp. Williams, G.C., 1957. Pleiotrophy, natural selection, and the evolution of senescence. Evolution 11, 398–411. White, P., 1965. Abnormal corallites. Science 150, 77–78. Work, T., Aeby, G., Coles, S., 2008. Distribution and morphology of growth anomalies in Acropora from across the Indo-Pacific. Dis. Aquat. Org. 78, 255–264. Zvuloni, A., Artzy-Randrup, Y., Stone, L., Kramarsky-Winter, E., Barkan, R., Loya, Y., 2009. Spatio-temporal transmission patterns of black-band disease (BBD) in a coral community. PLoS ONE 4 (4), e4993.