The biology and functional morphology of Septifer bilocularis and Mytilisepta virgata (Bivalvia: Mytiloidea) from corals and the exposed rocky shores, respectively, of Hong Kong

Regional Studies in Marine Science 25 (2019) 100454

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The biology and functional morphology of Septifer bilocularis and Mytilisepta virgata (Bivalvia: Mytiloidea) from corals and the exposed rocky shores, respectively, of Hong Kong Brian Morton School of Biological Sciences, The University of Hong Kong, Hong Kong Special Administrative Region

highlights • • • • •

Current classification of the Septiferinae. Comparative anatomical details of Septifer and Mytilisepta. Characteristic possession of an umbonal septum in both species. Unique accessory posterior adductor muscles in both species. Questions of convergent or parallel evolution evident between the two species.

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Article history: Received 12 September 2018 Received in revised form 27 November 2018 Accepted 27 November 2018 Available online 1 December 2018 Keywords: Shell morphology Anterior umbonal septum Accessory posterior adductor muscle No posterior pedal retractor muscles Evolution

a b s t r a c t There are fossil Mytiloidea assigned to the Septiferinae such as Coxesia mezzalirai and species of Admytilus and Assytilus. Extant species of the Septiferinae are assigned to Septifer and Mytilisepta and date from the end of the Triassic, >200 mya, and the end of the Cretaceous, >65 mya, respectively. All septiferines possess shells characterised by an external radially bifurcate ribbing and internal umbonal septa associated with which is the anterior adductor muscle. The Indo-West Pacific Septifer bilocularis and Mytilisepta virgata are described herein. Uniquely, both possess accessory posterior adductor muscles and posterior pedal retractor muscles that are either absent (M. virgata) or virtually so (S. bilocularis). It is hypothesised that the former muscles have evolved from modified pallial retractors. Gene sequencing evidence suggests that M. virgata is related to the Brachidontiinae and ongoing research further suggests that S. bilocularis is more related to the Mytilinae. This implies parallel evolution and argues that species of Septifer can be retained within the Septiferinae, as currently defined, whereas M. virgata, as the type species, should possibly be placed in its own subfamily – the Mytiliseptiferinae – and, again possibly, allied with the Brachidontiinae. But with both subfamilies retained within the Mytilidae. Uniquely too for the species-diverse marine Mytiloidea, M. virgata possesses ectopic pallial eyes located at the apex of papillae that line the inhalant aperture to the mantle cavity. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Some of the many representatives of the Mytiloidea are among the most conspicuous marine (and estuarine and freshwater) bivalves and species of Mytilus are especially obvious and of commercial economic value on rocky intertidal shores in both northern and southern boreal waters. As a consequence, there are a large number of published research papers on these species (Gosling, 1992). Studies upon all other mytiloideans, especially from warmer waters, are sparse (Soot-Ryen, 1955; Morton, 1973a,b, 1977, 1980; E-mail address: [email protected]. https://doi.org/10.1016/j.rsma.2018.100454 2352-4855/© 2018 Elsevier B.V. All rights reserved.

Morton and Puljas, 2017), although because too of their commercial value, species of Perna have received more attention (Choo, 1974) as have the invasive freshwater Limnoperna fortunei (Dunker, 1857) (Boltovskoy, 2015) and the estuarine, and also invasive, Xenostrobus securis (Lamarck, 1819) (Morton and Leung, 2015), plus the numerous species of the, often coral-boring, Lithophaginae (Wilson, 1979; Kleemann, 1990). Non-commercial taxa such as representatives of the Dacrydinae (Mattson and Warén, 1977) and other small inconspicuous taxa such as Modiolarca subpicta (Cantraine, 1835) (Morton and Dinesen, 2011), Trichomusculus semigranatus (Reeve, 1858) and the circum-boreal, adventitious crypt- and nest-constructing Crenella decussata (Montagu, 1808) and the tropical Arcuatula elegans (Gray, 1828), though interesting

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Abbreviations used in the figures AAM AM AN APAM APAM(1) APAM(2) APRM AU BG BN BY BYG CSSMG EE ES EXS F HT IA ID IEIMF IMF ISV K L LP MG MI MM MMF(1) MMF(2) MP OD OEIMF OMF ON P PAM PBRM PBRM(1-4) PBRM(1-6) PC PE PERM PG PL PN PPRM PPRM(F) PRM PS PSC PV

Anterior adductor muscle (or scar) Adductor muscle Anus Accessory posterior adductor muscle Accessory posterior adductor muscle (1) (or scar) Accessory posterior adductor muscle (2) (or scar) Anterior pedal retractor muscle (or scar) Auricle (of the heart) Byssal groove Byssal notch Byssus Byssus gland Conjoined style sac and mid gut Ectopic eye Eye stalk Exhalant siphon Foot Hinge tooth Inhalant aperture Inner demibranch of ctenidium Inner edge of the inner mantle fold Inner mantle fold Inter-siphonal valve Kidney Ligament Labial palp Mid gut Microvilli Mantle margin Outer sub fold of the middle mantle fold Inner sub fold of the middle mantle fold Mantle papilla Outer demibranch of ctenidium Outer edge of the inner mantle fold Outer mantle fold Optic nerve Periostracum Posterior adductor muscle (or scar) Posterior byssal retractor muscles Posterior byssal retractor muscles (1-4) (or scar) Posterior byssal retractor muscles (1-6) (or scar) Pigment cell Pericardium Pedal retractor muscles Pericardial gland Pallial line Pallial nerve Posterior pedal retractor muscle Posterior pedal retractor muscle (few fibres only) Pallial retractor muscle Pallial sinus Photosensory cell Pedal gape valve

R RAR RR RT SC SE SO SP V VA VM

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Rectum Radial shell ribbing Raised ridge Rejectory tract Siphonal cone Septum Socket Siphonal papillae Ventricle (of the heart) Vacuole Visceral mass

in their own rights, have received only basic attention (Morton, 1980, 1995a; Morton et al., 2016), Throughout the Pacific Ocean there are a number of species of what are located currently within the Septiferinae (Scarlato and Starobogatov, 1979) (Mytiloidea: Mytilidae) that, despite their particular ecological significance, have also been little studied. In terms of anatomy, however, both Fischer (1866, plate 4) and Pelseneer (1911, plate VI, figs 6 & 10) illustrated the internal structure of Septifer bilocularis (Linnaeus, 1758) (and S. excisus [Wiegmann, 1837] plate VI, figs 7, 8 & 9) in the latter case) but not in great detail though sufficient to recognise the presence of accessory posterior adductor muscles — although neither author defined them as such. This distinctive anatomical feature was later elaborated upon by Yonge and Campbell (1968) in relation to the evolution of the heteromyarian form in the Bivalvia and species of Septifer and, the convergently similar, Dreissena polymorpha (Pallas, 1771) (Dreissenidae) in particular. There are other research studies on the ecology of, notably, the rocky intertidal Septifer virgata (Wiegmann, 1837) (as will be described), but other than the above two studies and purely taxonomic papers, few others have concerned themselves with the Septiferinae as currently defined and the significance of the sub-family’s unique anatomical features. As with modern representatives of the Septiferinae, there are umbonally septate fossil mytilids, which provide clues to the age and origin of their modern descendants and these too will be identified and discussed herein. With so little written about the as currently defined Septiferinae, however, particularly with regard to the accessory posterior adductor muscles that characterise representatives of this sub-family, this study was initiated to rectify this deficiency. Currently defined because a number of genesequencing studies, for example that of Gerdol et al. (2017), have suggested removing M. virgata from the Mytilinae and relocating it in the Brachidontinae. This therefore is a comparative study of two Western Pacific species of septate mytilids – S. bilocularis and Mytilisepta virgata (Wiegmann, 1837) - undertaken with a view to determining anatomical similarities and differences between them and thereby coming to an opinion with regard to their similar current placement within the Septiferinae. Two particular aims of the study were: (i), to ascertain the origin of the accessory posterior adductor muscles that both possess and (ii), elucidate the comparative structures of the anterior interior septum that also characterise both taxa. 2. Materials and methods Individuals of M. virgata and S. bilocularis were obtained from the rocky intertidal zone of the highly wave-exposed shore of Shelter Island off the Sai Kung Peninsula in the waters of the eastern New Territories of Hong Kong and subtidal coral heads in the northeastern waters of Hong Kong, that is, Mirs Bay, respectively, in

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November 2016. Individuals were examined alive and as preserved specimens both of which were dissected to illustrate aspects of their morphology. Other individuals were fixed in 4% formalin and, following routine histological procedures, serially sectioned at 6 µm and the resulting slides stained in Ehrlich’s haematoxylin and eosin. 2.1. Lodgement of voucher specimens Voucher specimens of M. virgata and S. bilocularis collected from Hong Kong in 2016 (see above) have been deposited in the collections of the Mollusca Section of the Natural History Museum, London, and have the registration numbers NHMUK 20180059 (M. virgata) and NHMUK 20180060 (S. bilocularis). 3. Results 3.1. Taxonomy of the extant Septiferinae (as currently defined) Species of Septifer and Mytilisepta are generally assigned to their own subfamily, the Septiferinae (Mytilidae), although Amler (1999) elevated this to familial status (but this is not discussed herein). The subgenus Mytilisepta, with S. virgata (originally Tichogonia, Wiegmann, 1837) as the type species, was erected by Habe (1951) and elevated to generic status by Huber (2010), based on the genetic results of Matsumoto (2003). Coan and ValentichScott (2012), however, basing their conclusions on the shell characters of Septifer bifurcatus (Conrad, 1837) and Septifer zeteki Hertlein and Strong, 1946, both from the central west coast of North America, considered Mytilisepta to be a junior synonym of Septifer. In the Western Pacific, there are other species of Septiferinae (Récluz, 1848) (as currently defined) and which, for the purposes of this study, include S. bilocularis and M. virgata. Both these species occur throughout the Indo-West Pacific (Wang, 1988; Qi, 2004; Zheng et al., 2013). Additionally, some authors, for example Yang and others (2013, 2017) also record Septifer excisus (Wiegmann, 1837) from Chinese waters while Qi (2004) added Septifer pulcher Wang, 1983 and Septifer xishaensis Wang, 1983 (= Septifer rudis Dall, Bartsch and Rehder, 1938; Huber, 2010) and Xu and Zhang (2008) added Septifer keenae Nomura, 1936 (accepted as Mytilisepta keenae; Huber, 2010) to this number. In addition to S. bilocularis, S. excisus and M. virgata, S. keenae is also recorded from Japanese waters (Habe, 1977). The umbonal septum of S. excisus, however, resembles that of M. virgata (Yang and others, 2013, p. 166) and, indeed, other authors only record S. bilocularis and M. virgata from Chinese waters (Zhang, 2008), although Yang and others (2013, 2017) added S. excisus to these two taxa specifically from the South China Sea. Williamson (1898), Pilsbry and Raymond (1898) and Coan et al. (2000) recorded S. bifurcatus from California and the shores of much of the north-eastern Pacific. This taxon has been accepted as Mytilisepta bifurcata (Conrad, 1837) by Huber (2010). Coan and Valentich-Scott (2012), as noted earlier, considered Mytilisepta to be a junior synonym of Septifer. According to these authors, the range of this taxon extends south into the intertidal waters of Baja California Sur, Mexico, and it is, therefore, ecologically similar to M. virgata in East Asia, including Hong Kong. Additionally, Septifer zeteki Hertlein and Strong, 1946, which also occurs in Baja California Sur, can be regarded as an ecological equivalent of M. virgata, although its range extends further south into South America. Other nominal species recorded from the tropical Pacific and accepted as valid taxa include S. cumingii (Récluz, 1848), S. huttoni (Cossmann, 1916), S. ramulosus (Viader, 1951), S. rudis Dall, Bartsch and Rehder, 1938, S. rufolineatus (E.A. Smith, 1911) and S. torquatus (P. Marshall, 1918) (Huber, 2010). There is no significant literature about any of these taxa however.

Fig. 1. Mytilisepta virgata. This conspicuously black species is here illustrated on the precipitous rocky intertidal of Shelter Island of the Sai Kung Peninsula in Hong Kong.

Fig. 2. Photographs of the left shell valves of A, Septifer bilocularis and B, Mytilisepta virgata.

3.2. Biology Species of the Septiferinae, as currently defined, have been little studied except in Hong Kong where Lee and Morton (1985) included the two taxa here under investigation in a review of the known local species of Mytiloidea. In the warmer waters of the Indo-West Pacific, species of Septifer (Récluz, 1848) co-occur with subtidal corals and S. virgata replaces the more boreal species of Mytilus as the dominant intertidal band of zoning mussels, respectively, and including Hong Kong (Morton and Morton, 1983). Mytilisepta virgata Mytilisepta virgata occurs on the intertidal of highly exposed shores in Hong Kong’s south-eastern quadrant. Fig. 1 illustrates the species (black individuals) on the precipitous intertidal zone of Shelter Island of the Sai Kung Peninsula in Hong Kong. Despite the species’ exposed rocky shore habitat, Morton (1991) reported upon an unusual molluscan fouling community dominated by S. virgatus (= M. virgata) in a seawater outfall discharging into Victoria Harbour, Hong Kong. The species has no known other commercial significance or value and, as a consequence, there are few studies on this otherwise keystone zoning species on highly exposed rocky shores. In Hong Kong too, however, Morton (1995b) examined the population dynamics of M. virgata and showed that spawning is limited to two periods in spring (February to March) and autumn (September to December) with subsequent recruitment into the adult population. This study also showed that the species generally lives for about 4–5 years, although older individuals, possibly up to 12 years of age and, with a maximum shell length of 65 mm, occur as solitary individuals lower down the shore. Mortality in winter mainly affects newly recruited juveniles. A periodic heavy mortality of adults in summer is thought to be related to high rock

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1991). There is little else written about this species except for the taxonomic studies identified earlier although Albayrak and Çaglar (2006) record that it has been identified as an alien invader of the Mediterranean Sea. 3.3. Anatomy

Fig. 3. Juveniles of Septifer bilocularis and Mytilisepta virgata showing the distribution of byssal setae on the right (A and C) and left (B and D) shell valves, respectively.

temperatures at mid-day (>50 ◦ C), concurrent with low spring tides. Liu and Morton (1994) further examined the species’ temperature tolerances on the exposed shores of Cape d’Aguilar, showing it, again, to be typically highly resistant to mid-summer heat residing, as it does, in crevices, but susceptible to the above dry rock temperatures. Ong Che and Morton (1992) related the changes in structure and variations in abundance of the macro-invertebrate community associated with M. virgata to seasonal changes in climate within the, now, Cape d’Aguilar Marine Reserve while Seed and Brotohadikusomo (1994) studied the spatial variation in the molluscan community associated with the species in the same location. Concurrently, in Japan, Iwasaki (1995) compared the rocky intertidal communities associated with the vertically contiguous M. virgata and Hormomya mutabilis (Gould, 1861). In the upper M. virgata bed, crustaceans and bivalves were dominant in terms of both numbers of individuals and biomass whereas the lower H. mutabilis bed supported virtually no epizoans or mobile fauna. The H. mutabilis bed, however, contained a much greater amount of sediment than the M. virgata one and the biomass of six of the nine species dominant in the latter was correlated negatively with this. These basic studies on M. virgata were complemented by others, both ecological and physiological, on the species both in Hong Kong and elsewhere. For example, Seed and Lee (1995) showed how in Hong Kong, M. virgata is the prey of the shell-crushing crab Eriphia laevimana smithii Guérin, 1832 (= Eriphia sebana [Shaw and Nodder, 1803]) while Momoshima et al. (1985) reported upon the radioactive and stable cobalt concentrations in M. virgata and Mytilus edulis (Linnaeus, 1758) from the Kyushu Islands, Japan, showing that the former has a tendency to concentrate cobalt with growth and age whereas the latter does not. Wang and Dei (1999) examined the factors affecting trace element uptake in M. virgata. Sze and Lee (1995) demonstrated that in comparison with Perna viridis (Linnaeus, 1758), which depurates copper by producing large amounts of pallial mucus, M. virgata lacks this ability. Subsequently, Seed and Richardson (1999) compared the same two species in relation to the differing evolutionary traits they illustrate and later re-compared them to ascertain their relative values as environmental biomonitors and chronometers of environmental change (Seed and Richardson, 2003). Septifer bilocularis A second putative septiferine, that is, S. bilocularis (Linnaeus, 1758), occurs in Hong Kong where it is associated with coral heads, both living and dead, in the oceanic eastern, subtidal, waters of Hong Kong (Dudgeon and Morton, 1982), although it is a superficial nestler on such scleractinians and is not associated intimately with the gallery communities within the corals (Morton et al.,

3.3.1. The shells Representatives of the Mytiloidea show a high degree of uniformity in terms of their shell mineralogy and structure such that, with only a few possessing a middle layer of nacreous aragonite, notably, for example, Mytilus californianus Conrad, 1837 and Modiolus modiolus (Linnaeus, 1758), virtually all the other taxa investigated by Taylor et al. (1969), including S. bilocularis, had shells comprising two layers, that is, an outer layer of nacreous aragonite and an inner layer typically of either nacreous, prismatic or complex crossed-lamellar aragonite. Species of Septifer and Mytilisepta are characterised by a strongly triangular, heteromyarian, shell, when seen from the lateral aspects, and which is equivalve, ventrally flattened and dorsally peaked (Fig. 2). Representatives of these genera are also patterned by an irregularly bifurcated radial ribbing that confers them with greater compressive strength (Taylor and Layman, 1972). The shells of both species here under study are further marked by a thick periostracum that protects the underlying calcareous component of the shell from environmental and biotic agents of dissolution including chemically drilling gastropod predators but not predatory crabs (Seed and Lee, 1995). Finally, the shells of species of Septifer and Mytilisepta are characterised most distinctively by an anterior internal umbonal septum in each valve, to be described below, and between which is inserted the anterior adductor muscle (Yonge and Campbell, 1968). Such a septum provides the subfamily with its name. 3.3.2. The juvenile shells Ockelmann (1995) reviewed the ontogenetic characteristics of the Mytiloidea. The juvenile shells of some, perhaps many, mytiloids are characterised by byssal setae that are secreted from the byssal gland and planted on it by the foot. Byssal setae have been described for the marine species M. edulis (Board, 1983), all species of Dacrydium (Ockelmann, 1983) and Adula (Ockelmann and Dinesen, 2009) and M. modiolus (Dinesen and Morton, 2014) and the freshwater L. fortunei (Montalto and Molina, 2014). The setae possibly have a defensive function (Wright and Francis, 1984) and their detailed structure in Modiolus traillii (Reeve, 1857) has been described by Choo et al. (2014). Fig. 3 illustrates juveniles of S. bilocularis (Fig. 3A and B) and M. virgata (Fig. 3C & D), both with shell lengths of ∼10 mm and showing the distribution of byssal setae on the right and left shell valves, respectively. Huber (2010) used the character of no byssal setae in M. virgata to separate it from species of Septifer sensu stricto. This is not true, however and, in both species here under investigation, the setae are virtually all located on the ventral valve borders and may be up to 4 mm in length in the case of S. bilocularis but up to ∼8 mm in M. virgata. They also seem to be secreted in small clusters each probably, therefore, at the same time. 3.3.3. The adult shells Septifer bilocularis The adult shell of S. bilocularis grows up to 40 mm in length and 24 mm in height (Fig. 2A). It is also thick, ovate–elongate with terminal beaks and a broadly rounded posterior margin. The blue green shell valves are characterised by fine radially divaricating ribs, or lirae (Habe, 1951). Internally, there is an anterior umbonal septum upon which the anterior adductor muscle is located.

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Fig. 4. Diagrammatic illustrations of the shells of: A, Septifer bilocularis and B, Mytilisepta virgata as seen from the right lateral (left) and anterior (right) aspects and all drawn to the same scale. (a–b and x–y = the dorso-ventral heights and the greatest left–right widths of the shells, respectively). Also indicated are the angles subtended at the antero-ventral margin of each species with the horizontal. Fig. 5. Ventral views of the shells of: A, Septifer bilocularis and B, Mytilisepta virgata showing the location of the byssal gape in the latter and its absence in the former and their greatest left–right widths (x–y).

Mytilisepta virgata The shell of M. virgata grows to an adult length of 65 mm and a height of ∼26 mm, although this dimension is variable (Morton, 1995b). It is solid, trigonal in overall form and covered with a purple brown periostracum that is typically highly eroded anteriorly obscuring the broad radial ribbing and any growth lines (Fig. 2B). Such broad ribbing was used by Habe (1951, 1977) to separate M. virgata (and Mytilisepta) from S. bilocularis (and Septifer) with a shell sculpture of numerous radial lirae and which also has distinct commarginal growth lines. Soot-Ryen (1969) also used the different thicknesses of the radial ribs to separate Septifer from Mytilisepta. Internally too, like S. bilocularis, M. virgata possesses an anterior umbonal septum in each valve and between which the anterior adductor muscle is located. 3.3.4. Shell comparisons Fig. 4 represents diagrammatic illustrations, as seen from the right lateral and anterior aspects, of the shells of S. bilocularis (Fig. 4A) and M. virgata (Fig. 4B) both drawn to the same scale. In this figure, a–b and x–y represent the dorso-ventral heights and the greatest left–right widths of the shells, respectively. Also indicated are the approximate angles subtended at the antero-dorsal and antero-ventral junction of the shells of each species. In the coral dwelling S. bilocularis, the shell is antero-dorsally (a–b) tall but antero-ventrally indented creating an average antero-dorsal and antero-ventral subtended angle of ∼40◦ and the greatest shell width (x–y) is near basal. In the rocky intertidal M. virgata, the shell is antero-dorsally shorter and less inwardly curved anteroventrally than that of S. bilocularis giving it a lower overall profile and the greatest shell height (a–b) is thus located more posteriorly. The shell is also more flattened overall ventrally and, as a consequence, the average antero-dorsal and antero-ventral subtended angle of the antero-dorsal shell margin is greater at ∼65◦ .and the greatest shell width (x–y) is situated more mid dorso-ventrally than in S. bilocularis. The shells of S. bilocularis and M. virgata are illustrated in Fig. 5A and B, respectively, from the ventral aspect and show the location of the byssal notch (BN) in the latter and its absence in the former and the greatest left–right widths of the shells (x–y). As described

Fig. 6. Internal views of the left shell valves of: A, Septifer bilocularis and B, Mytilisepta virgata.

for Fig. 4, not only is the greatest shell width (x–y) situated more ventrally in S. bilocularis, but the greatest shell height is more anterior than in M virgata. Further, Fig. 5A shows that the greatest shell width of S. bilocularis is located further posteriorly than in M. virgata (Fig. 5B) where it is situated closer to its anterior byssal notch to provide a wider and flatter shell base (Fig. 4B) that effects a closer, firmer, contact with the substratum on the exposed rocky intertidal. The shell of S. bilocularis on the other hand is more modified for the habitation of coral crevices.

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Fig. 7. The hinge plates of the right and left shell valves, respectively, of: A and A1 , Septifer bilocularis and B and B1 , Mytilisepta virgata. Fig. 8. The siphons of: A, Septifer bilocularis and B, Mytilisepta virgata as seen from the posterior aspect.

Fig. 6 provides illustrations of the internal morphology of the left shell valves of S. bilocularis and M. virgata. The internal shell morphology of S. bilocularis (Fig. 6A) is closely similar to that of M. virgata (Fig. 6B) except that the umbonal septum has an almost straight internal margin indented ventrally and dorsally and with an anterior adductor muscle scar located on it (SE+AAM). As with M. virgata too, an accessory, but single, posterior adductor muscle scar (APAM) arches over the even smaller (than in M. virgata) true posterior adductor muscle scar (PAM) and a series of about four large blocks of paired posterior byssal retractor muscle scars (PBRM(1-4)). Again, there are no obvious posterior pedal retractor muscle scars. The shell of M. virgata (Fig. 6B) has a stout ligament (L) at the anterior end of which is an internal umbonal septum upon that sits the anterior adductor muscle (SE+AAM) with an anterior pedal retractor muscle scar (APRM) nearby dorsally. The pallial line (PL) is thin ventrally but thickens posteriorly at the position of the inhalant aperture (PS). Arising from the dorsal edge of this are two, united, accessory posterior adductor muscle scars (APAM(1) and APAM(2)) that extend postero-dorsally to lie above the relatively small true posterior adductor muscle scar (PAM) and a series of some six paired posterior byssal retractor muscle scars (PBRM(16)). There are no obvious posterior pedal retractor muscle scars. 3.3.5. The hinge plates The final growth stage of all mytiloids, the dissoconch, is formed by the juvenile individual and becomes the permanent shell of the adult. This shell phase in representatives of the Mytiloidea is typified by anterior dissoconch hinge teeth as in M. edulis, which possesses two (Ockelmann, 1995). The large M. modiolus (shell length >20 mm) (Dinesen and Morton, 2014) and the small Mytilaster minimus (Poli, 1795) (shell length <16 mm) (Morton and Puljas, 2017), however, have none. The hinge plates of the right and left shell valves, respectively, of S. bilocularis (Fig. 7A and A1 ) and M. virgata (Fig. 7B and B1 ) are in the form of the internal umbonal septum. In both species there are simple, dyssodont, hinge teeth associated with the septum. The umbonal septum of S. bilocularis is marginally convex and has the anterior adductor muscle located on it (SE+AAM). The small anterior pedal retractor muscle (APRM) is associated closely with the umbonal septum dorsally but does not attach to it. There are two hinge teeth (HT) anterior to the anterior adductor muscle in the left valve and these interlock with three more in the right valve. In M. virgata the umbonal septum is also relatively large, approximately S-shaped along its internal margin, and there is a single tooth (HT) in the left valve that is positioned beneath the

Fig. 9. Mytilisepta virgata. A, A single eye stalk on the inner mantle fold of the inhalant aperture; B, A transverse section through one of the ectopic pallial eyes of the inhalant aperture.

adductor muscle making this antero-ventral-most region of the shell swollen. The tooth fits into a socket (S) on the right valve similarly making the same region of this one swollen. The anterior pedal retractor muscle (APRM) is long and narrow and separate from the umbonal septum. 3.3.6. Internal anatomy The siphons The siphons of S. bilocularis and M. virgata, as seen from the posterior aspect, are illustrated in Fig. 8A & B, respectively. They are of the typical mytiloid form with an inhalant aperture (IA) that is confluent with the pedal gape. The exhalant siphon (EXS) in all species, typically in the form of a low cone (SC), is separate from the inhalant aperture by mantle fusions that involve the left and right inner mantle folds (IMF) only (Yonge, 1957, 1982, Type A). Similarly, where the inhalant aperture abuts the exhalant siphon

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Fig. 10. Transverse sections through the ventral mantle margins of: A, Septifer bilocularis and B, Mytilisepta virgata. B1 is a more detailed illustration of the periostracum of M. virgata.

its dorsal apex is characterised by an inter-siphonal valve (ISV) that is small and concave in S. bilocularis and small and ventrally pointed in M. virgata. Mantle papillae (MP) characterise the mantle margins of S. bilocularis, which also has papillae adorning the dorsal surface of the exhalant siphon (SP). Such papillae are either absent or, at least, less significant in M. virgata. The inner mantle folds of both species are marginally contoured and typically coloured brown with white markings. The dorsal regions of the open and flared inner mantle folds of the exposed rocky shore M. virgata are characterised by small eyes (Fig. 8B, EE) located atop small papillae or eye stalks (ES) that line, left and right, the inhalant aperture to the mantle cavity. One of these is illustrated in more detail in Fig. 9A. Each eye (EE) papilla is located at a crease and on a raised ridge (RR) in the epithelium linking the inner edge (IEIMF) with the outer edge of the inner mantle fold (OEIMF). Fig. 9B is a transverse section through one of the ectopic pallial eyes of M. virgata. Each one comprises a swollen outer epithelium that is lined marginally by microvilli (MI). These epithelial cells are pigmented (PC). The inner epithelium beneath the pigmented cells comprises a few (six in the section illustrated) of large, tall (80 µm), photosensory cells (PSC) each with a large and distinctive (40-50 µm) nucleus. From the bases of these cells extend nerve fibres that ultimately unite to form an optical nerve (ON). 3.3.7. The mantle margins Transverse sections through the ventral mantle margins of S. bilocularis (Fig. 10A) and M. virgata (Fig. 10B) show that they generally conform to the standard mytiloid plan of three folds: inner (IMF), middle and outer (OMF), with the middle typically separated into sub-folds — outer (MMF(1)) and inner (MMF(2)). The coral-dwelling S. bilocularis has an enlarged inner mantle fold that is penetrated by the inner branch of the pallial retractor muscles (PRM) while other muscle fibres penetrate the two middle sub-folds and the long outer fold, which secretes the periostracum (P). There is also a distinctive pallial nerve (PN). The mantle margin of M. virgata is essentially the same save for slight differences in fold sizes. The periostracum (P) here (Fig. 10B1 ) shows the same features as those described by Beedham (1958, fig. 1) for M. edulis, including a vacuolated (VA) area abutting the outer middle mantle fold (MMF(1)).

Fig. 11. Mytilisepta virgata. A, The pedal gape valve of the antero-ventral mantle margin. B, An illustration of how the pedal gape valve serves to keep the pedal gape sealed.

Fig. 12. The relationship between the ctenidia and labial palps and the posterior adductor and byssal retractor musculature of A, Septifer bilocularis and B, Mytilisepta virgata.

3.3.8. The pedal gape valve In the exposed rocky intertidal M. virgata, the pedal gape is unusually characterised by the presence on each inner mantle fold of a raised, fleshy, extension (Fig. 11A, PV) which is here considered to be a valve (or ‘washer’). This has never been described for any other mytiloid. Fig. 11B shows how, as seen from the ventral aspect, the pedal gape ‘washer’ serves to keep the large byssal notch (BN) and pedal gape sealed off from the exterior by being wrapped around the byssus (BY) and, presumably, the foot (F) when this is extended. It thus serves to seal the anterior mantle cavity when necessary.

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Fig. 13. The relationship between the pericardial complex and the posterior adductor and byssal retractor musculature of A, Septifer bilocularis and B, Mytilisepta virgata. In both A and B, the right posterior byssal retractor muscles have been pulled downwards (arrows) exposing the course of the conjoined style sac and mid and the returning mid gut between the left and the displaced right elements of these muscle blocks.

3.3.9. The organs of the mantle cavity The anatomies of many mytiloid species have been described (see Introduction) and all conform to a basic plan. In Fig. 12 the relationships between the ctenidia and labial palps, the posterior adductor muscle, the accessory posterior adductor muscles and the posterior byssal retractor musculature of S. bilocularis and M. virgata are examined. As in all studied mytiloids, the ciliation of the ctenidial surfaces of these two taxa are of type B(I) (Atkins, 1937). Like many other mytiloids too, the outer demibranchs of both species are some three or four filaments shorter at their anterior ends than the inner ones (Fankboner, 1971) and the ctenidial– labial palp junctions are all of Category I (Stasek, 1963). In the coral-dwelling S. bilocularis (Fig. 12A) the posterior adductor muscle is smaller than that of M. virgata, the posterior byssal retractor muscle system is bulkier and, in terms of the ctenidia, the inner demibranch is dorso-ventrally shorter than in M. virgata. The mantle margin (MM) is thick posteriorly and terminates close to the ventral extremity of the accessory posterior adductor muscle. In M. virgata (Fig. 12B) the ctenidia are dorso-ventrally tall, the outer demibranch (OD) taller than the inner (ID), the labial palps are small and the mantle margin (MM) is thickened posteriorly and abuts closely with the accessory posterior adductor musculature (APAM(1), APAM(2)). These muscles overlie the true posterior adductor muscle (PAM) and some of the posterior byssal retractor muscles (PBRM(1-6)). Neither S. bilocularis nor M. virgata possess ctenidial plicate Organs of Sabatier (Thomsen et al., 2018).

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3.3.10. The pericardial cavity and posterior byssal retractor muscle complex Morton (2015b) showed how the relationship between the organs of the pericardium and the arrangement of the posterior byssal retractor muscle complex varied between the representatives of the different families of the Mytiloidea. The relationship between the pericardial complex and the posterior adductor and byssal retractor musculature of S. bilocularis (Fig. 13A) and M. virgata (Fig. 13B) are both of Morton’s Category 2. In representatives of this category, the pericardium is situated anterior to a reduced number, between one (species of Lithophaga, Botula, Adula and Brachidontes) and six (Mytilus), of posterior byssal retractor muscle units. This category is broadly typical of the Mytilinae and related subfamilies, for example, the Botulinae, Adulinae, Lithophaginae and, now, the representatives of the putative Septiferinae described herein. The situation in the two species here under study is further illustrated in Fig. 13 in relation, in particular, to the posterior pedal retractor muscles and the accessory posterior adductor muscle. In both Fig. 13A and B, the right posterior byssal retractor muscles have been pulled downwards (curved arrows) exposing the course of the conjoined style sac and mid gut (CSSMG) and the returning mid gut (MG) between left and right muscle elements. In S. bilocularis (Fig. 13A), the posterior accessory adductor musculature (APAM) covers the conjoined style sac and mid gut (CSSMG) and separated mid gut (MG), the true posterior adductor muscle (PAM) and the posterior byssal retractor muscle blocks (PBRM(1-4)). But there are no posterior pedal retractor muscles and the pericardium and contained organs are pushed upwards against the anterior end of the accessory posterior adductor muscle. This is because this dorso-ventrally tall species is more antero-posteriorly foreshortened in relation to height. In M. virgata (Fig. 13B), the accessory adductor musculature (APAM(1), APAM(2)) again extends over the true posterior adductor muscle (PAM) and the posterior byssal retractor muscles (PBRM(1-6)). And the pericardium (PE) with the contained ventricle (V) and paired auricles and their epithelially-contained pericardial gland (AU+PG) is situated further anteriorly and anteroposteriorly aligned. The posterior pedal retractor muscles are reduced to but a few fibres (PPRM(F)) in this species. 4. Discussion 4.1. Fossil Septiferinae and other taxa with an umbonal septum Runnegar and Newell (1971, Fig. 18, A, B, D and E) illustrated and described Coxesia mezzalirai (Mendes, 1952) (Mytilidae), possibly the oldest representative of the Septiferinae, from the South American Permian (>250 mya) and compared its external shell form with that of freshwater species, that is, Sinomytilus harmandi (Rochebrune, 1881) and Limnoperna fortunei, and species of Congeria, Dreissena and Mytilopsis (Dreissenidae). Simões et al. (2015) similarly noted that species of Coxesia from an Upper Permian freshening event (Salvador and Simone, 2010) in a marginal marine habitat in the Rio do Rasto Formation of the Paraná River Basin in southern Brazil (Mendes, 1952; Simões and Kowalewski, 1998; Matos et al., 2017), share the common features of an externally smooth, sickle-shaped, shell (Fig. 14A) with S. harmandi described by Morton and Dinesen (2010). Internally, species of Coxesia, again for example C. mezzalirai, have a small umbonal septum and no hinge teeth (Fig. 14A1 ), although no anterior adductor muscle scar has been identified for this species as yet. Coxesia mezzalirai is not, however, restricted to the Paraná River Basin, but has been recorded from the Permian of Sao Paulo State by Carbonaro et al. (2014). Berezovsky (2015) described two new, much younger, genera and two new species of Admytilus and A. alius, and Assytilus and A.

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Fig. 14. A, An external view of the left shell valve of Coxesia mezzalirae. A1 , An internal view of the left shell valve of Coxesia mezzalirae (after Mendes, 1952). B, An external view of the left shell valve of Assytilus alpha showing the (highlighted) radial ribbing. B1 , A detail of the left (left) and right (right) hinge plates of Assytilus alpha (re-drawn after Berezovsky, 2015).

each ventral valve margin. Fig. 14B1 illustrates this ‘step’ on the left (left) and right (right) hinge plates of A. alpha. There are no hinge teeth however. Berezovsky (2015), Fig. 5[a]) also described both the genus Assytilus and A. alpha as possessing a posterior adductor muscle scar shaped like an inverted comma and a comma, respectively. Above the posterior adductor muscle scar of A. alpha, there appears to be, though not identified as such, a long accessory adductor muscle scar indicating that, alongside the evidence of an umbonal septum and bifurcating shell ridging, that this is a fossil representative of the as currently defined Septiferinae. Extant taxa currently assigned to the Septiferinae and species of Septifer date from the end of the Triassic >200 mya to the Recent (Coan et al., 2000). The fossil record for putative species of Mytilisepta extends from the end of the Cretaceous to the Recent, that is, about the last 65 million years. 4.2. Anatomical comparisons

Fig. 15. Simplified diagrams illustrating how from A, (dorsal view) and A1 (transverse section) a primitive limpet-like mollusc with lateral pallial retractor muscles and internal pedal retractor muscles, there has been the anterior–posterior division of the shell into two valves (B and B1) . This necessitated the modification in the Bivalvia of anterior-most and posterior-most pallial retractor muscles into adductor muscles and the reduction in the number of pedal retractor muscles (A and A1 ; B and B1 ) (redrawn after Morton, 1958). In the case of the species of Septifer and Mytilisepta (C and C1 ), the evolution of the heteromyarian form (resulting in the greater expansion of the postero dorsal region of the shell) has resulted in extra pallial retractor muscles being modified and cross united to form accessory posterior adductor muscles.

alpha (Mytilidae), both with an external radially bifurcate ribbing and internal umbonal septa, from the Middle Eocene (∼45 mya) of the Ukraine. The shells of both taxa are wedge-shaped, arcuate, strongly convex and have terminal umbones. The external surface is covered with many closely positioned dorsally bifurcated radial ribs and Fig. 14B is an external view of the left shell valve of A. alpha showing this (highlighted) septiferine-like ribbing. Possibly significantly too, representatives of Assytilus possess a small anterior adductor muscle located on a ‘step’, or small septum, on

Morton (2015b) showed that each phylogenetic lineage of the Mytiloidea has an uniquely correlated relationship between the pericardium and its contained organs, the heart and pericardial glands, and the posterior byssal and pedal retractor musculature. In all the so far studied species of the Mytilidae, the pericardium is situated anterior to the posterior byssal and pedal retractor muscles — the latter typically small, the former aligned in a row of either one or more units. There are six of these for example in Mytilus galloprovincialis (Lamarck, 1819) (Morton, 2015b). In M. virgata and S. bilocularis there are approximately six and four pairs of posterior byssal retractor muscle blocks, respectively. Because the heteromyarian shells of the two species are broadly similar but differing in overall detail, that is, relatively dorso-ventrally compressed and elongate (M. virgata) and tall (S. bilocularis), the pericardium is horizontal and separate (M. virgata) and closely abutting and aligned near vertically (S. bilocularis). On the basis of these characters, the two species can thus be assigned anatomically (but see below) as representatives of the Mytilidae (Morton, 2015b). The posterior byssal retractor complex needs to be viewed in terms of its own diverse functions. Where these muscles are strong and clustered centrally (either as one block or divided) they seem to function for pulling the animal down strongly onto the substratum, that is into coral crevices and exposed rocky surfaces in the cases of S. bilocularis and M. virgata, respectively. Conversely, in species of the Lithophaginae for example, the muscles are weak and focused anteriorly and posteriorly near their ends so that they are used to pull the animal back and forth in its burrow (Wilson, 1979). If this is an accurate assumption, then the position of the pericardial complex has probably changed in the diverse representatives of the byssally attached Mytiloidea to accommodate a diversified muscle function, not the other way around. It thus seems clear that it is the posterior byssal retractor complex which is the driver of the evolutionary anatomical changes seen in the numerous representatives of the Mytiloida, including the, as currently recognised, representatives of the Septiferinae described herein, not the pericardial complex. The two species described and discussed herein have also been shown to possess an internal umbonal septum with an anterior adductor muscle located between this feature in the left and right valves. Further, the two taxa possess an accessory posterior adductor musculature, which is unique to the Septiferinae — again as currently defined. 4.3. Origin of the accessory posterior adductor musculature It is first necessary to understand the origin of the uniquely septiferine character of the accessory posterior adductor musculature. Fig. 15 attempts to explain this. It is generally conceded that the

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Bivalvia evolved from a univalve ‘limpet like’ ancestor in which the conical shell (Fig. 15A and A1 ) became separated left and right dorsally by an antero-posterior division, the resulting two, possibly unequal, valves being cross-connected by a posteriorly aligned ligament and anterior and posterior adductor muscles (Yonge, 1947; Morton, 1958). These, in turn, were derived from ancestral pallial retractor muscles (Fig. 15B and B1 , PRM). Associated with this radical change in body form, the left and right array of, possibly four, pedal retractor muscles (PERM) were also reduced to paired anterior and posterior pedal retractor muscles (APRM, PPRM). Such evolutionary changes resulted in a form similar to that of the primitive bivalve Fordilla (Pojeta and Runnegar, 1974) and which Morton and Yonge (1964, fig. 14), described and illustrated would look like anatomically assuming an earliest protobranch ancestry. See Cope (2000), however, for a more recent interpretation of overall bivalve evolution. If the above scenario is broadly correct, however, then, in the case of species of Septifer and Mytilisepta (Fig. 15C & C1 ), the evolution of the heteromyarian form (and thus the greater expansion of the postero-dorsal region of the shell) has resulted in extra pallial retractor muscles being modified and cross-united to form an accessory posterior adductor musculature (APAM) such that although the conjoined crystalline style sac and mid gut (CSSMG) and the separated mid gut (MG) still remain beneath the (true) posterior adductor muscle (PAM) and between left and right posterior byssal retractor muscles (PBRM), the rectum (R) now passes above the original posterior adductor muscle (PAM) and below the accessory posterior adductor musculature (APAM). Again because of the adoption of the heteromyarian form, the pedal retractor muscles (PRM) of the burrowing ancestor now function as posterior byssal retractor muscles (PBRM) and in both mytilids herein investigated, the true posterior pedal retractor muscles (otherwise typical of most mytiloideans) are now reduced to either vestigial fibres (M. virgata) or are absent (S. bilocularis). But, is there any evidence that the accessory posterior adductor muscles are ‘modified’ pallial retractor muscles as envisaged by Yonge (1947) and Morton (1958)? There is one other example in the Bivalvia, that is, the Tellinoidea, where it is thought that pallial retractor muscles have evolved into, albeit simple, valve linking, ‘adductor’ muscles. Prior to the researches of Yonge (1949), earlier workers considered the cruciform musculature characteristic of the Tellinidae to be an accessory adductor muscle (von Ihering, 1900) and a regulator of blood pressure in the siphons (Hoffmann, 1914). Graham (1934a,b) dismissed the notion of the cruciform muscles being an accessory adductor muscle although, most recently, Arruda and Domaneschi (2005) have shown that there is an accessory adductor muscle located between the anterior and posterior branches of the cruciform muscle of Macoma biota (Arruda and Domaneschi, 2005). Further, Morton (2017) has shown that the cruciform musculature of Gari costulata (Turton, 1822) is more complex than originally shown for other taxa with a greater number of cross-valve attachments. Both of the above observations are of interest because these tellinoid cruciform muscles may provide a clue as to the origin of the bivalve adductor muscles, that is, they were derived (in evolutionary terms) from cross-connected pallial retractor muscles. And, they may also explain the evolution of the posterior accessory adductor muscles of representatives of the as currently recognised Septiferinae (Mytilidae) and as hypothesised herein. A fossil mytiloid similar to the taxa herein discussed and, for example, similar to the Middle Miocene species of Assytilus (Berezovsky, 2015) is probably a septiferine because of its radially bifurcate shell ribbing with a small septum and vestigial anterior adductor muscle scar. Similarly, the smooth shelled species of Coxesia, also with an umbonal septum but with no (described) indication of an anterior adductor muscle scar (Soot-Ryen, 1969)

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may yet be proven to be a yet older Permian ancestor of the, again as currently recognised, Septiferinae (Runnegar and Newell, 1971). Finally, as noted earlier in this paper the subgenus Mytilisepta, with M. virgata (originally Tichogonia, Wiegmann, 1837) as the type species, was erected by Habe (1951) and elevated to generic status by Huber (2010). Coan and Valentich-Scott (2012), however, considered Mytilisepta to be a junior synonym of Septifer. This is because, in summary, the only anatomical features which separate the two putative genera are: (i), differences in the strength of their external shell ribbing; (ii), differences in the size and form of the internal shell septum and the arrangement of the hinge teeth thereupon; (iii), the posterior pedal retractor muscles are reduced to but a few fibres in M. virgata and are absent in S. bilocularis and (iv), there is a pedal gape valve and posterior ectopic pallial eyes in M. virgata but not in S. bilocularis. These two latter differences thus seem to relate to the cryptic occupation of exposed rocky shores in the former species and are perhaps anti-predator features and thus have no comparative phylogenetic significance unlike characters (i) and (ii) in particular (Habe, 1951; this study). There is, however, further significance in the possession by M. virgata of ectopic pallial eyes located at the apices of siphonal papillae. That is, some representatives of the Mytiloidea possess true cephalic eyes located on the bases of the anterior-most filaments of the ctenidia, for example, M. edulis (Morton, 2008). However, the only other mytiloid shown to date to possess ectopic pallial eyes is the freshwater L. fortunei where they are located on the intersiphonal valve (Morton, 2015a). In both cases, such eyes are simple eyespots, which typically serve a defensive function by initiating a shadow reflex of valve closure as demonstrated most recently for L. fortunei by Duchini et al. (2015). This is, further, therefore, the first record of ectopic pallial eyes in a marine mytiloid. 4.4. The genetic evidence Until recently, the classification of the Bivalvia at the family level using mainly shell characters by Bouchet et al. (2010) concluded that the Mytilidae comprised eight subfamilies including the Septiferinae as originally defined by Scarlato and Starobogatov (1979). Such a classification was broadly accepted by Morton (2015b) on the basis of internal anatomical characteristics. Earlier, however, Matsumoto (2003), had suggested, on the basis of gene-sequencing data, that M. (as Septifer) virgata was somewhat distinct from S. excisus but still united within the Mytilidae. Subsequently, Trovant et al. (2015), again using DNA evidence showed that M. virgata and M. bifurcata from the western and eastern Pacific, respectively, were genetically different from S. bilocularis also from the western Pacific. Again on genetic evidence, Gerdol et al. (2017) removed M. virgata from the Mytilinae and relocated it in the Brachidontiinae. Combosch et al. (2017) using a 5-gene Sanger-based approach suggested that M. virgata was most closely related to Brachidontes exustus (Linnaeus, 1758) and included, generally, within the Mytilidae. Most recently, Liu et al. (2018) using both mitochondrial and nuclear genes, have argued that S. excisus is most closely related to species of Modiolus, M. edulis, P. viridis (Linnaeus, 1758) and M. senhausia all within the Mytilinae and more distantly to species of Brachidontes but, again, still within the Mytilinae. The genetic evidence regarding the presently accepted location of M. virgata within the broader context of the Mytiloidea is equivocal but seems to possibly favour a relationship (however close) with the Brachidontiinae. The conclusion of Trovant et al. (2015) suggesting that Eastern Pacific species of Mytilisepta and Western Pacific M. virgata are genetically distinct from S. bilocularis, for example and however, raises the possibility of either convergent or parallel evolution between the two genera. Either of these may be a distinct possibility given the wide disparity in the fossil record between the two taxa, that is >100 million years.

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5. Conclusions Two (known) representatives of the Septiferinae, as currently defined, inhabit Hong Kong waters – S. bilocularis nestling singly in crevices in coral heads and M. virgata occurring as small clusters in the intertidal zone of highly exposed rocky shores. Save for clear differences in anterior hinge teeth structure, the two species share virtually identical anatomical features both in the form of the shell and the internal anatomy. Such features include, for the shell, a bifurcatingly radiate sculpture (broader in M. virgatus and narrower, as ‘lirae’, in S. bilocularis) and an internal anterior umbonal septum on each valve and between which is situated the anterior adductor muscle. The occurrence of byssal setae on the juvenile shells (in particular) of both taxa refutes the distinguishing character of Huber (2010) separating S. bilocularis (with setae) from M. virgata (without). Internally, both S. bilocularis and M. virgata possess the uniquely ‘septiferine’ character (Yonge and Campbell, 1968; this study) of a large posterior accessory adductor muscle probably derived from modified pallial muscles (this study). In both too, significant posterior byssal retractor muscles are either absent (S. bilocularis) or virtually so (M. virgata). On the basis of anatomical evidence, therefore, the current location of the species within the Septiferinae as a subfamily of the Mytilidae would seem to be justified. Notwithstanding, recent genetic evidence suggests that the similarities between the two taxa may possibly be the result of either convergent or parallel evolution. If this is so, and bearing in mind the long time difference in the fossil record between the two genera, then the results of this study highlight a remarkable example of such an evolutionary phenomenon. As described herein, however, the fossil record for the, again as currently accepted, Septiferinae identify a highly conservative shell and internal morphological anatomy, that seems to have changed little since the subfamily’s origin(s) until today. This conservatism is possibly a characteristic of the Mytilidae in general and, if so, would suggest that in the Septiferinae, genetic separation between species of Septifer and Mytilisepta has been extraordinarily slow. Current genetic research casts a clearer light on this (Morton and Leung in preparation) and suggests that S. bilocularis can be retained within the Septiferinae, as currently defined, whereas species of Mytilisepta, including M. virgata, as the type species, should possibly be placed in a new subfamily, again possibly, allied to the Brachidontiinae — the Mytiliseptiferinae. Both subfamilies should, however, the latter if and when erected, be retained within the Mytilidae. Acknowledgements I am grateful to Vriko Yiu (School of Biological Sciences, The University of Hong Kong) and Priscilla Leung (State Key Laboratory in Marine Pollution, City University of Hong Kong) for assistance in collecting S. bilocularis and M. virgata, respectively, in Hong Kong. Thanks to Gabriel Lee (also City University of Hong Kong) for permission to reproduce Figs 1 and 2. I am finally indebted to Dr Sanja Puljas (University of Split, Croatia) for her kindness in sectioning the specimens of the two species reported upon herein. References Albayrak, S., Çaglar, S., 2006. On the presence of Siphonaria belcheri Hanley, 1858 [Gastropoda: Siphonariidae] and Septifer bilocularis (Linnaeus, 1758) [Bivalvia: Mytilidae] in the Iskenderun Bay (SE Turkey). Aquat. Invasions 1, 292–294. Amler, M.R.W., 1999. Synoptical classification of fossil and Recent Bivalvia. Geol. Palaeontol. 33, 237–248. Arruda, E.P., Domaneschi, O., 2005. New species of Macoma (Bivalvia: Tellinoidea: Tellinidae) from southeastern Brazil, and with a description of its gross anatomy. Zootaxa 1012, 13–22.

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