BULLETIN DE L'INSTITUT ROYAL DES SCIENCES NATURELLES DE BELGIQUE BIOLOGIE, 72-SUPPL.: 105-108, 2002 BULLETIN VAN HET KONINKLIJK BELGISCH INSTITUUT VOOR NATUU RW ETENSCHAPPEN BIOLOGI E, 72-SUPPL.: 105-108, 2002
On the taxonomic status of the brittle-star A1nphipholis squamata (DELLE CHIAJE, 1828)
S. DUPONT
Summary used to separate species: enzyme activity, genetic dis tances, etc. [29]. Amphipholis squamata is one of the 15 species of echinoderms of the The small ophimoid species Amphipholis squamata Belgian fauna. Recent works on the intraspecific variability suggest that this species might constitute a complex of sympatric colour (DELLE CHI AJ E, 1828) is one of the 15 echinoderm species morphs. This short paper aims to review the research on this field of the Belgian fauna. It is a good illustration of the and to clari ty the taxonomic status of this brittle-star. difficulty to design correctly a taxon and it constitutes an interesting model to discuss the validity of the taxo nomical parameters (genetic, physiology, morphology, etc.). Resume Amphipholis squamata Amphipholis squamata est une des 15 especes d'echinodermes de Ia faune beige. Des travaux recents sur Ia variabilite intraspecifique suggerent que cette espece pourrait constituer un complexe d'especes A. squamata is probably "the most widely mentioned surs vivant en sympatrie. L'objectif de cet article est de synthetiser les Amphiurid brittle-star in scientific literature" [28, 39]. recherches realisees dans ce domaine et de clarifier le statu! taxono mique de cette ophiure. Various aspects of its biology have been studied in several areas of the world since the 19111 century. First known description was published in 1805 by VIVIANI [42 ] who described the luminous capabilities of the ophiuroid. Introduction Since this period, numerous papers appeared regularly in the literature but for fifty years, the frequency of The species is the lowest level of classification. In sexu publications increased exponentially. The unexpected ally-reproducing organisms, it is a group of individuals interest for this small ophiuroid could be explained by that can usually or potentially interbreed and which are its amazing biological properties: A. squamata is co reproductively isolated from other such groups [3 , 29, 30, smopolitan [21, 22, 26], bioluminescent [4, 42], poly 31]. After centuries of species descriptions, numerous chromatic [2, 11, 12, 13, 14, 16, 17, 18, 19, 20, 40], mistakes were discovered. Most of the time, a species hermaphrodite, brood protecting [1 , 2 1] ; both setting was described several times under different names be and outcrossing occur for reproduction [1 8, 20, 36], etc. cause of its polymorphism or its large geographical dis A. squamata is believed to be the only ophiuroid with tribution. For example, the red seaweeds Porphyra umbi a world-wide distribution excepted in polar regions licalis and Conchocelis rosea are very common on the [21, 22, 26]. As a consequence of this distribution, the coast of Europe. They have contrasting morphology since species appeared in the literature under a variety of Porphyra have the form of an upright sheet or leaf generic and specific names over the last two hundred whereas Conchocelis is a branch, fi lamentous seaweed years and more than thi rty synonyms were proposed which is able to bore into the calcareous shells of mol [1 , 5]. Surprisingly, global populations of A. squamata luscs and barnacles. Carpospores of Porphyra were cul are morphologically uniform [21 , 24, 38]. It appeared tured in laboratory and it quickly became clear that the to vary but little between distant populations. For fi lamentous algae produced were identical with Concho example, no differences were observed in morphology celis [41]. Numerous mistakes were then corrected and between animals from Britain and New Zealand [2 1]. the quantity of known species decreases with the time, Reason why CLARK underlined the importance to stabi constituting a virtual biodiversity crisis. Fortunately, a lize the specific name and the name Amphipholis squa new taxonomy appeared with modern biology. New tools mata (DELLE CHIAJE, 1828) was formalized by the Inter such as genetic markers induced modifications in the national Commission on Zoological Nomenclature in classifi cation of the taxons. New cri terions were then 1987 [5]. I I I 06 S. DUPONT
A complex of species? sampling site. We used the same method in order to demonstrate that body colour and luminous capabilities Recent works on eco-physiology of A. squamata report are genetically determined. Genetic structure is homo loose inter- and intrapopulational variability at all levels genous for each colour morph within a population while from population to genotype. it shows inter-population variations for a same variety (i) The species displays various colours of disc and [ 19]. anns. Eleven colour m01·phs were described in different stations around the world [2 , 11 , 12, 13 , 14, 16, 17, 18, 19, 20, 40]. Since adult body colour remains constant through One species? time and since juveniles present always the same body colour than its parent, BfNAUX & BOCQUET suggested that A. squamata is then a complex of sympatric colour colour morphs are genetically determined and that most mm·phs. Within a population, these mor-phs are geneti of the reproduction occurs by selfing [2]. cally determined and huge differences between them (ii) The species is bioluminescent. It emits visible light were observed in their life-history traits and biolumines when mechanically stimulated. Tl1e nervous control of cence proper1ies. Two hypotheses arise: (a) the "sibling the light emission was studied and several differences in species hypothesis" : this ophiuroid represents different the physiology of luminescence were observed between species I iving in sympatry or (b) the ''on e species hypoth colour morphs [6, 7, 8, 9, I 0, 27] . Recently, it have been esis": it is a single highly variable species, which " har demonstrated that each colour morph possesses its own bours unprecedent levels of genetic divergence" [37]. luminous capabilities [11 , 12, 13 , 14, 16, 17, 18, 19].ln DEHEYN et al. [II , 14] , working on bioluminescence laboratory conditions, DEH EYN eta!. [11] have confirmed and polychromatism, and SPONER et al. [38], working the results of BrNAUX & BOCQUET on inheritance of body on genetic macro-variations, support the "sibling species colour and they have shown that luminous capabilit_i~s are hypothesis". Nevertheless, other works on morphology also transmitted to the offspring even in presence of and reproduction of the species give useful information individuals of other colour m01·phs. They suggest that supporting the "one species hypothesis". cross-fertilization can only happen between individuals (i) If the species have been reproductively isolated for of the same variety. On the basis of these observations, millions of years, as expected by the huge genetic differ some authors suggest, "The 'colour-luminescence' link ences, some fixed mm-phological differences are ex appeared clearly fixed (...) and is suggested to be of pected between individuals. Using scanning electron mi genetic origin. The species 'A. squamata' may then be croscopy, SPONER et al. [38] have carried out a morpho a mosaic of genetically different ·entities (the varieties) metric study of the micro-morphology of A. squamata rather than a unique cosmopolitan taxonomic entity'' specimens of contrasting genotype. Surprisingly, only [14] and "ophiuroid colour varieties are reproductively few differences were observed between them. isolated sibling species" [ 11]. (ii) Amphipholis squamata is a simultaneous hermaph (iii) Differences between the colour morphs were also rodite [ 1, 21, 25, 35]. Both testis and ovary are paired in observed for life-history traits. We observed two colour the same genital bursae and reach matmity simulta m01·phs, co-existing in the same population, with differ neously [1, 34]. Because of this anatomy, the species ent reproductive period (one in Autumn and another one was suspected of s~lf-fertilization [34] and/or of parthe mainly in Spring). This observation suggests that colour nogenesis [4, 32]. More recently, the possibility of selfing morphs are nearly reproductively isolated and that sym was demonstrated both in laboratmy [11] and natural [18, patric speciation could occur [ 15, 16]. 20, 36] conditions. POULIN et al. [36] have also demon (iv) In 1989, MuRRAY has performed the first genetic strated that outcrossing could occur. Using data on in analysis on this species. He has demonstrated that the heritance of body colour and luminous capabilities in species genetic identity is low (31 %) between individuals laboratory conditions, DEHEYN et al. [11] suggested that: from UK and USA with a high genetic polymorphism that ''If cross-fertilization could indeed normally occur ( . .) it can reach 14 different alleles for some loci [33]. Another was not successfitl between the beige and the black study of the genetic diversity at high geographical levels variety (...) It is possible that cross-fertilization is only was realized by SPONER et al. [38]. Using mitochondrial successfitl between individuals of the same variety". DNA sequencing they revealed high genetic differences Using a similar method, we have showed that outcrossing among populations since they observed deep divergence between different colour morphs could occui· in natural of up to 23.4 % which is ten times higher than reported conditions since we have observed juveniles with lumi levels of intraspecific divergence in other echinoderms. nous capabilities different from their parent [ 18, 20]. On the basis of these observations, they proposed: "di Only few data are available on the frequency of the vergent clades are likely to represent different species" . monoparental reproduction. Some authors suggest that Genetic variation was assessed at smaller geographic selfing might be the main way of reproduction [2] when scale by POULAfN et al. [36]. Using RAPDs markers, they others affirm that it constitutes only an alternative way of have demonstrated that both selfing and outcrossing were reproduction [11]. PouLI N et al. [36] said: "seljing and used for reproduction in A. squamata. Moreover, they outcrossing rates probably depend on the spatial distri observed a strong genetic diversity, even in the same bution of local population and the type of habitat''. I I On the taxonomic status of the brittle-star Amphipholis squamata (DELLE CHIAJE, 1828) 107
Conclusion phipholis squamata (Echinodermata: Ophiuroidea). Bioi. Bull., 152: 12-25. Amphipholis squamata has a worldwide distribution 5. CLARK, A.M., 1987. Asterias squamata DELLE CHIAJE, 1828 being absent only in polar region. It occupies a wide (currently Arnphipholis squamata; Echinodermata, Ophiuroi range of habitats from intertidal zone down to 1330 m dea): proposed conservation of the specific name. Bull. Zoo!. depth [21 , 22, 26]. It is constituted of dense local popula Nom., 44 (4): 246-247. tions in various biotopes: under stones of tide pools, in 6. DE BREMAEKER, N., 1999. Nervous control of the lumines phanerogam meadows (Zoostera, Cymodocea, etc.), in cence in the ophiuroid Amphipholis squamata (DELLE CHIAJ E, seaweeds ( Chaetomorpha, Corallina, etc.), endosymbio 1828)(Echinodermatn). PhD Thesis, Universite catholique de Louvain, Belgium. tic of sponges, in coral reefs, etc. In 1987, CLARK pro posed to stabilize the name Amphipholis squamata and to 7. DE BREMAEKER, N., BAGUET, F. & MALLEFET, J., 1999. suppress synonyms [5]. Since this period, numerous Characterization of acetylcholine-induced bioluminescence in Amphipholis squamata (Echinodermata: Ophiuroidea). Be/g. J. works were performed in various fields. They showed Zoo!. , 129 (2): 353-362. that an important intraspecific variability is the mle at all 8. DE BREMAEKER, N., BAGUET, F. & MALLEFET, J., 2000. the studied levels, from gene to populations. Moreover, Effects of cathecholamines and purines on the luminescence this variability is linked to the polychromatism. For ex of the ophiuroid Amphipholis squamata (Echinodermata). J. ample, within a population each colour morph possesses Exp. Bioi., 203: 20 15-2023. its own luminous capabilities, which are genetically de 9. DE BREMAEKER, N., BAGUET, F., THORNDYKE, M. & MA L termined [ 19]. On the basis of these observations, several LEFET, J ., 1999. Modulatory effects of some amino acids and authors suggest that A. squamata represents different neuropeptides on luminescence in the brittlestar Amphipholis sibling species, each colour morph being a new species squamata. J. Exp. Bioi. , 202: 1785-179 1. [ 11 , 14, 18]. Moreover, genetic stmcture shows important 10. DE BREMAEKER, N., MALLEFET, J. & BAGUET, F., 1996. inter-population variation for the same colour morph Luminescence control in the brittlestar Amphipholis squamata: [1 9]. On the basis of these criterions, A. squamata rnust Effect of cholinergic drugs. Camp. Biochem. Phys., ll5C: 75-82. be divided in dozens species. On the other hand, global II. DEHEYN, D. & JANGOUX, M., 1999. Colour varieties as populations of A. squamata are morphologically unifonn sibling species in the polychromatic ophiuroid Amphipholis and no fixed morphological differences were observed squamata (Echinodermata): evidence from inheritance of body between individuals with contrasting genotypes [2 1, 24, colour and luminescence characters. J. Exp. Mar. Bioi. Ecol., 37]. Moreover, we have demonstrated that outcrossing is 234: 219-234. possible between colour morphs [ 18, 20]. Therefore, on 12. DEI-IEYN, D., 1998. Etude de Ia bioluminescence chez the basis of the biological definition of the species, we l'Ophiure Amphipholis squamata (DE LL E CH IAJE, 1828) propose to consider Amphipholis squamata as a unique (Echinodermata): Structure des sites photogenes; modalites species. This species ' 'harbours unprecedent levels of de l'expression et fonction de Ia bioluminescence. PhD The genetic divergence" [37] inducing a high level of intras sis, Universite Libre de Bruxelles, Belgium. pecific variability. This variability is probably the clue to 13. DEHEYN, D., MALLEFET, J. & JANGOUX, M. , 1997. Intra explain its incredible ecological success. specific variations of bioluminescence in a polychromatic po pulation of Amphipholis squamata (Echinodermata: Ophiuroi dea). J. Mar. Bioi. Ass. U.K., 77: 1213-1222. Acknowledgements 14. DEHEYN, D., MALLEFET, J. & JANGOUX, M. , 2000. Evidence from polyclu·omatism and bioluminescence that cosmopolitan I apologize to the many excellent researchers whose work I have not ophiuroid Amphipholis squamata might not represent a unique been able to cite directly in this short review. l would li ke to thank Prof. taxon. CR. Acad. Sci. Paris, 323 : 499-509. Jerome M ALLEFET for giving me the opportunity to work on this 15. DuPONT, S. & MALLEFET, J. , 1999. Population dynamics of amazing brittle-star and Yannick DEWAEL who made valuable com Amphipholis squamata ments on this manuscript. Research supported by FRIA grant. (Echinodermata: Ophiuroidea) in the Oliveri-Tindari's lagoon system, Sicily. In: CANDIA CARNEVAL LI , M.D. & BoNASORO, f._ (eds), Echinoderm Research 1998, Balkema, Rotterdam: 65-70. References 16. DUPONT, S. & MALLEFET, J., 2000. Luminous capabilities and life-history traits of the polychromatic ophiuroid Amphi J. ALVA, V. , 1996. Reproducti on, developpement, incubation pholis squam.ata in two isolated populations from Oliveri et dynamique de population de l'ophiure Amphipholis squama Tindari lagoon system, Sicily. Hydrobiologia, 440: 137- 144. ta (Echinodermata) en baie de Seine. PhD thesis, Universite 17. DUPONT, S., 1998. Characterization of bioluminescence Libre de Bruxelles, Belgium. variations in an isolatt>d population of Amphipholis squamata 2. BINAUX, R. & BOCQUET, C., 197 1. Sur le polychromatisme (Ophiuroidea, Echinodermata) from Oliveri-Tindari's lagoon de I' ophiure Amphipholis squamata (DELLE CHIAJE). CR. A cad. (Sicily). Master thesis, Universite catbolique de Louvain. Sci. Paris, 273: 1618-16 19. 18. DUPONT, S., 2000. Bioluminescence as an indicator of 3. BLONDEL, J., 1995. Biogeographie- Approche ecologique et genetic variability. evolutive. Masson, Paris.