© 2000 by The Society for Integrative and Comparative Biology
Evolution of Starfishes: Morphology, Molecules, Development, and Paleobiology. Introduction to the Symposium1
1 Department of Geology, University of Illinois at Urbana-Champaign, Urbana, Illinois, 61801
2 Department of Invertebrates, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024
3 Department of Invertebrate Zoology and Geology, California Academy of Sciences, Golden Gate Park, San Francisco, California 94118-4599
 |
SYNOPSIS |
|---|
TOP
 SYNOPSIS THE NATURE OF THE...TAXA AND MORPHOLOGYPHYLOGENETIC INTERPRETATIONSREFERENCES |
|---|
Among starfishes, fascinating life cycles and complex morphological patterns have evolved within a familiar but unusual basic body plan. In spite of these rich complexities, available phylogenetic interpretations conflict, and the history of this important group remains uncertain. The symposium brought together current perspectives on phylogeny, the implications of certain poorly known aspects of asteroid morphology, and consideration of significant events that preceded the diversification of extant asteroid groups in the Mesozoic.
It has become axiomatic in modern organismal biology that strongly supported phylogenetic reconstructions are crucial to the understanding of biological pattern and progress. Although asteroids exhibit complex morphologies, life cycles, and behaviors that indicate their status as ideal model organisms in the study of marine invertebrates, their evolutionary history remains obscure. This is in part due to a lack of treatment by researchers but also in part due to inherent limitations of the available data.
In the following pages, the state of inquiry into the study of relationships among starfishes is arranged in three sections: the nature of the fossil record (an introduction to modern starfishes), taxa and morphology, and phylogenetic interpretations.
|
THE NATURE OF THE FOSSIL RECORD |
|---|
|
TOP
SYNOPSISTHE NATURE OF THE...TAXA AND MORPHOLOGYPHYLOGENETIC INTERPRETATIONSREFERENCES |
|---|
In general, the rather meager fossil record is difficult to interpret. Spencer (1951)
recognized a small group of Paleozoic stelleroid genera as the class Somasteroidea, which he considered to be ancestral to both surviving stelleroid classes (i.e., Ophiuroidea and Asteroidea). His perspective is widely accepted, although understanding of early phylogenetic events is sketchy because representative fossils are few and also because the skeletons of most recorded somasteroids are delicate and tend to be poorly preserved.
Biologists have stressed external characters in the taxonomy of asteroids, and paleontologists have followed this lead in assigning fossil taxa to extant orders. Unfortunately, homeomorphy among exterior characters is widespread. Spencer and Wright (1966) suggested some separation of Paleozoic from younger fossils at the subordinal level, and Blake (1987) and Gale (1987) both fully separated Paleozoic from post-Paleozoic species.
Fossils and issues critical to the diversification of the crown-group have not received directed attention. The fossil record, although modest, is significant, and future research using available phylogenetic techniques will do much to elucidate starfish diversity, history, and relationships. D. BLAKE begins this symposium with discussion of some difficulties entailed in the interpretation of the fossil record of asteroids, and he suggests two crown-group apomorphies.
|
TAXA AND MORPHOLOGY |
|---|
|
TOP
SYNOPSISTHE NATURE OF THE...TAXA AND MORPHOLOGYPHYLOGENETIC INTERPRETATIONSREFERENCES |
|---|
Most familial- and ordinal-level taxon concepts for asteroids were developed during the latter half of the 19th and the first quarter of the 20th centuries, although new subdivisions continued to be suggested (e.g., Asterodiscididae Rowe, 1977
). Generic content of families has been broadly stable, although some genera have been transferred, and further rearrangement can be expected with future reevaluation. Two classification schemes have been most influential at the ordinal level, that of W. P. Sladen (1889), who recognized two orders based primarily on the size of the marginal ossicles (Phanerozonia and Cryptozonia) as well as other aspects generally related to skeletal robustness, and that of E. Perrier (1884, 1891, 1894), who delineated five orders (Spinulosa, Velata, Paxillosa, Valvata, Forcipulata), stressing a variety of characters still emphasized in classifications today. Several smaller orders have been recognized for one or two distinctive families: the Notomyotida Ludwig, 1910, for the Benthopectinidae; the Brisingida Fisher, 1928, for the Brisingidae; and the Zorocallida Downey, 1970, for the Zoroasteridae. In general, the terms of Perrier have dominated, although there is some use of the smaller ordinal units (e.g., Spencer and Wright, 1966; Blake, 1987; Clark and Downey, 1992).
Taxon diagnoses traditionally have emphasized characters visible in complete specimens, although features visible only through dissection have received some careful consideration (e.g., Agassiz, 1877; Viguier, 1879; Fisher, 1911). A goal of the present symposium was to draw attention to available but incompletely exploited sources of information that are available for taxonomic and phylogenetic analyses.
Differing interpretations of the phylogenetic significance of certain characters exist for all groups of organisms. Asteroids are particularly problematic because they bear many strikingly serialized ossicle systems that invite speculation on system origins. Too often, debate lacks an underlying set of principles for the evaluation of homologies; R. MOOI AND B. DAVID apply a recently developed system of echinoderm homologies (summarized in Mooi and David, 1997) to asteroid morphology. These authors here integrate developmental and phylogenetic perspectives in an evaluation of homologies of body wall elements, and focus on the mouth frame, a matter of enduring debate among students of stelleroids.
F. HOTCHKISS evaluates patterns of insertion of supernumerary arms in asteroids using the presumably fixed marker points of the anus and madreporite, and, more importantly, developmental patterns of the primary lobes in the rudiment. "Suckered" and "pointed" tube feet are a traditional guide to taxon affinities, but M. VICKERY demonstrates a greater diversity of form. Both approaches will shed light on the adaptive significance and functional morphology of asteroids. Further, the unexpected complexity of these very different characters indirectly draws attention to other available categories of characters. Jangoux (1982), for example, began documentation of the many differences in gut structure found among living asteroids. Although taxonomists traditionally have stressed overall skeletal relationships, some authors have emphasized the importance of form and occurrence of individual ossicle types. Viguier (1879) noted the value of the odontophore, an unpaired interbrachial ossicle located behind each pair of mouth angle ossicles, and McKnight (1975) suggested that odontophore presence is an apomorphy of asteroids. Further developing a theme of Fell (e.g., 1963), Kano (1995) underscored the usefulness of the superambulacral, an ossicle extending between the dorsal abradial surface of the ambulacral and lateral ossicles of some asteroids. Blake (1973) argued that individual ossicle types are potentially diagnostic at the species level. Ossicle form would seem to lend itself particularly well to modern computer imaging and morphometric techniques, which will provide not only taxonomic and phylogenetic insights, but also a key to understanding asteroid functional morphology.
|
PHYLOGENETIC INTERPRETATIONS |
|---|
|
TOP
SYNOPSISTHE NATURE OF THE...TAXA AND MORPHOLOGYPHYLOGENETIC INTERPRETATIONSREFERENCES |
|---|
Phylogenetic interpretation of recent asteroids is contentious, although species-level treatments are rare in the literature. Notable exceptions are the species-by-species reconstructions of the histories of Astropecten and Luidia by Döderlein (1917
, 1920). It would be valuable to begin modern cladistic analyses of these species-rich and widely distributed genera with Döderlein's careful summaries.
Papers by both Spencer (1914–1940) and Schuchert (1915) reflected deep-seated phylogenetic opinions, but because of the period in which they worked their approaches lacked a consistent, rigorous methodological perspective.
A vigorous debate on character significance and asteroid phylogeny took place in the pages of Nature during the early 1920s. The discussion centered on the issue of whether or not Astropecten and related genera could be considered primitive (the scope of taxa also was under discussion.) Principals were E. W. MacBride (1921; 1923a, b) and T. Mortensen (1922, 1923) but also involved were F. A. Bather (1921a, b; 1923), G. F. Gemmill (1923), and indirectly through their correspondence, other important asteroid systematists of the time, W. K. Fisher, H. L. Clark, R. Koehler, and L. Döderlein. The issue on taxon position was not resolved to the satisfaction of the debaters, and for his part, Mortensen simply wearied of the issue and went on to other topics (F. J. Madsen, 1983, personal commun. to DBB).
In a series of papers, H. B. Fell (e.g., 1963) argued that the living genus Platasterias is a surviving somasteroid and that Luidia is a surviving basal asteroid, which Fell assigned to the otherwise extinct order Platyasterida. Astropecten and the Paxillosida were the next step in Fell's phylogenetic scheme, a view in keeping with the earlier perspective of Mortensen. Fell's inference on the nature of Platasterias was quickly challenged (Madsen, 1966), and Blake (1982) argued in brief that recent asteroids (including Platasterias and Luidia) lack close affinities with Paleozoic genera, a view supported by Blake (1987) and Gale (1987).
Blake (1987) and Gale (1987) offered differing phylogenetic and classification schemes for asteroids. Blake was closer to the views of MacBride whereas Gale was more in accord with those of Mortensen. Based on molecular data, Lafay et al. (1995), Wada et al. (1996), and Littlewood et al. (1997) to varying extents supported the Mortensen-Gale perspective. Smith (1997, p. 345) found that "The current situation concerning asteroid phylogeny is extremely unsatisfactory," a view based on disagreements in interpretation of morphological characters and weak overall signals from then-available molecular data.
The treatments of Blake (1987) and Gale (1987) were at the familial and ordinal level. That is, selected genera and species were reviewed and character states generalized across the family. In essence, the families were treated as individuals for phylogenetic analysis. Molecular studies to date have generalized even more broadly. Luidia and Astropecten, for example, have been selected to stand for the Paxillosida, although the order is morphologically diverse. Many astropectinids (unlike Astropecten itself and Luidia) are found below wave base where different conditions might be expected to have influenced evolution (e.g., MacBride, 1921, 1923a, b; Blake, 1988). The Asterinidae is aligned with the spinulosacean families Echinasteridae and Solasteridae in the perspectives of Fisher (1911), Spencer and Wright (1966), Lafay et al. (1995) and Wada et al. (1996) but with the valvatacean families Goniasteridae and Ophidiasteridae in the views of Sladen (1889) and of Blake (1987). Asterina served as the proxy for the Valvatida in the analysis of Lafay et al. (1995) whereas Wada et al. (1996) slightly expanded the valvatidan sampling by including Pseudarchaster and Certonardoa, as well as Asterina. Generalizing across the family and ordinal levels has value in that such approaches can suggest major trends and identify areas of disagreement. Nevertheless published work now has carried this approach as far as it is useful to take it. Asteroid evolution is too complex, and there has been too much convergence to continue to use family- and ordinal-level taxa as individuals. It is now appropriate to proceed to at least generic and more preferably species level phylogenetic analyses.
In the present symposium, D. FOLTZ et al. comprehensively combine morphological and molecular approaches in an evaluation of a morphologically highly varied and complex genus, Leptasterias. In expansion of past approaches, C. MAH begins a morphological analysis of the Forcipulatacea at the genus level. F. HOTCHKISS clearly indicates phylogenetic data are available in the developmental sequence of arm addition in multiarmed asteroids. Finally, E. KNOTT AND G. WRAY significantly broaden taxonomic sampling and develop significant new perspectives in their molecular study of asteroid phylogeny.
The classification and phylogeny of asteroids has been contentious, yet the major impediment to understanding is lack of sufficient research attention. This symposium demonstrates that new and more detailed approaches can significantly broaden understanding of the history of this complex group of invertebrates.
|
FOOTNOTES |
|---|
1 From the Symposium Evolution of Starfishes: Morphology, Molecules, Development, and Paleobiology presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 6–10 January 1999, at Denver, Colorado.
|
REFERENCES |
|---|
|
TOP
SYNOPSISTHE NATURE OF THE...TAXA AND MORPHOLOGYPHYLOGENETIC INTERPRETATIONSREFERENCES |
|---|
Agassiz, A. 1877. North American starfishes. Mem. Mus. Comp. Zool, Harvard College, 5136p.
Bather, F.A. 1921. aEchinoderm larvae and their bearing on classification. Nature, 108:459-460.
Bather, F.A. 1921. b(comment on E. W. MacBride, Echinoderm larvae and their bearing on classification, Nature, 108:529–530.). Nature, 108:530.
Bather, F.A. 1923. Echinoderm larvae and their bearing on classification. Nature, 111:397.
Blake, D.B. 1973. Ossicle morphology of some Recent asteroids and description of some west American fossil asteroids. Univ. Calif. Pub.Geol. Sci, 104.
Blake, D.B. 1982. Somasteroidea, Asteroidea, and the affinities of Luidia (Platasterias) latiradiata. Palaeontology, 25:167-191.
Blake, D.B. 1987. A classification and phylogeny of post-Palaeozoic sea stars (Asteroidea: Echinodermata). J. Nat. Hist, 21:481-528.[CrossRef]
Blake, D.B. 1988. Paxillosidans are not primitive asteroids: A hypothesis based on functional considerations. In R.D. Burke, P.V. Mladenov, P. Lambert and R.L. Parsley (eds.)Echinoderm biol, p309-314A. A. Balkema, Rotterdam.
Clark, A.M., and M.E. Downey. 1992. Starfishes of the Atlantic. Chapman and Hall, London794p.
Döderlein, L. 1917. Die Asteriden der Siboga-Expedition 1: Die Gattung Astropecten und ihre Stammesgeschichte. Siboga-Expeditie Mono, 46:a1-191.
Döderlein, L. 1920. Die Asteriden der Siboga-Expedition 2: Die Gattung Luidia und ihre Stammesgeschichte. Siboga-Expeditie Mono, 46:b193-293.
Downey, M.E. 1970. Zorocallida, new order, and Doraster constellatus, new genus and species, with notes on the Zoroasteridae (Echinodermata: Asteroidea). Smith. Contrib. Zool, 64:18p.
Fell, H.B. 1963. The phylogeny of sea-stars. Phil. Trans. Roy. Soc., London, Ser. B, 246:386-435.
Fisher, W.K. 1911. Asteroidea of the North Pacific and adjacent waters. Bull. U.S. Nat. Mus, 76:420p.
Fisher, W.K. 1928. Asteroidea of the North Pacific and adjacent waters. 2. Forcipulata (Part). Bull. U. S. Nat. Mus, 76:267p.
Gale, A.S. 1987. Phylogeny and classification of the Asteroidea. Zool. J. Linn. Soc, 89:107-132.
Gemmill, J.F. 1923. (comment on E. W. MacBride, Echinoderm larvae and their bearing on classification. Nature 110:806–807.). Nature, 111:47-48.
Jangoux, M. 1982. Digestive systems: Asteroidea. In M. Jangoux and J.M. Lawrence (eds.)Echinoderm nutrition, pp235-272A. A. Balkema, Rotterdam.
Kano, Y.T. 1995. Systematic studies on the skeletal system of sea stars, with special reference to the superambulacral plate. Unpublished Ph.D. Diss., Niigata University, Japan191p.
Lafay, B., A.B. Smith, and R. Christen. 1995. A combined morphological and molecular approach to the phylogeny of asteroids (Asteroidea: Echinodermata). Syst. Biol, 44:190-208.[CrossRef]
Littlewood, D.T.J., A.B. Smith, K.A. Clough, and R.H. Emson. 1997. The interrelationships of the echinoderm classes: Morphological and molecular evidence. Biol. J. Linn. Soc, 61:409-438.[CrossRef]
Ludwig, H. 1910. Notomyota, eine neue Ordnung der Seesterne. Sber. Preuss. Akad. Wiss, 23:435-466.
MacBride, E.W. 1921. Echinoderm larvae and their bearing on evolution. Nature, 108:529-530.
MacBride, E.W. 1923. aEchinoderm larvae and their bearing on evolution. Nature, 111:47.
MacBride, E.W. 1923. b(comment on Th. Mortensen, Echinoderm larvae and their bearing on classification. Nature 111:322–333.). Nature, 111:323-324.
McKnight, D.G. 1975. Classification of somasteroids and asteroids (Asterozoa: Echinodermata). J. R. Soc. NZ, 5:13-19.
Madsen, F.J. 1966. The Recent sea-star Platasterias and the fossil Somasteroidea. Nature, 209:1367.[Medline]
Mooi, R., and B. David. 1997. Skeletal homologies of echinoderms. In J.A. Waters and C.G. Maples (eds.)Geobiology of echinoderms, pp305-335The Paleontological Society Papers, Vol3:.
Mortensen, T. 1922. Echinoderm larvae and their bearing on evolution. Nature, 110:806-807.
Mortensen, T. 1923. Echinoderm larvae and their bearing on evolution. Nature, 111:322-323.
Perrier, E. 1884. Mémoire sur les étoiles de mer recueillis dans la Mer des Antilles et la Golfe de Mexique. Nouv. Archs. Mus. Hist. Nat. Paris, 6:127-276.
Perrier, E. 1891. Echinodermes 1. Stellerides. Mission Scientifique du Cap Horn, 1882–1883, 6:Zool. (3)198p.
Perrier, E. 1894. Stellérides. Expéd. Scient. Travailleur-Talisman, 3:G. Masson, Paris431p.
Rowe, F.W.E. 1977. A new family of Asteroidea (Echinodermata), with the description of five new species and one new subspecies of Asterodiscides. Rec. Aust. Mus, 31:187-233.
Schuchert, C. 1915. Revision of Paleozoic Stelleroidea with special reference to North American Asteroidea. Bull. U.S. Nat. Mus, 88:312p.
Sladen, W.P. 1889. Report on the Asteroidea collected by the Challenger. Sci. Rpts. Challenger, Zool, 30:894p.
Smith, A.B. 1997. Echinoderm phylogeny: how congruent are morphological and molecular estimates. In J.A. Waters and C.G. Maples (eds.)Geobiology of echinoderms, pp337-355The Paleontological Society Papers, Vol3:.
Spencer, W.K. 1914–1940. British Palaeozoic Asterozoa. Palaeont. Soc. Lond., Mem, 540p.
Spencer, W.K. 1951. Early Palaeozoic starfishes. Phil. Trans. Roy. Soc., London, Ser. B, 235:87-129.
Spencer, W.K., and C.W. Wright. 1966. Asterozoans. In R. C. Moore (ed.)Treatise on Invertebrate Paleontology, Pt. U, Echinodermata 3. pp4-107UGeological Society of America and University of Kansas Press, Lawrence.
Viguier, C. 1879. Anatomie comparée du squelette des Stellérides. Archs. Zool. Exp. Gén, 7:33-250.
Wada, H., M. Komatsu, and N. Satoh. 1996. Mitochondrial rDNA phylogeny of the Asteroidea suggests the primitiveness of the Paxillosida. Mol. Phylogen. Evol, 6:97-106.[Medline]
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||