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Take a close look at the photo above. What kind of animal do you think this is? [The photo comes from here, but don't look there just yet, because that would be cheating.]
Some of the more observant among you may have noticed the five rays visible on the animal, and so you would have correctly decided that this is an echinoderm, seen from the underside. Echinoderms are the phylum of marine animals that includes crinoids (sea lilies and feather stars), asteroids (sea stars or starfish), ophiuroids (brittle stars), echinoids (sea urchins) and holothuroids (sea cucumbers). The five rays are the ambulacra - furrows lined with the tube feet that the echinoderm uses for walking on, or for passing food particles to the central mouth. Before I reveal exactly what kind of echinoderm this is, though, I'll show you another photo of the same specimen (from the same site) seen from the side:
By now, it should be pretty obvious which of the five living classes of echinoderms this is. So if you guessed "starfish"* - you're absolutely right. This specimen is, in fact, the type specimen of Podosphaeraster toyoshiomaruae Fujita & Rowe, 2002. Podosphaeraster is an extremely unusual asteroid known from the western Pacific and north-east Atlantic that has abandoned the typical star-shape of most members of its class, and adopted a near-spherical form much more similar to that of an echinoid. If you were to look closely at the specimen, you would be able to see a difference from a typical echinoid in that the ambulacral furrows only go halfway up the side of the sphere, rather than all the way up as in echinoids.
*Kevin Zelnio is going to kill me for calling it a starfish instead of a sea star. Tough.
The way in which Podosphaeraster has evolved its unusual form is relatively simple. The development of the plates that normally form the dorsal (aboral) surface of the flattened star has been greatly reduced relative to those that form the ventral (oral) surface. The reasons why this unusual morphology has evolved in Podosphaeraster, however, are unknown. Though five species have been described to date, specimens of Podosphaeraster are few and far between. All species are small (the largest specimens are little over a centimetre in diameter) and there is evidence that they live in habitats that are not conducive to easy collecting - among sponges or rocky ground in depths of 85 - 615 m. It may be adapted to living in cracks or crevices in these habitats.
For all its unusualness, Podosphaeraster is not unique. A fossil family of asteroids, the Sphaerasteridae, also developed a similar globose form by the reduction of the aboral surface. Also, Smith (1997) suggested that echinoids could have also evolved from a star-like ancestor in just this way. If true, what might seem an interesting but inconsequential oddity in the asteroid world could actually be very significant in understanding how another of the major modern animal groups came into being.
REFERENCES
Fujita, T., & F. W. E. Rowe. 2002. Podosphaerasteridae fam. nov. (Echinodermata: Asteroidea: Valvatida), with a new species, Podosphaeraster toyoshiomaruae, from southern Japan. Species Diversity 7: 317-332.
Smith, A. B. 1997. Echinoderm larvae and phylogeny. Annual Review of Ecology and Systematics 28: 219-241.
Interesting about the ancestral echinoderms possibly looking like starfish (or sea stars, whatever). But is there any reason to favor that idea, or is it simply a possibility.
ReplyDeleteThis is also interesting, from Smith's (1997) abstract:
[L]arval morphology displays more homoplasy than adult morphology, and ... early developmental patterns are remarkably flexible.
Kind of topsy-turvy--ontogeny inverts phylogeny! Once again, echinoderms show themselves to be some of the weirdest organisms on the planet.
See, I would have said "sand dollar," although I have no idea what a living sand dollar looks like.
ReplyDeleteWas I looking at a living sand dollar?
I was gonna guess it was a sea urchin.
ReplyDeleteA living sand dollar doesn't look too much different from a dead sand dollar. Unlike the long erect spines of more classic sea urchins, sand dollar spines are really tiny and laid flat, and instead of totally changing the appearance of the living animal they just give it a furry appearance (though not a furry texture). Besides, sand dollars are dorsoventrally flattened in side view, while Podosphaeraster, like Mickey Mouse's ears, is round whichever direction you look at it from.
ReplyDeleteInteresting about the ancestral echinoderms possibly looking like starfish (or sea stars, whatever). But is there any reason to favor that idea, or is it simply a possibility.
Not direct evidence, as far as I know. Among living echinoderms, some authors (including, I believe, Smith) have regarded the brittle stars as more closely related to the Echinoidea + Holothuroidea (Echinozoa) clade than to the asteroids based on larval morphology, but I think the supporters of an Asteroidea + Ophiuroidea clade currently hold the fort. [The similarities in larvae between ophiuroids and echinozoans may be convergences, or they may simply be plesiomorphies - the outgroup, crinoids, are so different from all other echinoderms that they are effectively uninformative in this regard).
The fossil edrioasteroids, which probably include the ancestors of the modern non-crinoid echinoderms, were stout, sessile animals with a star-like pattern on the dorsal feeding surface, and it has been suggested that the free-living echinoderms evolved from edrioasteroids through loss of the sessile attachment, flattening of the theca and inversion of the life orientation so that the oral surface became ventral. This would probably give rise to a somewhat starfish-like animal. However, while I think this model does currently hold the floor without any real contenders, I don't know if there's actually any direct fossil evidence to support it. And in the end, as we have seen in so many other cases in biology, plausability does not necessarily guarantee accuracy.
I just don't get how you, of all people, can sit here and espouse paraphyletic nomenclature! I'm disappointed. Maybe this is an issue we need to raise on the Taxacom listserve.
ReplyDeleteShouldn't that be polyphyletic nomenclature? I'm afraid, though, that you're facing something of an insurmountable obstacle - "starfish" just sounds better to say than "sea star".
ReplyDeleteSeriously, though, it's exactly because of this sort of thing that we distinguish between biological nomenclature and vernacular nomenclature. Biological nomenclature follows rules, vernacular nomenclature does not.
Well, I know this is all a bit tongue-in-cheek, but, I'm not so sure scientific nomenclature can claim superiority over vernacular nomenclature here. I mean, calling an asteroid echinoderm a "starfish" is surely no worse than calling a genus of stem-whale "Basilosaurus", right?
ReplyDeleteYou're right about having rules, of course (although none of these apply to Asteroidea yet--but they will once the PhyloCode goes into effect). But as for the names themselves go, both types of nomenclature are open to arbitrariness, inaccuracy, etc. There's no way around it except to realize that a name is just a name. And I think most people realize this. They know that starfish, crayfish, silverfish, etc. aren't fish--they just have "fish" in their names.
Chris, that is the second time you have corrected me lol. thanks - polyphyletic...
ReplyDeleteMike said "But as for the names themselves go, both types of nomenclature are open to arbitrariness, inaccuracy, etc."
As someone who works with the public often enough to know the misconceptions people have about sea life, I certainly beg to differ. Arbitrariness does not equal inaccuracy, so lets strive to be accurate with our arbitrary boundaries. A name conveys a definition. While I dream of the day when the general public can call out the linnaean adjectives and nouns that more accurately describe a given organism, I realize there is a certain utility to vernacular nomenclature.
The definition of fish of very different from that of an echinoderm. The star part of the name is a noun that describes a defining morphology trait for the taxon. By adding fish, one is suggesting that the definition of fish applies to the organism in question, which is blatantly false.
I think I need to write up my arguments in a paper. If I do that one day, I should like you and Mike to critique it for me :)
A name conveys a definition.
ReplyDeleteI have to disagree. A name may be associated with a definition, but if it is forced to convey a definition then 1) nomenclature would become very unstable, and 2) what would be the point of definitions, anyway?
I mean, I'm all for striving to get a close match between etymology and definition, but I don't really think these names should be thrown out:
Agouti (not an agouti)
Ambrosia (inedible)
Basilosaurus (not a lizard, or even a sauropsid, but a stem-whale)
Beipiaosaurus inexpectatus (totally predicted by phylogenetic bracketing)
Fossa (not a fossa)
Gymnosperma (an angiosperm)
Mustela africana (South American)
Oviraptor (the eggs it was supposedly "stealing" were its own)
Pan troglodytes (does not live in caves)
Pongo pygmaeus (is actually quite large)
Puffinus (not a puffin)
Tetrapoda (includes snakes, glass lizards, sirens, sirenians, cetaceans, etc.)
more here, toward the bottom
So scientific nomenclature is no less guilty than vernacular literature here. (It does beat it on precision, though--or at least phylogenetic nomenclature does.) I don't think a name can be required to reflect a definition (although it's nice when it does). I mean, what else are we going to call "silverfish"? And why do we drive in a parkway but park in a driveway, etc.?
To bring it back to the subject at hand, Podosphaeroaster is certainly not star-shaped! (In fact , the name is kind of an oxymoron.)
If I do that one day, I should like you and Mike to critique it for me :)
Be glad to!
(For the record, I have nothing against the term "sea star" and wish it all the best, but old habits die hard, and I don't find myself using it naturally.)
Actually, Mike, you've touched on a point that I've had problems with in the past, though it's not entirely connected to what Kevin and I are arguing about. I should add that you obviously agree with what I'm about to say, but I have come across students (and even researchers) who don't realise it. Once a formal biological name is coined, it exists independently of its etymology. The etymology may be helpful in remembering things about the taxon, and it may have historical interest, but it doesn't dictate the correctness of the name. This is why I personally get annoyed when a paper coins a replacement name for a higher taxon because it is "inappropriate". I don't think we should give a rat's that, for instance, Ornithopoda have feet that are less bird-like than Theropoda. You can think of formal names in this sense like personal names - I could still be called Christopher were I not a Christian, and my partner's sister Sarah is not required to be permanently cheerful.
ReplyDeleteThe case of vernacular names is a little different in this regard, but Kevin is making me really think about this. Do people really associate starfish with fish? Personally, I would say that I think of "starfish" as a term in its own right, independent of its origin - they're "starfish", not "star fish". Similarly, "jellyfish" is a different term from "fish made of jelly". But am I typical in this regard? Are the majority of the general public making associations that I'm not?
This is why I personally get annoyed when a paper coins a replacement name for a higher taxon because it is "inappropriate".
ReplyDeleteWell, to be fair, in zoology nobody regulates taxa above family level (...yet). But, yes, I agree.
I could still be called Christopher were I not a Christian
Good point! I'm an atheist, yet my given names mean "honoring Dyeus" and "next to 'El" (both chief gods later synonymized with the concept of "God").
Personally, I would say that I think of "starfish" as a term in its own right, independent of its origin - they're "starfish", not "star fish".... But am I typical in this regard?
I'm in the same boat here. To me, having "fish" in the name of "starfish", "jellyfish", "shellfish" (itself an odd category), "silverfish" (a land animal!), "crayfish" (which is actually a back-formation from French "écrevisse"), etc. doesn't mean that they're fish any more than a "water bear" is a bear or a "flying fox" is a fox. But is that how the general public sees it? I have to admit I'm not really sure.
arse! I wrote a nice long comment clicked publish and something about a duplication error. wtf!? i only clicked once. anyways the comment was send to blogger limbo and I'm not going to try to rewrite it all. grrr
ReplyDeletethe short: the public doesn't always get it right. I worked for monterey bay aquarium and people thought penguins were fish.
conclusion: there is no disincentive to being accurate, so scientists should start using unambiguous terminology. public gets info about world from scientists (or the bible in which they are hopeless), hence they will use the terminology that we use (eventually)
that was much more succinct, minus my drawn out stories.
Well, in general I agree, but there has to be some kind of line somewhere. For example, nobody considers sea horses to be actual horses (do they?) or silverfish to be actual fish (right?). We don't need to rename those, even if "sea star" instead of "starfish" or "koala" instead of "koala bear" might be good ideas.
ReplyDeletethere's a lot of stuff in this blog post that is actually not fully correct. Podospheraster is something I know about... I'll have to write about it one of these days...
ReplyDeletethere's a lot of stuff in this blog post that is actually not fully correct.
ReplyDeleteWell, don't be a tease - how have I offended? :-)