I just thought that I'd show you something that I alluded to briefly nearly a year and a half ago in my post on Planctosphaera. This is the giant phoronid larva described by Temereva et al. (2006 - phoronids are small filter-feeding worms related to brachiopods), as illustrated in a figure from that paper:
For comparison, the animals to its right are more normal phoronid larvae (Actinotrocha is a form genus for such larvae, as it is not generally possible to identify a particular larva with its mature adult form). Phoronids are not the only marine animals for which such giant larvae have been found. If you've read the other post, you may recall that Planctosphaera was such an example. There's also the famed giant leptocephalus larvae, similar to the ten-centimetre (at most) leptocephalus larvae of eels or tarpons but reaching lengths of over six feet. Findings of giant larvae have lead to speculations about the existence of truly gigantic adults (particularly, it hardly needs saying, in the case of the leptocephali), but these adults remain as yet undiscovered. Many researchers suspect that giant larvae are not spawned from giant adults, but instead are pathological larvae of more normal-sized species that have failed to mature in the proper manner.
Even if the majority of giant larvae are merely abortive freaks, they are not without interesting implications for our understanding of evolution. Temereva et al.'s giant phoronid larva differed from other phoronid larvae in more than mere size. It also possessed a more fully developed circulatory system, as well as rudimentary gonads (which normally don't appear in phoronids until maturity). It takes little imagination to see the next step leading to a phoronid larva attaining full maturity while maintaining its larval form. It would not be the first known case - in 1928, Heath described Graffizoon lobatum, an animal very similar to the larva of a polyclad flatworm except for its possession of fully-developed gonads (as a reminder of our lack of familiarity with marine life, Graffizoon does not seem to have been recorded since).
For comparison, this is what adult phoronids look like (Phoronopsis viridis, from UCMP):
How difficult would it be to recognise the relationship between animals potentially only separated by a single generation?
REFERENCES
Heath, H. 1928. A sexually mature turbellarian resembling Müller's larva. Journal of Morphology and Physiology 45 (1): 187-207.
Temereva, E. N., V. V. Malakhov & A. N. Chernyshev. 2006. Giant actinotroch, a larva of Phoronida from the South China Sea: the giant larva phenomenon. Doklady Akademii Nauk 410 (5): 712-715 (transl. Doklady Biological Sciences 410: 410-413).
Just a brief post - the air-conditioning in our office hasn't been working these past two days, I'm currently sitting in thirty degree-plus heat, and consequently I simply haven't the mental strength to compose anything more. Weather in Perth is evil.
ReplyDeleteIf it's any consolation to you, my beard froze while I walked to uni around noon today.
Does the entire Animal kingdom descend from something that had distinct larval/adult life stages? Is this seen as a pure accident of history, or is it somehow essential? Have any reptile or mammal species retained or resurrected the practice?
ReplyDeleteE.g. do fetal marsupials qualify, technically? I don't mean analogous stages, but actual ontological identity. Indeed, do fish larvae really qualify on that basis?
Non-bilaterian animal groups (sponges, cnidarians) have separate larval stages, but these are different to and simpler than those of bilaterians. It has been suggested (but still not well established) that bilaterians as a whole arose from cnidarian-like ancestors through neoteny of the planula larva.
ReplyDeleteWithin the bilaterian clade, it's something of an open question whether indirect (separate larva) or direct (no larva) development is ancestral. Many lophotrochozoans, hemichordates and echinoderms have what is called a "type A" larva, with ciliated bands in the larva and a complete morphological revamp between larva and adult. However, ecdysozoans and chordates lack type A larvae, and their "larval" forms (if present) are much more similar to the adults. So to answer the second part, fish larvae are not larvae in the way that phoronid larvae are. I suspect that whether new-born marsupials are comparable to the larval stage of fishes would depend to a certain extent on your interpretation - marsupial development is largely comparable to placental development, but with the young ejected from the womb at an earlier stage.
Thanks, Chris. That's a lot to digest. Bilaterans, you and I in particular, then, may actually still be larvae, having failed to pupate and metamorphose into a respectably sessile adult form.
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