Chitonous Confusion

Lepidochitona cinerea, photographed by Rokus Groeneveld.

Chitons have been featured at this site once before, in a brief post on the family Ischnochitonidae. Today's post is focused on another chiton family, the Lepidochitonidae.

As noted in the earlier post, the field of chiton classification is a confusing place. The lepidochitonids were characterised by Kass & Van Belle (1985; under the name Lepidochitoninae, as a subfamily of the Ischnochitonidae) as chitons with slit-bearing valves, no extra-pigmentary eyes (i.e. no cuticular eyes outside the aesthetes, which are sensory canals in the valves), and a girdle that appears nude or has non-overlapping scales. Contrary to Kass & Van Belle's classification, however, the lepidochitonids do not appear to be immediately related to the ischnochitonids. Lepidochitonids have what are called abanal gills, in which new pairs of gills are only added during development in front of the first pair to develop (so the largest pair of gills is the furthest back), but ischnochitonids have adanal gills, in which new gill pairs are added both in front of and behind the original pair. The significance of this distinction has been corroborated by molecular analysis (Okusu et al. 2003). As for the composition of the Lepidochitonidae itself, Eernisse et al. (2007) referred two genera found in California, Cyanoplax and Nuttallina, to this family, but referred a third erstwhile lepidochitonine Tonicella to the family Mopaliidae, indicating the non-monophyly of the previously recognised grouping. In support of this, they cited in-progress molecular analyses. However, the detailed results of these analyses have yet to appear in print, so we are still unsure what the final face of the Lepidochitonidae will be.

Gould's baby chiton Cyanoplax dentiens, photographed by Gary McDonald.

The species of Cyanoplax are also of interest because of their varying reproductive habits. Four species in this genus studied by Eernisse (1998) are free spawners, releasing eggs into the water column where they hatch into free-swimming larvae that later settle and metamorphose elsewhere. Three other species, in contrast, are brooders, retaining their eggs to hatch at a later stage in development, bypassing the planktonic stage and reaching maturity close to their parent. Two of these brooding species, C. caverna and C. fernaldi, are also the only known examples of simultaneous hermaphrodites among chitons, seemingly able to fertilise their own eggs. As well as in larval development, brooding and free-spawning Cryptoplax species also differ in characters of the eggs, with the eggs of free-spawning species being more ornate than those of brooding species. This is of note as egg ornamentation has been suggested as a phylogenetically significant character in chitons; though this view has also been corroborated by molecular analysis (Okusu et al. 2003), the example of Cyanoplax recommends caution. The contrast between spawning vs brooding species in Cyanoplax also resembles situations found in other marine genera: starfish and annelid worms, for instance, each include examples of closely related yet developmentally distinct taxa.

REFERENCES

Eernisse, D. J. 1988. Reproductive patterns in six species of Lepidochitona (Mollusca: Polyplacophora) from the Pacific coast of North America. Biological Bulletin 174 (3): 287-302.

Eernisse, D. J., R. N. Clark & A. Draeger. 2007. Polyplacophora. In: J. T. Carlton (ed.) Light and Smith Manual: The Intertidal Invertebrates of Central California to Oregon, 4th ed., pp. 701-713. University of California Press: Berkeley.

Kaas, P., & R. A. Van Belle. 1985. Monograph of Living Chitons (Mollusca: Polyplacophora) vol. 2. Suborder Ischnochitonina. Ischnochitonidae: Schizoplacinae, Callochitoninae & Lepidochitoninae. E. J. Brill/Dr W. Backhuys.

Okusu, A., E. Schwabe, D. J. Eernisse & G. Giribet. 2003. Towards a phylogeny of chitons (Mollusca, Polyplacophora) based on combined analysis of five molecular loci. Organisms Diversity & Evolution 3: 281-302.