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.

With Plate and Girdle (Taxon of the Week: Ischnochitonidae)

Some chitons are surprisingly colorful. This is Ischnochiton virgatus from southern Australia, as photographed by Leon Altoff.

In the familiarity stakes, chitons occupy the fourth spot among the generally-recognised mollusc classes after gastropods, bivalves and cephalopods (which is a little unfair as there are more living chiton than cephalopod species). All living chitons share a similar general morphology with a central linear series of eight overlapping shell valves, surrounded by a fleshy girdle that (depending on species) may or may not be covered with small spicules. Like limpets, chitons live attached to marine rocks where they graze on algae. Go down to any rocky beach and you'll have no difficulty finding them in large numbers.


A bright blue example of Lepidozona radians, photographed by Ron Wolf.

The classification of chitons has been shifted around a bit in recent years. Earlier systems seem to have been based primarily on the morphology of the shell valves but more recent classifications have also looked at other features such as the soft anatomy (the most commonly cited reference seems to be Sirenko, 2006, but unfortunately I don't have access to it at present). The family Ischnochitonidae as recognised by Kaas & Van Belle (1985, 1987), for instance, has been divided into a number of families; Sirenko's Ischnochitonidae appears to be roughly equivalent to Kaas & Van Belle's subfamily Ischnochitoninae. Phylogenetic studies indicate that Ischnochitonidae in the older sense is para- or polyphyletic (for instance, in the tree of Wilson et al., 2010).


Stenoplax conspicua is a large ischnochitonid, growing up to about ten centimetres long. Photo by J. P. McKenna.

Kaas & Van Belle (1985, 1987) distinguished the Ischnochitoninae from other chitons by the lack of comb-like projections on the insertion plates (lateral extensions of the valves that anchor the valve in the surrounding girdle), slits present on the insertion plates but not corresponding in number or position to the radial ribs of the valve, and with smooth, sharp teeth on the insertion plates that are not thickened at the edges of the slits. The girdle is covered with scales. Ischnochitonids can be found at all depths, from the shoreline to the deep sea - Stenosemus chiversi has been recorded at a depth of over 4500 m (Schwabe, 2008).

REFERENCES

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.

Kaas, P., & R. A. Van Belle. 1987. Monograph of Living Chitons (Mollusca: Polyplacophora) vol. 3. Suborder Ischnochitonina. Ischnochitonidae: Chaetopleurinae, & Ischnochitoninae (pars). Additions to Vols 1 & 2. E. J. Brill/Dr W. Backhuys.

Schwabe, E. 2008. A summary of reports of abyssal and hadal Monoplacophora and Polyplacophora (Mollusca). Zootaxa 1866: 205-222.

Sirenko, B. I. 2006. New outlook on the system of chitons (Mollusca: Polyplacophora). Venus 65 (1-2): 27–49.

Wilson, N. G., G. W. Rouse & G. Giribet. 2010. Assessing the molluscan hypothesis Serialia (Monoplacophora + Polyplacophora) using novel molecular data. Molecular Phylogenetics and Evolution 54 (1): 187-193.