Ischnochitonidae

Ischnochiton australis, copyright Nuytsia@Tas.

Belongs within: Ischnochitonina.
Contains: Stenoplax (Stenoradsia), Ischnochiton, Lepidozona.

The Ischnochitonidae are a group of chitons in which the dorsum of the intermediate valves is divided into distinct lateral and central areas, often by a diagonal rib (Smith 1960).

With plate and girdle
Published 25 January 2010
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).

Chitonous confusion
Published 26 February 2013
Lepidochitona cinerea, photographed by Rokus Groeneveld.

As noted above, 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.

Systematics of Ischnochitonidae

Characters (from Smith 1960): Ovate to elongate chitons of variable size. Tegmentum of intermediate valves divided into lateral and central areas by diagonal rib (commonly indistinct) extending from apex to anterior outer angle of valves on each side. Articulamentum of head and tail valves multiple-slitted, that of intermediate valves with single slit or in some groups with two or more slits on each side. Teeth sharp-edged, not pectinated, grooved, or buttressed on outside. Eaves not porous. Sutural laminae sharp and well developed.

<==Ischnochitonidae
    |--Schizoplax Dall 1878S60 [Schizoplacidae, SchizoplacinaeBRW98]
    |    |--*S. brandtii (Middendorff 1846) [=Chiton brandtii]S60
    |    `--S. multicolor Dall 1920O27
    |--IschnochitoninaeBRW98
    |    |--IschnochitonSF77
    |    |--LepidozonaBRW98
    |    `--Stenochiton Adams & Angas 1864BRW98, S60 [incl. Zostericola Ashby 1919S60]
    |         |--*S. juloides Adams & Angas 1864 [=Stenoplax (*Stenochiton) juloides]S60
    |         |--S. cymodocealisBRW98
    |         `--S. longicymbaBRW98
    `--Lepidochitoninae [Lepidochitonidae, Trachyderminae]BRW98
         |--Particulazona milneiBRW98
         |--Subterenochiton Iredale & Hull 1924BRW98, S60 [Subterenochitonidae]
         |    |--*S. gabrieli (Hull 1912) [=Ischnochiton gabrieli]S60
         |    `--S. bednalliBRW98
         |--Paricoplax Iredale & Hull 1929P61
         |    |--*P. crocina (Reeve 1847) [=Chiton crocinus]P61
         |    |--P. mortenseni (Odhner 1924)P61
         |    |--P. perscrutanda Iredale & Hull 1929P61
         |    `--P. profundior Dell 1956P61
         |--Icoplax Thiele 1893P61
         |    |--*I. punicea (Gould 1846) [=Chiton puniceus]P61
         |    |--I. chathamensis Dell 1960P61
         |    |--I. empleura (Hutton 1872)P61 [=Callochiton empleurusF27]
         |    |--I. kapitiensis Mestayer 1926P61 [=Callochiton kapitiensisF27]
         |    |--I. subeudoxa Iredale & Hull 1930P61
         |    `--I. sulculata (Suter 1907)P61 [=Callochiton sulculatusF27]
         `--Lepidochitona Gray 1821 (see below for synonymy)S60
              |--L. cinerea (Linné 1767) (see below for synonymy)S60
              |--L. alba Linnaeus 1767O27
              |--L. aleutica (Dall 1878) [=Trachyradsia aleutica, L. (Spongioradsia) aleutica]O27
              |--L. berryanaBRW98
              |--L. cavernaeGC99
              |--L. corrugataGR98
              |--L. dentiens (Gould 1846)O27 [=Chiton (Onithochiton) dentiensO27, C64, Trachydermon dentiensC64]
              |--L. fernaldiBRW98
              |--L. flectens (Carpenter 1864)O27 [=Ischnochiton (Trachydermon) flectensC64, O27]
              |--‘Trachydermon’ hartwegii Cpr. 1855C64
              |--‘Trachydermon’ interstinctusC64
              |--L. monterosatoiBRW98
              |--‘Trachydermon’ nuttallii Cpr. 1855C64
              |--‘Trachydermon’ pseudodentiens Carpenter 1864C64
              |--L. sharpei (Pilsbry 1896) [=Trachydermon sharpii]O27
              `--L. thomasiBRW98

Ischnochitonidae incertae sedis:
  Oligochiton Berry 1922S60
    `--*O. lioplax Berry 1922S60
  Tonicina Thiele 1906S60
    `--*T. zschaui (Pfeffer 1886) [=Chiton zschaui]S60
  Basiliochiton Berry 1918 [incl. Lophochiton Berry 1925 non Ashby 1923, Ploiochiton Berry 1926]S60
    |--*B. heathii (Pilsbry 1898) [=Mopalia heathii]S60
    `--B. lobium Berry 1925O27
  Gurjanovillia Jakoleva 1952F77
    |--*G. albrechti (Schrenck 1867) [=Chiton albrechti]F77
    `--G. lindberghi Jakovleva 1952F77
  Cyanoplax Pilsbry 1892 [incl. Mopaliella Thiele 1909, Mopaliopsis Thiele 1893]S60
    |--*C. hartwegii (Carpenter 1855)S60 (see below for synonymy)
    |    |--C. h. hartwegiiO27
    |    `--C. h. nuttalli (Carpenter 1855) [=Chiton nuttalli, Lepidochitona (Cyanoplax) hartwegii nuttalli]O27
    |--C. fackenthallae Berry 1919 [=Lepidochitona (Cyanoplax) fackenthallae]O27
    |--C. lowei (Pilsbry 1918) [=Trachydermon lowei, Lepidochitona (Cyanoplax) lowei]O27
    `--C. raymondi (Pilsbry 1894) [=Trachydermon (Cyanoplax) raymondi, Lepidochitona (C.) raymondi]O27
  Stenoplax Carpenter in Dall 1879S60
    |  i. s.: S. circumsenta Berry 1956PP78
    |         S. heathianaGW02
    |--S. (Stenoplax)S60
    |    |--*S. (S.) limaciformis (Sowerby 1832) [=Chiton limaciformis]S60
    |    |--S. (S.) biarcuata (Dall 1903) [=Ischnochiton (Stenoplax) biarcuata]O27
    |    |--S. (S.) conspicua (Carpenter in Pilsbry 1892)S60, O27 [=Ischnochiton (Stenoradsia) conspicuusO27]
    |    `--S. (S.) fallax (Carpenter 1892) [=Ischnochiton (Stenoplax) fallax]O27
    `--S. (Stenoradsia)S60

*Cyanoplax hartwegii (Carpenter 1855)S60 [=Chiton hartwegiiS60, Ischnochiton hartwegiiC64, Lepidochitona (Cyanoplax) hartwegiiO27]

Lepidochitona Gray 1821 [=Lepidochiton Thiele 1928 non Carpenter 1857, Leptochitona (l. c.); incl. Adriella Thiele 1893, Craspedochilus Sars 1878, Spongioradsia Pilsbry 1894, Trachydermon Carpenter 1864]S60

Lepidochitona (Lepidochitona) cinerea (Linné 1767) [=Chiton cinereus; incl. C. marginatus Pennant 1777, *Lepidochitona marginata]S60

*Type species of generic name indicated

References

[BRW98] Beesley, P. L., G. J. B. Ross & A. Wells (eds) 1998. Fauna of Australia vol. 5. Mollusca: The Southern Synthesis. Part A. Australian Biological Resources Study: Canberra.

[C64] Carpenter, P. P. 1864. Supplementary report on the present state of our knowledge with regard to the Mollusca of the west coast of North America. Report of the British Association for the Advancement of Science 33: 517–686.

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.

[F77] Ferreira, A. J. 1977. A new species of chiton from the Aleutian Islands (Mollusca: Polyplacophora). Veliger 20 (1): 27–29.

[F27] Finlay, H. J. 1927. A further commentary on New Zealand molluscan systematics. Transactions and Proceedings of the New Zealand Institute 57: 320–485.

[GC99] Giribet, G., & C. S. Carranza. 1999. Point Counter Point. What can 18S rDNA do for bivalve phylogeny? Journal of Molecular Evolution 48: 256–258.

[GR98] Giribet, G., & C. Ribera. 1998. The position of arthropods in the animal kingdom: a search for a reliable outgroup for internal arthropod phylogeny. Molecular Phylogenetics and Evolution 9: 481–488.

[GW02] Giribet, G., & W. Wheeler. 2002. On bivalve phylogeny: a high-level analysis of the Bivalvia (Mollusca) based on combined morphology and DNA sequence data. Invertebrate Biology 121 (4): 271–324.

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.

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.

[O27] Oldroyd, I. S. 1927. The Marine Shells of the West Coast of North America vol. 2 pt 3. Stanford University Press: Stanford University (California).

[PP78] Poorman, F. L., & L. H. Poorman. 1978. Additional molluscan records from Bahía de Los Angeles, Baja California Norte. Veliger 20 (4): 369–374.

[P61] Powell, A. W. B. 1961. Shells of New Zealand: An illustrated handbook 4th ed. Whitcombe and Tombs Limited: Christchurch.

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.

[S60] Smith, A. G. 1960. Amphineura. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt I. Mollusca 1: Mollusca—General Features, Scaphopoda, Amphineura, Monoplacophora, Gastropoda—General Features, Archaeogastropoda and some (mainly Paleozoic) Caenogastropoda and Opisthobranchia pp. I41–I76. Geological Society of America, and University of Kansas Press.

[SF77] Smith, A. G., & A. J. Ferreira. 1977. Chiton fauna of the Galápagos Islands. Veliger 20 (2): 82–97.

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.

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