Polyplacophora
From Palaeos
| MOLLUSCA | |
| Taxonomy | Phylogeny |
o Eutrochozoa `--o MOLLUSCA `-+--Solenogastres `--+--Caudofoveata `--o Testaria `--+--o Polyplacophora `--o Conchifera |--Tryblidiida |?-- Stenothecoida `--+?--Bivalvia (if molecular phylogeny) `==Helcionelloida (paraphyletic) | |--+-- †Yochelcionellidae | | `?-- Cephalopoda | |? -- †Criconarida | `-+?--Bivalvia (if transitional fossils) | `-- †Rostroconchia | `-- Scaphopoda `--+--†Paragastropoda `-- Gastropoda | |
Polyplacophorans, or chitons, are a class of persistently primitive marine molluscs commonly occurring on rocks and seaweed in the intertidal zone. They have also been found at depths down to 5,000 meters. There are about 600 recent species.
Chitons do not vary much in form. All have eight shell plates (although a Palaeozoic lineage, the Septemchitonida, had seven), are generally oval in outline and have a flattened body, usually with the greatest thickness of about one-fourth their greatest width.
The chiton shell normally consists of eight, articulating calcareous valves with the joints between them running perpendicular to the axis of the body. These valves are constructed in such a way that the animal can often roll up when disturbed or removed from the rock. The parts of the shell are shown here.
drawing © xxxx by G. J. Browning from Marine Molluscs of Victoria
There is a creeping foot, a primitive feature in molluscs, and the radula is of the "docoglossan" type, like limpets (Patellogastropoda) and Tryblidiida. The radula are mineralized with magnetite (a compound of iron). The head is poorly developed, and visible only from the ventral side (again like the Tryblidiida). A band of muscular tissue, the girdle, runs along the dorsal periphery of the animal (see illustration above). The girdle commonly has embedded in it small calcareous spines, scales or spicules, not unlike the spicules of the very primitive and wormlike Caudofoveata and Solenogastres (Aplacophora). Indeed, at one time the Aplacophora and chitons were grouped in the same class, the Amphineura. However it is now known that there are many differences between these types of primitive molluscs, despite the shared primitive features (symplesiomorphies) that unite them.
Chitons lack normal molluscan sense organs like statocysts, cephalic eyes, and tentacles. The chief sense organs are a subradular organ and the "aesthetes". These latter, which among living molluscs are unique to chitons, are special mantle shells found within tiny vertical canals in the upper tegmentum (upper shell layer) [Barnes 1980 p.386]. Interestingly, very similar canals are found in the shell of Tryblidium reticulatum (Silurian period, family Tryblidiidae, Tryblidiida) [Knight and Yochelson 1960 p.77], although they are absent in the recent Neopilina (presumably lost when the animal migrated to deep water). The aesthetes are supplemented by tactile and photoreceptor cells in the mantle and girdle. The animal is thus able to detect light, which it responds negatively to. These shy creatures only become active at night, when they creep over rocks scraping algae and other microscopic organisms off the surface with their radula. Like limpets, the animal will often return to its resting place for the day.
Again as with the Tryblidiida, with which the chitons would seem to be related, the animal is metameric or pseudometameric, with repeated gills and shell muscles. This led to the view that these animals (and thus the Mollusca as a whole) evolved from a segmented annelid-like ancestor. However, analysis of chiton embryo, larval, and postlarval development and metamorphosis shows that the "segmented" or serial nature of the polyplacophoran shell musculature is a secondary condition, and an unsegmented ancestor at the base of molluscan evolution is more likely [Wanninger and Haszprunar 2001].
Fossil chiton specimens are almost always loose, disarticulated valves. They have been found in rocks of the late Cambrian onwards, but are rare, probably because chitons tend to occupy environments where the possibility of preservation is not high. There are about three extinct orders, mostly Palaeozoic.
Phylogeny
<==Polyplacophora [Placophora] | i. s.: Heloplax | Enetoplax | Arctoplax | Acanthochiton fascicularis | Septemchiton | Cobcrephora Bischoff 1981 | Nuttallina californica | Sypharochiton pelliserpentis | Lepidochitona | |--L. cavernae | `--L. corrugata | Acanthopleura japonica |--Matthevia [Mattheva, Matthevina] |--+--+--Glaphurochiton | | `--+--Lepidopleurinae [Lepidopleurida, Lepidopleuridae, Lepidopleurina] | | | |--Lepidopleurus Risso 1826 | | | | |--L. cajetanus | | | | |--L. cancellatus | | | | `--L. intermedius Salvini-Plawen 1968 | | | `--Leptochiton asellus | | `--+--Callochiton septemvalvis | | `--+--+--Cryptoplax japonica | | | `--Acanthochitona [Acanthochitonina] | | | |--A. crinita | | | `--A. garnoti | | `--+--+--Tonicella lineata | | | `--+--Mopalia muscosa | | | `--Cryptochiton stelleri | | `--+--+--Chaetopleura apiculata | | | `--Callistochiton antiquus | | `--+--Ischnochitonina | | | |--Stenoplax heathiana | | | `--Ischnochiton comptus | | `--+--Lorica volvox | | `--Chiton | | |--C. olivaceus | | `--C. tulipa | `--+--Echinochiton dufoei | `--Multiplacophora [Hercolepadida] | |--Polysacos Vendrasco, Wood & Runnegar 2004 | | `--*P. vickersianum Vendrasco, Wood & Runnegar 2004 | |--Strobilepis Clarke 1888 | | `--S. spinigera Clarke 1888 | |--Diadeloplax paragrapsima | |--Protobalanus hamiltonensis | |--Hercolepas signata | `--Aenigmatectus `--Chelodes
* Type species of genus indicated
References
Castro, P., & M. E. Huber. 1997. Marine Biology, 2nd ed. WCB McGraw-Hill: Boston.
Giribet, G., D. L. Distel, M. Polz, W. Sterrer & W. C. Wheeler. 2000. Triploblastic relationships with emphasis on the acoelomates and the position of Gnathostomulida, Cycliophora, Plathelminthes, and Chaetognatha: A combined approach of 18S rDNA sequences and morphology. Systematic Biology 49: 539-562.
Giribet, G., & C. S. Carranza. 1999. Point Counter Point. What can 18S rDNA do for bivalve phylogeny? Journal of Molecular Evolution 48: 256-258.
Giribet, G., A. Okusu, A. R. Lindgren, S. W. Huff, M. Schrödl & M. K. Nishiguchi. 2006. Evidence for a clade composed of molluscs with serially repeated structures: Monoplacophorans are related to chitons. Proceedings of the National Academy of Sciences of the USA 103 (20): 7723-7728.
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.
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.
Monniot, F. 1962. Recherches sur les graviers a Amphioxus de la région de Banyuls-sur-Mer. Vie et Milieu 13: 231-322.
Salvini-Plawen, L. v. 1986. Lower Mollusca. In Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) (L. Botosaneanu, ed.) pp. 148-152. E. J. Brill / Dr. W. Backhuys: Leiden.
Sutton, M. D., D. E. G. Briggs, D. J. Siveter & D. J. Siveter. 2001. An exceptionally preserved vermiform mollusc from the Silurian of England. Nature 410: 461-463.
Sutton, M. D., L E. Holmer & L Cherns. 2001. Small problematic phosphatic sclerites from the Ordovician of Iapetus. Journal of Paleontology 75 (1): 1-8.
Vendrasco, M. J., T. E. Wood & B. N. Runnegar. 2004. Articulated Palaeozoic fossil with 17 plates greatly expands disparity of early chitons. Nature 429: 288-291.
Credits
MAK020929; phylogeny CKT071209
