Chiton

Abstract: Most species of Polyplacophora are found in shallow, coastal water, and their distribution and ecology is generally well known. On the contrary, information related with distribution, biology and ecology of deep-sea chitons is scarce. Specimens of the deep-sea mollusks fauna were obtained during sampling operations of western Mexico (TALUD project) aimed at the study of invertebrate fauna occurring under the Oxygen Minimum Zone (OMZ). The material reported herein is part of the Polyplacophora fauna collected with a benthic sledge off the west coast of the Baja California Peninsula, in the eastern Pacific, Mexico. The deep-sea chiton Tripoplax cf. balaenophila Schwabe & Sellanes 2004 was collected in 530–625 m depth, in the following environmental conditions: dissolved oxygen, <0.25 ml/l; temperature, 6.44 °C; salinity, 34.47. It represents a range extension of 63° of latitude (ca 8280 km) to the north for this species previously known only from the type locality, off Concepcion, Chile. Tripoplax cf. balaenophila is associated with the lower boundary of the Minimum Oxygen Zone occurring off western Mexico, an habitat characterized by severe hypoxic conditions and colonized by well-adapted species that are generally abundant. Assignment of species to Tripoplax or Lepidozona is still a conflictive issue. The existing discrepancy in slit number and the assignment of “balaenophila” to Tripoplax cannot be solved without a more thorough review of both genera. The disjunct distribution of the genera Tripoplax in the eastern Pacific is also an interesting biogeographical issue.

Abstract: X-ray diffraction patterns show that the mature denticles of three extant chiton species are composed of the mineral lepidocrocite and an apatite mineral, probably francolite, in addition to magnetite. Each of the three minerals forms a discrete microarchitectural unit of the chiton denticles. This is the first indication that lepidocrocite is precipitated by marine organisms and an apatite mineral by chitons.

Abstract: Recent theory has suggested that in systems in which resource taxa compete, the abundance of a consumer specialized on one prey should be positively affected by the presence of a specialized consumer of the other prey. We have shown that the type of competition between prey (i.e., hierarchical vs. symmetrical) strongly affects the nature of the interaction between consumers. In the low intertidal zone on San Juan Island, Washington, two species of acmaeid limpets overlap substantially in habitat and food use with the large chiton Katharina tunicata. The limpets in this region consume microalgae, especially diatoms, while the chiton is a generalist, eating both microalgae and macroalgae. The experimental removal of Katharina from two large areas resulted (over 3 yr) in a dramatic rise in macroalgal cover and the disappearance of limpets. Artificially increased densities of Katharina at another site led to greater limpet abundances and increased reproductive output of both limpets and chitons. The probable ...

Abstract: In order to build а natural classification of the chitons, а new approach is proposed that uses not only the shells, as usual, but also other suitable features including aesthetes, girdle, radula, gills, glands, egg hull projections, spermatozoids etc. Several previous classifications are discussed. А brief review of the evolution of the Polyplacophora is given and а new classification of the chitons is proposed. The roles of the articulamentum and the reductions in the tegmentum in chitons are discussed. The evolutionary line of the reduction of slits is shown for the superfarnily Cryptoplacoidea. Specifically, the genera Heтiarthruт, Weedingia and Choriplax, which have unslitted valves, have Ьееп removed from the order Lepidopleurida and reassigned to the order Chitonida within the Cryptoplacoidea. Affinities of these and other genera within the Cryptoplacoidea are discussed.