The organisation of the Placophora is compared with that of the Conchifera and of the aplacophoran Solenogastres and Caudofoveata. This analysis, with special emphasis on the threefold regionated alimentary tract and the excretory system, not only reveals a close relationship of the Placophora with the Tryblidia (part of paraphyletic *Monoplacophora*), but also recognises that the Placophora and Conchifera represent a monophyletic group of Testaria. Synorganisationally, other characters shared (as *Aculifera*) only with the aplacophoran molluscs represent plesiomorphies rather than synapomorphies. In contrast to some earlier assumptions, the Placophora cannot be regarded as being derived from the conchiferan level; rather, there is a well-defined organisational sequence of gradually additive characters (anagenesis) from the a-placophoran to the poly-placophoran and to the mono-placophoran (conchiferan) configuration. The organisation of the Placophora thus plays a key role in bridging the conservative aplacophoran and the derived conchiferan evolutionary levels within molluscs.
A progression of modes of dorsal exoskeleton formation can be discerned from the Precambrian to the present in mollusc-like and molluscan forms. Examined here are the known modes in Polyplacophora, Aplacophora, Neopilina, Nautilus, Unionid bivalves and other Conchifera, and the probable modes in Kimberella, Halkieria, Wiwaxia, Maikhanella, Multiplacophorans, and Acaenoplax. The steps in the evolution of sclerites and shells seem to be from (1) mantle cuticle alone (Kimberella), to (2) development of sclerites that fuse within a cuticle matrix (Maikhanella), to (3) development of sclerites or sclerites and shells embedded in cuticle formed over the body, the sclerites formed within invaginations of individual cells and the shells by accretion onto a properiostracum (Halkieria, Wiwaxia, Polyplacophora including the Multiplacophora, Aplacophora, and Acaenoplax), to (4) loss of sclerites and development of periostracum at the mantle edge and shell formed by aragonite columns within cells that then become attached to periostracum at the mantle edge (Nautilus, probably Neopilina), to (5) crystallization in a vacuolated middle layer of periostracum (Unionid bivalves, Mytilus), to (6) crystallization directly onto periostracum from extrapallial fluid. In 4, 5, and 6 there is loss of sclerites, the shell is not embedded in mantle cuticle covering the dorsum, there is greater variation in morphology of the foot for locomotion, and there is a concomitant loss of iteration of the exoskeleton. Acaenoplax and Wiwaxia are considered not to belong to either the Mollusca or Annelida, but to the clade Spiculata that includes all the forms under discussion.
In order to build a natural classification of the chitons, a 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. A brief review of the evolution of the Polyplacophora is given and a 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 superfamily Cryptoplacoidea. Specifically, the genera Hemiarthrum, Weedingia and Choriplax, which have unslitted valves, have been 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.
The last common ancestor of Chitonida, the largest order of chitons, evolved unique mechanisms of fertilizing elaborate eggs with dart-like sperm that are typical of the group. In contrast Leptochiton asellus and other Lepidopleurida [sensu Sirenko 1993], have retained the plesiomorphic condition with smooth-hulled eggs and sperm with prominent acrosomes. The mechanism of fertilization in L. asellus is expected to be similar to scaphopods and some other mollusks, because of basic similarities in sperm and egg design. By this mechanism, the acrosome reaction releases enzymes that digest a large hole in the jelly layer and vitelline layer. An acrosomal process polymerizes and extends the inner acrosomal membrane down to fuse with an egg microvillus. A fertilization cone is raised up through the vitelline layer and engulfs the sperm including the nucleus, centrioles, mitochondria and part of the flagellum. The transition to elaborate egg hulls and reduced acrosomes, which characterize all Chitonida, may be evident as intermediate stages among certain lepidopleurids, such as Deshayesiella curvata and Hanleya hanleyi, which have eggs with smooth hulls but sperm with smaller acrosomes on short nuclear filaments. Furthermore, Callochiton dentatus is of special interest, as it has retained a similar egg to D. curvata but its sperm is derived, like all other chitonids. Fertilization follows the chitonid pattern, in which a tiny acrosome digests a hole in the vitelline layer that permits only the injection of chromatin into the egg. The fertilization cone remains below the vitelline layer and so does not engulf the body of the sperm. Thus sperm organelles seem to be abandoned on the egg surface in a bag of sperm membrane. This suggests that centrioles, as well as mitochondria, of Chitonida are maternally inherited. New characters developed for Callochiton dentatus, Deshayesiella curvata and Hanleya hanleyi, have enabled a revision of previous phylogenies. The revised analysis suggests that Callochitonidae is the sister taxon to all of Chitonida and is not part of the order Lepidopleurida [sensu Sirenko, 1993]). Furthermore, Lepidopleurida appear to be a paraphyletic grouping. The Chitonida are still clearly divisible into the two suborders, Chitonina and Acanthochitonina.
The major lateral radula teeth of chitons (Mollusca: Polyplacophora) are composite materials, incorporating a variety of biominerals within an organic scaffold. While magnetite is ubiquitous to these teeth in all Polyplacophorans whose radulae have been described to date, this is not the case for the biominerals of the tooth core. In situ analsysis, using energy dispersive spectroscopy, to determine the distribution of elements in chiton teeth, and Raman spectroscopy, to identify the biominerals present, has been undertaken in the mature teeth of seven chiton species representing three families in the suborder Chitonina. The results show the tooth core to be comprised of a variety of elements, with the main biominerals identified as limonite, lepidocrocite and hydroxyapatite. Along with Ischnochiton australis, all five representatives of the Chitonidae deposit an apatitic mineral in their tooth core, while Plaxiphora albida does not deposit any calcium biomineral. With the exception of Acanthopleura echinata, the hydrated iron (III) oxide, limonite, is found in all species, including I. australis, which has relatively small amounts of iron in its tooth core. The lack of any evidence for a phosphate mineral in species that possess high levels of phosphorus in their core, challenges the long accepted notion that the presence of phosphorus implies its deposition as a biomineral. The combined techniques of energy dispersive spectroscopy and Raman spectroscopy provide a simple and effective means to evaluate, in situ, the biomineralisation strategies employed by chitons. While the results from this study are inconclusive in determining whether biomineralisation strategies reflect phylogenetic affinities in chitons, an extension of the study to include a wider range of chiton taxa could provide a basis for the utilisation of radula tooth biomineralisation as a systematic tool in this class of molluscs.
The chiton fauna of the Magellanic region is investigated based on material from various expeditions and collecting trips made between 1958 and 2003. In total, 15 species of chitons, including one probably undescribed species are recognized in this region. Among them, 14 species inhabit the Magellan Strait, Beagle Channel and Estados Island, and 11 species live near the Falkand Islands. Comparisons of these faunas reveal that the shallow water chiton fauna of the Falkland Islands is the most impoverished in the Magellan Strait. Eleven other species that were mentioned in the literature for the Magellanic region are discussed. Four brooding species were found with eggs and juveniles in their pallial grooves, and the brooding habit of Ischinochiton stramineus and Tonicia lebruni are reported for the first time.
A new deep-sea chiton, Ferreiraella tsuchidai n. sp., is described from the Philippine Basin, at a depth of 5567 m. The new species resembles F. caribbensis Sirenko, 1988 from the Caribbean Sea at a depth of 6780 m by having rather raised valves that lack insertion plates. The similarity between the present species and the distantly distributed F. caribbensis may support Sirenko's hypothesis of the origin and radiation of the family Ferreiraellidae.
Three species of Polyplacophora, Callistochiton granifer, Chiton (Tegulaplax) hululensis and Leptoplax unica, are recorded from Papua New Guinea for the first time. The occurrence of Lucilina lamellosa in New Guinea is also confirmed from new material. Specific diagnostic features are illustrated and short notes are given on each species. In addition, a compilation of Polyplacophora known from the New Guinea region is provided.
Morphological, genetic and ecological analyses were performed on the Japanese chiton Acanthopleura japonica. Two morphological forms were identified. Spots were not observed on the shell plates of A. japonica collected from rocky shores around northern Japan (Form A). Two large black to dark-brown spots were observed in each shell plate, although sometimes not in the head plate, in populations from the Inland Sea, in the southwestern part of Japan (Form B). It is easy to discriminate Form A from Form B solely on the basis the pattern of the shell surface in both young and old specimens, despite the old ones being somewhat eroded. The 705 bp nucleotide sequence of mitochondrial cytochrome oxidase subunit I (COI) region of Form A was 7.66% different from that of Form B. Form-specific restriction fragment length polymorphism (RFLP) patterns were consistently observed when PCR-amplified COI region was digested with restricted enzyme FokI. These two haplotypes co-occurred on some shores in central Honshu. Form A tended to be abundant in the seaward quadrants and co-occurred with the barnacle Tetraclita japonica, whereas Form B was abundant on the landward quadrants and co-occurred with a population of the rock oyster Saccostrea kegaki at Nabeta Bay. There was a tendency for Form A to attach to the side of a rock facing the sea, whereas Form B were distributed on the same rock facing the shore, suggesting that the microhabitat of Form A and Form B may be different. Although Form A and Form B hybridize artificially, there is a possibility that these two haplotypes are reproductively isolated. We consider Form A and Form B to be in a sibling relationship.
Moving patterns and homing behavior in Acanthopleura gemmata and A. tenuispinosa were investigated on the rocky shore of Sesoko Island, Okinawa, Japan. In the daytime, both A. gemmata and A. tenuispinosa moved only when washed by sea water, while at night they moved not only when washed by water but also when they were exposed to the air. Almost all chitons rest in a fixed 'home' site in the daytime and during periods when the rocks are submerged. They do not move when strong sunlight heats the rocks in their habitat either. Surface temperatures of dry rocks under such conditions have been measured as high as 64.8 C. When under water, they suffer the risk of predation by fish or other carnivorous invertebrates. Their movement patterns can therefore be explained as avoidance of heat and desiccation of rock surfaces and predation. Homing behavior in these species was observed throughout the period of study. Homing and moving patterns of A. gemmata and A. tenuispinosa were studied to compare daytime and nighttime activity, including when they go out and when they come back 'home'. The nighttime activity was longer than the daytime activity, and activity patterns were slightly different between these two species. The speed of movement was not significantly different between when they go out and when they return. Vying for use of locations as 'home' and cognitive behavior were observed, suggesting that cognition of geographic features is found among chitons.
Population density and size composition of Acanthochitona defilippii were investigated over a six-year period on a boulder shore at Amakusa, Japan. The vertical distribution range was restricted to the mid and low intertidal zone. The density on the shore varied haphazardly in space and time, but the highest density occurred around the mean tide level. The density showed a slightly decreasing trend over the 6 years at the mean tide level. Seasonal changes in density were detected in individuals of the 2-4 mm size class (fifth valve width), with high density in winter and low density in summer; however seasonal change in the density of smaller individuals (<2 mm) was not apparent from the data. Analyses of size frequency histograms showed bimodal or unimodal patterns, but it was difficult to detect seasonal change of size mode caused by the growth of individuals, or seasonal recruitment of small individuals. These seasonal patterns, which do not show clear modal changes in density fluctuation or size composition, may be caused by low recruitment rate, low growth rate, and/or low rate of mortality.
Chitons are known for their ability to incorporate significant amounts of magnetite (Fe_3O_4) into their radular teeth. Previous studies have classified the mineralization process according to the developmental sequence of the second lateral teeth. In this study, the radular teeth of a chiton Acanthopleura japonica were analyzed by X-ray diffraction (XRD) and electron probe micro analysis (EPMA) in order to examine the composition and two-dimensional distribution of minerals in the tooth cusps. Crystalline components detected by XRD were magnetite (Fe_3O_4), goethite (α-FeOOH), lepidocrocite (γ-FeOOH) and hydroxyapatite (Ca_5(PO_4)_3(OH)). EPMA revealed a negative correlation between Fe and (Ca, P) in the two-dimensional elemental distribution within a tooth cusp. These results indicate that A. japonica exerts a sophisticated level of control over the biomineralization of mineral species during tooth development, from their crystalline condition to their spatial and sequential deposition. The maturation of the second lateral teeth in A. japonica is here divided into five stages.