Egg hull morphology in two chitons (Polyplacophora) from the Southwestern Atlantic Ocean

ITUARTE, C.; LIUZZI, M. G.; CENTURIÓN, R.

MALACOLOGIA

INST MALACOL

Lugar: Annarobor, Michigan; Año: 2010 vol. 53 p. 167 - 174

0076-2997

Polyplacophorans with planktotrophic development (free spawners) are known to have eggs with elaborate extra cellular coverings currently known as egg hulls (Eernisse & Reynolds, 1994). In these species, hulls are commonly ornamented with projections in the form of cupules, cups, cones, flaps or spines. Members of Lepidochitonidae (Lepidochitona spp., currently classified as Cyanoplax spp.) from the Pacific coast of North America were shown to have reduced, nearly smooth, egg hull sculpturing, which is a reduction of a cone-like hull in free-spawning relatives (Eernisse, 1988). Leptochiton asellus, a species of Leptochitonidae (Lepidopleurida) have eggs with smooth jelly-like hulls (Buckland-Nicks & Hodgson, 2000; Buckland-Nicks, 2008). Some brooder species such as Chiton nigrovirescens, have eggs with hulls provided with short spines that serve to maintain eggs packed within the pallial groove (Buckland-Nicks & Brothers, 2008). Then, the shape and the length of hull projections would be indicative of the mode of development, i.e. brooders vs. free spawners (Eernisse, 1988). Variations in the morphology of the egg hull are known to be of taxonomic value (Eernisse 1984; Sirenko, 1993; Eernisse & Reynolds, 1994). Recently, the morphology of hull projections was recognized as a valuable character in the study of phylogenetic affinities (Buckland-Nicks, 2006, 2008; Sirenko, 2006). During oogenesis, follicle cells are known to be probably responsible for the shaping of the egg hulls, acting as moulds for the depositing of mucopolysaccharides and proteins that constitute the egg hull layers (Eernisse & Reynolds, 1994). Buckland-Nicks & Reunov (2009) reported the involvement of both oocyte and follicle cells in the formation of the egg hull in a chitonid, Rhyssoplax tulipa. The existence of a close relationship between the shape of egg hulls, morphology of sperm and mode of fertilization has been reported (Buckland-Nicks, 1993, 1995, 2006, 2008). All chiton species with complex egg hulls have dart-like sperm with reduced acrosomes, usually at the tip of a nuclear filament of variable length; conversely, lepidopleurid species with smooth egg hulls have sperms with prominent acrosomes (Hodgson et al., 1988; Pashchenko & Drosdov, 1998; Buckland-Nicks 2006, Buckland-Nicks & Brothers, 2008). Knowledge of the hull morphology and characteristics of the follicle epithelium of genera and species from the Southwestern Atlantic is scant. Sirenko (1993, 2006) provided general sketches of the hull morphology of many chitons among them, species of the genus Chaetopleura, Plaxiphora and Tonicia. However, details of the morphology of SW Atlantic species are entirely unknown. Buckland-Nicks (2008) and Buckland-Nicks & Brothers (2008) reporting on the fertilization process and evolution in chitons, briefly described and illustrated the hull morphology of species of the genera Chaetopleura and Callochiton from the Northern Hemisphere. The present study describes the ultrastructure of the egg hulls of Chaetopleura isabellei (d´ Orbigny, 1841) and Plaxiphora aurata (Spalowsky, 1795) two common species along Argentine shores. A peculiar behavior of the ooplasm during early stages of oogenesis in P. aurata is described. This characteristic, to date only reported in species belonging to Acantochitonina, could represent a reproductive trait unique for this clade, reinforcing the phylogenetic coherence derived from morphology. Our results suggest that knowledge of details of the oogenesis process, a dramatic event in defining life history of a species, may be of help in defining phylogenetic relationships among chitons. Morphology of the egg hull of P. aurata here reported, strongly suggests the need for a new placement of Plaxiphora, outside Mopaliidae.