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

Abstract

Polyplacophorans with planktotrophic development (free spawners) are known to have eggs with elaborate extra-cellular coverings 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., now 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) has eggs with smooth jellylike 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). Thus, the shape and the length of hull projections is indicative of the mode of development, that is, brooders vs. free spawners (Eernisse, 1988).