Light and scanning electron microscopic observations on the developmental biology of the common starfish, Asterias rubens Linné (Echinodermata: Asteroidea)

Abstract

Larval development and metamorphosis of Asterias rubens Linné, 1758 were re-examined by means of light and scanning electron microscopy. The main morphological feature of the larva is in succession: 1) the particulate feeding apparatus, 2) the brachiolaria complex, and 3) the adult primordium. The tripod-shaped brachiolaria complex is composed of three tubular brachiolaria arms, a sucker-shaped attachment disc, and domed and triangular papillae. The ready-to-settle larva displays a substratum-testing behaviour in which the three arms and the disc may be involved in both cue perception and temporary adhesion. Permanent attachment of the larva to bottom substrata is performed by the disc. During metamorphosis larval structures are replaced with juvenile structures and these morphological alterations result in changes in body plane and ecology. The process commences while A. rubens is still in the plankton by the appearance of five hydrocoel lobes and five terminals in the posterior part of the early brachiolaria. The larval body serves as an endogenous energy reserve in the period without a functional digetive system. Absorption of dissolved organic matter through an identified outer extracellular matrix, may supply energy. After differentiation of a Iocomotory apparatus consisting of the five tube foot systems, the adult mouth appears as an open crevice.