A NON-SWIMMING ASCIDIAN LARVA - PROTOSTYELA LONGICAUDA (STYELIDAE)

摘要

The ascidian Protostyela longicauda broods larvae that have a unique, non-locomotory tail. In hatched larvae, the tail is a simple strand of tunic without cellular components or external fins, extremely elongated and anteriorly directed. Before hatching, the tail consists of a mass of secretory cells that release fibrillar material to the tunic. Notochord, neural tube, and muscle cells are absent, but presumptive chordal cells are found at earlier embryonic stages. The anterior part of the trunk bears three conical adhesive papillae and five pairs of ampullae, which elongate at metamorphosis. The tunic of the trunk and tail is sticky from the first moment after hatching and adheres readily to any object; the papillae make contact later. This species attaches to algal fronds in wave-swept, rocky intertidal habitats. Field observations on adult populations revealed a strongly aggregated, philopatric pattern of distribution, as well as within-patch patterns of colonization of some algal species as substrate. Restricted dispersal of short-lived larvae in these communities diminishes the risk of being swept away and is consistent with the aggregated pattern found. We suggest that, upon release of the larva, the sticky tail rapidly becomes entangled with algae, thereby allowing the papillae, oriented at about 30 degrees to the tail, to contact the substrate and achieve permanent fixation. We therefore interpret these non-swimming larvae as facilitating philopatry. [References: 40]