A Non-Swimming Ascidian Larva: Protostyela longicauda (Styelidae)

Abstract

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? to the tail, to contact the substrate and achieve permanent fixation. We therefore interpret these non-swimming larvae as facilitating philopatry. Additional key words: tunicate larvae, caudal complex, dispersal, distribution, philopatry Structural and morphological variations in invertebrate larval forms, driven by strong selective pressures, reflect profound differences in the biology of the species-in duration of larval period, in feeding mode, dispersal ability, and mortality-even in taxonomically close groups (reviewed in Giangrande et al. 1994). A vast literature is concerned with debating the adaptive value of diverse larval strategies (reviewed in McEdward 1995). Indeed, major developmental and morphological changes in larval forms may accompany corresponding changes in life-history strategies of the species (Wray 1995). Among ascidians, the typical tadpole larva has a locomotory tail, sensory structures (ocellus, otolith) for orientation, and papillae and ampullae for anchoring. Adult organs are present in a rudimentary state (Cloney & Torrence 1984; Svane & Young 1989). In some species, adult features develop precociously (a phenomenon called adultation by Jeffery & Swalla 1992); in others, the locomotor tail system is enhanced (caudalization, Jeffery & Swalla 1992); and in some a To whom correspondence should be addressed. cases, the free-swimming larval stage is completely suppressed (anural development). In general, larvae of solitary ascidians tend to be simpler than those of colonial species, which more often feature adultation and/or caudalization. While some biologists focus on the developmental mechanisms (Jeffery & Swalla 1990, 1992; Satoh 1994), these phenomena also have life-history consequences. Adultation reduces the interval between hatching and metamorphosis, while caudalization enhances swimming and hence dispersal. For dispersal, the most drastic dichotomy is between urodele and anural modes of development (Bates 1993; Satoh 1994). Anural forms have been reported in some members of the family Molgulidae and in two species of Styelidae (Pelonaia corrugata MILLAR 1954 and Polycarpa tinctor MILLAR 1962). Most anural species are unattached inhabitants of sand-flat substrates, which are far more homogeneous than hard substrates (Berrill 1931, 1975). But some anural species inhabit high-energy habitats, such as shallow subtidal communities along rocky shores, and appear specifically adapted to such environments (Young et al. 1988). Recently, another anural species has also been reported to inhabit rocky, This content downloaded from 157.55.39.58 on Sun, 11 Dec 2016 04:28:38 UTC All use subject to http://about.jstor.org/terms