Induction of Metamorphosis in Larvae of a Sponge

Abstract

Elevated exogenous KC1 and CsCl are effective artificial metamorphic triggers for many, but not all, invertebrate larvae. Neither taxonomic nor functional patterns in their effectiveness, however, are apparent. In our experiments, elevated exogenous KC1 promoted metamorphosis of larvae of the demosponge Aplysilla sp., but only when applied simultaneously with a biofilmed artificial substratum. Although the presence of this artificial substratum alone significantly enhanced settlement over seawater controls, its effectiveness was further augmented significantly in the presence of elevated KC1, indicating a synergistic interaction. Exposure of larvae to a 2-min pulse of 30 mM KC1 promoted metamorphosis as effectively as continuous exposure to 30 mM KC1. Moreover, 30 mM CsCl proved an effective trigger of metamorphosis both in the absence and presence of a biofilmed substratum. As with KC1, the CsCl data indicate the presence of a synergistic interaction between CsCl and the biofilmed substratum. This report provides preliminary insights into the induction of settlement and metamorphosis of sponges, metazoans which lack established sensory and neuronal elements, and it permits comparison with responses of eumetazoans. Additional key words: Porifera, Aplysilla, KC1, CsCl, settlement Sponges represent a grade in metazoan evolution for which the presence of neuronal cells and differentiated receptor cells is unconfirmed (Harrison & De Vos 1991). Additionally, sponge larvae are considered generally nonspecific in substratum preferences at settlement and metamorphosis (Bergquist 1978). The receptor site(s) and the internal pathway(s) of the metamorphic response in sponges remain unexplored. Elevated concentrations of exogenous K+ (as KC1) are effective in inducing larval settlement and metamorphosis in species of a wide range of marine invertebrates including a hydrozoan (Spindler & Muller 1972; Muller & Buchal 1973), a polychaete (Yool et al. 1986), at least 15 gastropods (Baloun & Morse 1984; Yool et al. 1986; Pechenik & Heyman 1987; Hubbard 1988; Davis et al. 1990; Todd et al. 1991; Inestrosa et al. 1993; Pechenik & Gee 1993; Campos et al. 1994; Gibson & Chia 1994; Yang & Wu 1995), a bivalve (Nell & Holliday 1986), a phoronid (Herrmann 1979), a brachiopod (Freeman 1993), 5 bryozoans (Eiben 1976; Stricker 1989; Wendt & Woollacott 1995), and 3 echinoderms (Cameron et al. 1989; Pearce & Scheibling 1994). Evidence indicates, however, that supplemental K+ is not a global inducer as aTo whom correspondence should be addressed. it is ineffective in stimulating settlement and metamorphosis in species of certain other marine invertebrates (at least at the concentrations tested) including 2 corals (Morse et al. 1988), 2 bivalves (Eyster & Pechenik 1987; Gustafson et al. 1991), 2 echinoderms (Rowley 1989; Johnson et al. 1991), and 2 ascidians (Grave & Nicoll 1940; Lynch 1961). In the barnacle Balanus amphitrite, supplemental K+ even inhibits settlement (Rittschof et al. 1986). Inspection of these data reveals little in the way of phylogenetic patterns in larval responses to K+. Only in 3 taxa (echinoderms, gastropods, and bryozoans) are responses of more than 3 species established. In gastropods and bryozoans, excess K+ was uniformly effective as an inducer of settlement and metamorphosis. In contrast, data on echinoderms (and smaller data sets on cnidarians and bivalves) indicate that larvae of some species within each group are affected by elevated K+ whereas others are not. Responses of larvae within deuterostome and protostome assemblages are also mixed. Finally, where response to elevated K+ varies within a clade, it does not correlate with overall differences in known features of developmental and larval biology. At present, an encompassing synthesis of K+ induction remains elusive from phylogenetic perspectives. To expand on this taxonomic baseline This content downloaded from 157.55.39.186 on Sun, 09 Oct 2016 06:18:30 UTC All use subject to http://about.jstor.org/terms Woollacott & Hadfield and to address the induction pathway in sponges, we examined responses of larvae of the demosponge Aplysilla sp. to elevated exogenous K+ and Cs+. Aplysilla sp. retains its early developmental stages and releases short-lived parenchymella larvae that are anenteric and, presumably, incapable of feeding on particulate matter (Woollacott & Hadfield 1989 [as Dendrilla cactus]; Woollacott & Pinto 1995). Most of the larval surface is comprised of a uniform field of flagellated (monociliated) cells. The anterior end (that facing forward during swimming) is cap-like and formed by larger flagellated cells. The posterior pole bears a tuft of long flagella. On release, larvae are initially strongly positively phototactic, but the strength of this response gradually diminishes during the swimming period.