. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 262 A. J. YOOL ET AL. 100. 10 20 Excess K+ (m/W) 30 FIGURE 6. Induction of attachment of larval Haliotis rufescens by excess potassium, when present in addition to the chemical inducer GABA. The concentration of GABA was 4 X 10~7 M. The percentage of larvae attached was scored at 22 h (O), 48 h (A), and 71 h (D) of continuous exposure, with other details as in the legend for Figure 1. For values obtained in the absence of GABA, see Figure 2. or the response of P. sibogae larvae to coral extract (Table I A). The effect of TEA
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 262 A. J. YOOL ET AL. 100. 10 20 Excess K+ (m/W) 30 FIGURE 6. Induction of attachment of larval Haliotis rufescens by excess potassium, when present in addition to the chemical inducer GABA. The concentration of GABA was 4 X 10~7 M. The percentage of larvae attached was scored at 22 h (O), 48 h (A), and 71 h (D) of continuous exposure, with other details as in the legend for Figure 1. For values obtained in the absence of GABA, see Figure 2. or the response of P. sibogae larvae to coral extract (Table I A). The effect of TEA on the response of A. undosa larvae has not yet been tested. DISCUSSION The gastropods Phestilla sibogae, Haliotis rufescens, Astraea undosa, and the poly- chaete Phragmatopoma californica differ in their native habitats. This difference is reflected in the dissimilar nature of the metamorphic signals recognized by the larvae. Larvae of Phestilla, a small subtropical carnivore, settle and metamorphose on the prey species Porites compressa in Hawaiian coral reefs (Hadfield, 1977). Larvae of the macroalgal herbivores Haliotis and Astraea metamorphose on substrates covered by various crustose red algae, on which the juveniles feed in the intertidal and subtidal zones of the southern California coast (Morse et al, 1979; 1980a; and unpub. obs.). Larvae of the filter-feeding tubeworm Phragmatopoma metamorphose gregariously in response to the anterior tube material of conspecific adults (Jensen and Morse, 1984). Certain neuroactive compounds applied externally have been demonstrated to induce larval metamorphosis in several species. These inducers are specific, and have been suggested to act either as chemical analogs of the natural substratum-derived inducers, or as precursors or active components within a signaling pathway. Choline (at X iO~2 M, as choline chloride) induces metamorphosis of 60-85% of P. sibogae larvae (Hadfieid, 1978; Hadfield et al., in prep.), but has no indu
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