Reference: Biol. Bull., 147: 573-585. (December, 1974) LARVAL SETTLEMENT OF A SYMBIOTIC HYDROID : SPECIFICITY AND NEMATOCYST RESPONSES IN PLANULAE OF PROBOSCIDACTYLA FLAVICIRRATA SYEN DONALDSON Friday Harbor Laboratories, Friday Harbor, U'asliington 98250 Planktonic larvae of many benthic invertebrates preferentially settle and meta-morphose on specific substrates (Williams, 1964; Newell, 1970). Frequently, the planktonic larvae of symbionts possess exceptionally well-developed physiological mechanisms for substrate selection. Such mechanisms presumably evolved be-cause, for these species, only a small proportion of the surfaces that a larva might contact are suitable for settlement (Davenport, 1955). I have studied planulae of the symbiotic hydroid Proboscidactyla flavicirrata (Brant). The structure of a planula larva is extremely simple, yet the planulae of Proboscidactyla settle with great specificity (Campbell, 1968). Sabellid polychaetes are the only known hosts of Proboscidactyla (Gosse, 1857; Uchida and Okuda, 1941; Hand, 1954; Brinkmann and Vannucci, 1965; Calder, 1970). Proboscidactyla colonies are found on the rims of sabellid tubes where their gastrozooids can contact the host's plume of tentacular cirri. According to Campbell (1968), Proboscidactyla planulae begin settlement by adhering with nematocysts to the pinnules that branch from the tentacular cirri. Later, when the sabellid retracts its tentacular cirri into its tube, the planulae somehow transfer to the rim of the tube where they metamorphose. In this report I confirm and extend Campbell's work. Two new findings are of particular importance. First, planulae use nematocysts both to attach to pinnules as Campbell (1968) suggested and to transfer to the tube rim. Between the time of attachment to a pinnule and transfer to the tube rim, the effective stimulus for nematocyst discharge shifts from pinnule to worm tube. Secondly, planulae settle and metamorphose on either of two sympatric species of sabellids, one of which does not support Proboscidactyla colonies. MATERIALS AND METHODS During June, July, and August of 1973, the medusae of Proboscidactyla were collected from surface waters, and the sabellids Schisobranchia insignis and Eudistylia vancouvcri from the undersides of mooring floats near the University of Washington Friday Harbor Laboratories. Nearly mature Proboscidatyla medusae sometimes ripened when fed Artemia larvae. The medusae, embryos, and planulae were kept in glass finger bowls or Syracuse dishes partially submerged in an open-circulation sea table and the water in the holding containers was changed at least every third day. The medusae shed their gametes at night and developing embryos were rou-tinely transferred to Syracuse dishes the next morning. Zygotes were sometimes 573
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