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The t r o c l d gastropod Gibbula cinerana spawns in response to a rlse in sea water temperature A simple laboratory bioassay a warm water shmulus can therefore be employed to de t e rm~ne the competence of adult s n a ~ l s to spawn at any given tlme d u r ~ n g the year The bioassay has been used over 1 yr, at approximately monthly intervals to estabhsh the spawning season of a population of G clnerarla in NE England The main spawning season was indicated between late June and early September Spawning could also be induced in females in winter Results are discussed in relation to previous, contrasting, reports regarding the spawning season of G cinerarla Gibbula cjneraria ( L . ) is gonochoristic and spawns directly to the sea where fertilisation occurs. Reports of the spawning season of G. cineraria present a confusing picture of events (see Underwood 1972 for a review). For example, Fretter & Graham (1962) consider that G. cineraria spawns throughout the year at Plymouth (SW England), while in Scotland (Elmhirst 1922) and North Wales (Thompson cited in Underwood 1972), spawning reportedly occurs in spring. To clarify this situation, a laboratory study was performed using a simple and rapid bioassay (Clare 1987). The assay employs a 'warm water stimulus' and is based upon Underwood's (1972) observation that Gibbula cineraria spawns when the sea water temperature rises above 12°C. The assay involves transferring 30 specimens, which have been maintained below 12"C, to individual bowls containing 100 m1 of sea water (also below 12°C) and allowing them to equilibrate to room temperature (ca 20°C). The bowls are examined after 24 h for spawned gametes and all snails are dissected to determine the condition of the gonad. The gonads are scored in 3 categories based on a visual examination. These categones are: (1) spent, i.e. the digestive ' Present address: Duke University Marlne Laboratory, Pivers Island, Beaufort, North Carolina 28516, USA gland is clearly visible through the gonad; (2) partly spent, i.e. patches of digestive gland are visible, and (3) gravid, i.e, the gonad is either entirely green in females or cream/white in males. This method allows the gonad condition to be rapidly evaluated. The specimens used in this study were collected at low water from the intertidal zone at Cullercoats, NE England, throughout 1986 and during the early months of 1987. Fig. 1 shows the results of bioassays performed on the day of collection. Fenlale snails had 2 potential spawning periods; one during the summer and the other during late winter. Male snails, however, spawned in response to a temperature rise in summer, but not during late winter.

Laboratory-induced spawning of the gastropod Gibbula cineraria as an indicator of field spawning