TI: Algal associations of Tricolia pulus, Lacuna vincta and Cerithiopsis tubercularis (Gastropoda) with special reference to the settlement of their larvae.
AU: Fretter,-V.; Manly,-R.-(Univ.-Reading,-Dep.-Zool.,-Reading,-UK)
SO: J.-Mar.-Biol.-Assoc.-U.K., 1977 57(4), 999-1017
LA: English
AB: Populations of Tricolia pullus (L.), Lacuna vincta (Montagu) and Cerithiopsis tubercularis# (Montagu) were studied at four intertidal areas in the vicinity of Plymouth. The larvae of Tricolia and Lacuna metamorphosed on algae, chiefly rhodophyceans, on which recently-settled and older individuals occurred; their first food was diatoms and sediment and later plant tissues. The early settlers of Lacuna, which is an annual, appeared after previous generations had gone from the shore and it is assumed that organic substances released by the algae or the lack of repellants are of prime importance in initiating their metamorphosis. The data available indicate that Tricolia is also an annual, but the limits of generations are obscured by extended breeding periods. An analysis of the population of Lacuna with respect to shell height showed that during the initial weeks of benthic life the snails remained on the algae on which they settled. The most favoured was Lomentaria articulata, especially extensive growths in sheltered gullies. Mortality was high and the number of potential spawners in February 1972 was less than 4-.-5% of the population of the previous July, a month when numbers are maximal. Whereas the abundance of Lacuna was inversely proportional to the degree of exposure of the shore, a relationship was far less evident for Tricolia which occurred more frequently on the basal areas of algae rather than fronds and has a higher intake of sediment. Veligers of the sponge-feeder Cerithiopsis settled at the base of rhodophyceans associated with Halichondria and Hymeniacidon. Some snails were on these weeds in every month, particularly in spring and early autumn when mature individuals were collected from fronds as well as basal area. These periods of high numbers may be related to breeding activities. Dense tufts of red algae offering shelter and some degree of surface rigidity are the most frequent settlement sites. Larvae trapped within these growths have greater opportunity to encounter appropriate surfaces, to which their firm attachment by secretion from the foot is an essential precursor of metamorphic changes.