Seaweed palatability and selective grazing by littoral gastropods

The vertical zonation and microdistribution of the dominant gastropod herbivores was examined at a sheltered site in the Firth of Clyde. The flat winkles littorina obtusata and littorina mariae, constitute potential competitors on the low-shore. On the mid-shore, the limpet patella vulgate, may compete with the common winkle, littorina littorea, but Patella is not abundant under a dense fucoid canopy - conditions which favour the common winkle. The response of L. littorea to various stimuli was assessed in a series of controlled laboratory experiments. Changes in light intensity and the wetting/drying effects of immersion/emersion, prompted changes in the level of littorinid activity. Such behaviour is paralleled in the field, where immersion/emersion by the tide generally illicits a rapid response. A high degree of niche separation is evident between common and flat winkles. This is reflected in food preferences and adaptations to the feeding apparatus. The two sibling species of flat winkle are inhabitants of the algal canopy and prefer to feed on the perennial thalli. Subtle adaptations to the radular teeth permit the flat winkles to excavate the fucoid lamina efficiently. In contrast, the common winkle possesses denticles ideally suited to feeding on the adult foliose thallus and consequently, the ephemeral algae may constitute an important seasonal food source on some shores. Both the flat winkle and the common winkle graze algal germlings efficiently and littorina littorea displays a high degree of selectivity in laboratory choice experiments. On the shore, the microdistribution of the littorinids in relation to the availability of potential food sources, is closely related to laboratory- determined preferences. The flat winkles are confined largely to the algal canopy, while the common winkle forages predominantly on the rock surface and amongst the ephemeral understorey species, only occasionally venturing into the canopy. Where macroalgal foods and benthic diatoms/microalgae are scarce, the opportunist common winkle can adapt and utilise unattached, "drift" material deposited by the tide. The anti-herbivore defences of the macrophytic algae are based on the contrasting strategies of escape in time and space Ci"-selected species) and the utilisation of metabolically expensive structural and chemical defences (K-selected species). The latter confer varying degrees of immunity to excessive grazing damage. Thallus form and toughness both represent potential structural grazing inhibitors and consequently influence grazer preferences markedly. The nature of the algal cell wall may affect the digestibility of the plant, as the cell wall must be breached to allow the animal's digestive enzymes access to the cell contents. Both littorina littorea and littorina obtusata digest a relatively high proportion of ingested cellulose when feeding on preferred foods. Laboratory experiments with aqueous algal extracts demonstrate the important role of "taste" in food selection. The presence of noxious or attractive allelochemics is probably the prime motivation behind selective grazing at the germling phase of the algal life cycle. The phenolic content of the fucoid extracts has been examined closely and discussed in the light of current theories concerning the digestibility-reducing activities of the intracellular phenolics. Polyphenols and other plant secondary chemicals are often exuded into the surrounding seawater. "Long-distance" detection of exudates may influence both the microdistribution and the vertical zonation of littorinids on the shore. The chemosensory capabilities of the littorinids were tested in a series of controlled laboratory experiments. Both littorina littorea and littorina obtusata responded to the presence of Ascophyllum exudate. While L. littorea was repelled, L. obtusata was strongly attracted to the source of exudation