The importance of plant defence abilities to the structure of subtidal seaweed communities: The kelp Laminaria longicruris de la Pylaie survives grazing by the snail Lacuna vincta (Montagu) at high population densities

Following mass mortalities of sea urchins, Strongylocentrotus droebachiensis (Mueller), in the rocky subtidal of the Atlantic coast of Nova Scotia, dense seaweed beds dominated by the canopy-forming kelp Laminaria longicruris de la Pylaie developed for the first time in several years. The numerically abundant grazers in the seaweed beds when sea urchins were rare were the mesogastropod Lacuna vincta (Montagu), the limpet Notoacmaea testudinalis (Mueller), and the chiton Tonicella rubra (Linne). The limpet and chiton are unable to graze the macroscopic sporophytes of Laminaria, and a manipulation experiment showed that they have no significant effect on the recruitment of this kelp. In contrast, Lacuna grazes the macroscopic sporophyte and several of its population characteristics suggested that it might be a potentially important grazer of Laminaria. Snails can occur in high densities and several invasions of an area may occur during a single season; they manifest strong preference for Laminaria in the subtidal region; post-metamorphic individuals have considerable ability for dispersal; and predation by their principal predator, the labrid fish Tautogolabrus adspersus (Walbaum) is limited in time and space. Despite this potential, the estimated direct consumption of kelp laminae by snails was negligible, amounting to only ~0.05% of the total blade biomass (fresh wt) that was available during their season. However, the snails' overall impact on the Laminaria canopy was significant. Becasue grazing was concentrated on the frilled margins of laminae, and the holdfast, stipe and intercalary meristem were largely avoided, grazing promoted the tearing away of the margins of blades, resulting in a significant reduction in canopy area without increasing kelp mortality. Since there was no grazer-induced mortality of Laminaria, the canopy was closed following the growth burst of the kelps in early spring, and there was little opportunity for understorey species to benefit from the temporarily high light levels. If the distribution of grazing by the snails had been more uniform and caused significant mortality of Laminaria, major changes in the structure of the seaweed assemblage would have occurred. We suggest that the pattern of Lacuna grazing on Laminaria is goverened by the distribution of anti-herbivore chemicals in the plants, and variations in the toughness and nutritional quality of different regions of the plant. Avoidance of the intercalary meristem correlates with high levels of polyphenols in this area. Polyphenol concentration is uniformly low throughout the rest of the plant. The distribuition of snail grazing on the stipe and holdfast, and the central and lateral portions of the laminae correlates with the toughness and nutritional quality of these parts. The nature of the anti-herbivore defenses of Laminaria, which are effective against grazing by Lanuna but not by the urchins, indicates a trade-off between high specific growth rates and reproductive output (and thus superior competitive ability) versus development of defences. This strategy is consistent with cost-effective arguments relating resource allocation and fitness.