Invasive bryozoan alters interaction between a native grazer and its algal food

The epiphytic bryozoan Membranipora membranacea encrusts the surface of kelp blades, causing recurrent large-scale defoliation events in kelp beds off the Atlantic coast of Nova Scotia, Canada. The gastropod Lacuna vincta grazes kelp, creating perforations that weaken blade tissues and increase the fragmentation rate. We assess the interaction between M. membranacea and L. vincta by measuring the grazing rate of snails on bryozoan-encrusted and non-encrusted kelp (Saccharina latissima) in no-choice and choice experiments in the laboratory conducted in November and December 2010. There was no effect of diet on grazing rate in no-choice experiments. In choice experiments, however, L. vincta grazed significantly more non-encrusted than encrusted kelp (7.1 versus 1.1 mg snail−1 d−1), and grazing rate of non-encrusted kelp was almost twice that in the no-choice experiment (3.8 mg snail−1 d−1), indicating that snails may avoid colonies of M. membranacea on partially encrusted kelp blades. We found no effect of diet on growth, reproduction and survival of snails maintained for four weeks on encrusted or non-encrusted kelp. By concentrating grazing damage on non-encrusted areas of blades, L. vincta may act synergistically with M. membranacea to increase the likelihood of blade breakage and canopy loss. This indirect effect of the invasive bryozoan could augment its direct effect on the standing biomass of native kelp beds and detrital export to adjacent communities.

fish–macrofauna interactions in a kelp (laminaria hyperborea) forest

predator–prey interactions between fish and macrofauna in kelp (laminaria hyperboea) forest were examined. fish were caught in gill-nets and fish traps at two sites and throughout the diurnal cycle. their stomach contents were analysed. the most abundant fish species, except pollachius virens, were caught in larger numbers at night than during daytime. the examined fish fed throughout the diurnal cycle and kelp-associated fauna were found to be important prey. pollachius virens was caught in the same numbers throughout the diurnal cycle and contained mainly pelagic prey, in larger numbers during the day than at night. small labrids were specialists, feeding mainly on the two numerically dominant kelp-associated gastropods rissoa parva and lacuna vincta. the larger labrus bergylta, gadus morhua and myoxocephalus scorpius were generalists, feeding on a large range of kelp-associated invertebrates. some fish species showed preference for particular vertical levels when feeding in the kelp forest. the most mobile and/or the largest invertebrates were most heavily preyed upon.

Intraplant habitat and feeding preference of two gastropod herbivores inhabiting the kelp Laminaria hyperborea

The habitat and feeding preference of two gastropod mesoherbivore species, Ansates (Helcion) pellucida and Lacuna vincta, inhabiting the kelp Laminaria hyperborea were investigated by studying the distribution, habitat choice and food preference of the herbivores between new and old kelp fronds. Nitrogen content and chemical defence (phlorotannin) level of the kelp fronds were measured in order to determine whether they were correlated with the feeding preference of the herbivores. The habitat choice of Lacuna vincta was correlated with the food preference of this species, while the habitat choice of A. pellucida probably was due to some other factor, e.g. competition or mortality. Ansates pellucida preferred to reside on the old fronds of Laminaria hyperborea, but consumed an equal amount of both tissue types. Lacuna vincta, however, preferred the new Laminaria hyperborea fronds both as habitat and food. There was a large variation in the phlorotannin content among the tissue samples of L. hyperborea. However, there were no overall statistically significant differences in tissue nitrogen or phlorotannin content between the new and the old L. hyperborea fronds.

Grazing by the gastropod, Lacuna vincta, in the lower intertidal area at Musquash Head, New Brunswick, Canada

Lacuna vincta appeared at Musquash Head, New Brunswick, in large numbers from June to August 1981 principally on lower midlittoral and infralittoral fringe Fucus edentatus. The population reached a mean maximum of 280/m2 in June; by mid September the animals had disappeared. The L. vincta grazed heavily and deposited egg masses on Fucus edentatus but little on other species. All fine and filamentous and encrusting algal species and rock were avoided for egg deposition. Egg mass abundance peaked with the population in June with a mean of 180/m2. Mean egg production in June was 83000/m2.Both snail and egg mass distributions were correlated with the percentage cover of F. edentatus, reaching a maximum at 40–75 cm above mean low tide level.The snails ranged in length from 2–7 mm, consisting of three cohorts from different larval settlements. Growth was rapid in July averaging 0·6 mm/month slowing later.The food plant Fucus edentatus showed a heavy impact of the herbivore. Weight reductions attributable to grazing rose from 56 % in June to 79 % in August and grazed plants tended to thicken rather than elongate with growth. Grazing removed about 79 % of the net production of F. edentatus which averaged 61 g dry wt/m2/day. The impact of the L. vincta population on F. edentatus seems extraordinarily severe.