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Colour-infrared (CIR) imagery was evaluated for its usefulness to test hypotheses about grazing in the intertidal gastropods Nerita atramentosa (Reeve) and Bembicium nanum (Lamarck). Tiles of natural sandstone were placed onto the shore to accumulate micro-algae for a period of 6 weeks. Then, in the laboratory, three replicate tiles were assigned into each of 6 treatments (Nerita and Bembicum at densities of 3 or 6 animals, plus a control and a control for artifacts due to cages). CIR images were acquired before and three times after the animals were placed on the tiles. NIR/red ratio images, which are used to estimate micro-algal biomass, were generated from calibrated CIR data. Nerita at the greater density were more effective at removing microalgae than at the smaller density. The latter were more effective than Bembicium at either density, which did not significantly decrease algal biomass from that in controls. These data are consistent with previous field observations and indicate that CIR imagery is an effective method for testing hypotheses about grazing by intertidal gastropods. D 2005 Elsevier B.V. All rights reserved.

Shellfish collection by humans is a form of selective predation, with the larger individuals of a few target species being taken, and can have direct and indirect effects. We investigated the indirect effects of collection by experimental removals of two target species, Cellana tramorerica (Sowerby) and Nerita atramentosa (Reeve) from a rocky shore in south eastern Australia. Removals of single species were compared with the removal of both species and frequent removals were compared against once-off removal events. The removal of Cellana resulted in an increase in micro-algal abundance. This increase was greater where Cellana were frequently removed than where once-off removal of the species occurred. The removal of Cellana also promoted the immigration of adult Cellana, but the overall population was dominated by juveniles. Immigration of juvenile Cellana was high in all cases and was not affected by removals of either target species. The removal of Nerita, whether once-off or frequent, resulted in a decrease in micro-algae, but did not affect the migration of other species. The combined removal of Cellana and Nerita did not affect microalgal abundance, a result that was successfully predicted from the summed effect of the single species removals. In addition, the combined removal had no effect on the abundance of other species. The single and combined removal of these two gastropod species did not influence the species richness at this study site.

Terrestrial hermit crabs (Coenobita compressus (H. Milne Edwards)) on Isabel island, Mexico, prefer Nerita (Ritena) scabricosta (Lanark 1822) over other species of shells. Nerita scabricosta, the lightest species of shell, has the highest internal volume/weight ratio (IV/W ratio) of all species available at the island. Heavier shells (low IV/W ratio) are more costly to carry and may restrain growth. We hypothesized that crabs will search for shells with high IV/W ratios to save energy, and predicted that in every individual shell-exchange crabs will prefer a shell with a higher IV/W ratio. Observations of spontaneous aggressive shell-exchange interactions in nature between crabs carrying preferred shells (with high IV/W ratio) and crabs using less preferred species of shells (low IV/W ratio) support this idea. By inducing individual shell exchanges in the field, we experimentally confirmed the preference for shells with high IV/W ratio, and we also showed that crabs prefer larger shells over those they were bearing. Moving to a larger but not heavier shell may facilitate growth though saving energy that would otherwise be spent in locomotion. High growth rates seem to be advantageous because size is a correlate of fertility, since large males apparently have more access to females, and large females produce larger clutches, thus indicating the possible reproductive benefits for preferred light and large shells.

Populations of three coexisting intertidal gastropods, Nerita atramentosa Reeve, Bembicium nanum (Lamarck) and Cellana tramoserica (Sowerby), were sampled from a shore in Botany Bay, New South Wales, from July 1972 to September 1973. The recruitment and growth rates of each species were analysed from size frequency distributions. Mortality of each age cohort, and longevity, were estimated from analyses of the densities of Nerita atramentosa and Bembicium nanum . Nerita atramentosa showed no significant mortality during the first two years on the shore, but high mortality (at an instantaneous rate of 0.084 deaths/individual/month) after reaching the age of reproductive maturity, which was 20 months from settlement and at a mean shell-length of 13.5 mm. No growth could be detected after reproductive maturity was reached. Longevity of N. atramentosa was estimated as 3–5.5 years. Bembicium nanum juveniles showed higher mortality (at an instantaneous rate of 0.233 deaths/individual/month) than reproductively mature animals (0.060 deaths/individual/month). Reproductive maturity was reached at a mean shell-breadth of 11.0mm, i . e ., about ten months after settlement on the shore. Longevity was estimated as from 4–8 years. Cellana tramoserica showed different growth patterns depending on the time of settlement. The three species showed different patterns of growth and life history relating to variable recruitment (which was demonstrated in all three species) and different rates of mortality of age cohorts.

The oxygen consumption of three species of Malaysian mangrove gastropods was measured in air and sea water at the temperatures commonly recorded in the mangrove. The experiments in air were carried out after the animals had regained fluid lost from the mantle due to handling. Fluid loss can have considerable effects on rate of oxygen consumption. Nerita arliculata (Gould) was found throughout the mangrove and experiences from 50 to 92% aerial exposure. It has a gill and a ratio of aerial to aquatic respiration rates of 2.7 at 28°C. 50% of the animals can survive underwater for 72 h at 28°C. The other two species, Cerithidea obtusa (Lamarck) and Cassidula aurisfelis (Brugiere) experience over 95% aerial exposure, have their mantle cavities modified as lungs and have air : water respiration rate ratios of 5.5 and 6.0, respectively. 50% can survive from 48 to 36 h underwater at 28°C. Acclimated animals have Q10's of about 1.6 in air and 1.4 in water. The respiratory physiology of the snails is compared with that of rocky shore species.

Intraspecific competition for food within and between age-classes of the intertidal grazing gastropod Nerita atramentosa Reeve was investigated using experimental cages on a rock platform in Botany Bay, New South Wales. Adults (mean length 21.0 mm) and juveniles (mean length 12.36 mm) were confined separately and together, at densities of 10, 20 and 50 per cage. The lowest density was the same as the mean density of Nerita on the shore. Cages were maintained for 146 days from November, 1973 to April, 1974. Each animal was numbered and its growth rate recorded over the period of the experiment. Dead animals were replaced by others of the same size to maintain densities, but replacements were not included in analyses. Proportional mortality of each size-class in each cage, growth rates and wet tissue weights at the end of the experiment were analysed. Adult animals did not grow. Growth rate of juveniles decreased with increasing density. Mortality of juveniles was not significantly different at different densities, but mortality of adults increased linearly as density increased. Wet tissue weights of adults and juveniles decreased with increasing density. In adults there was a decrease in weight at densities of 20 and 50 per cage. In juveniles, tissue weight increased with growth, from the start of the experiment except at densities of 50/cage where tissue weight remained constant and growth was absent. For all three factors, there were no differences between the effects of increasing density by additional members of the same age-class or by members of the other age-class. Adults and juveniles have, therefore, identical effects on the growth rates of juveniles, mortality of adults and wet tissue weights of both age-classes. Population density regulation in Nerita is possible, since high densities (e.g., due to increased juvenile recruitment) will cause reduced growth of juveniles and increased mortality of adults, until the densities are lowered. This effect of increased density is compared with intraspecific competition reported in other intertidal gastropods.

The densities of populations of Nerita atramentosa Reeve, Austrocochlea constricta Lamarck, Bembiciuin nanum (Lamarck), and Cellana tramoserica (Sowerby) were recorded in 0.25 m2 quadrats on landward and seaward halves of a sheltered and a moderately exposed rock platform from June 1972 to June 1973. The data have been subjected to variance and multiple regression analysis. Differences between densities of each species on the two shores and in the two areas of each shore are discussed with respect to the regressions on littoral height. Densities of Nerita on the sheltered shore, Austrocochlea and Bembicium on both shores, and Cellana, on the exposed shore, increased with height. The density of Cellana on the sheltered shore decreased as height increased. The density differences in the landward and seaward areas on the two shores were attributable to the effect of height for Nerita and Austrocochlea. Height was the major variable affecting the density of these two species on the sheltered shore. Bembicium and Cellana densities on both shores, and Austrocochlea density on the exposed shore, are not primarily determined by height. On the sheltered shore, Nerita and Cellana increased in density with increasing cover of water in rock pools. On the exposed shore, Bembicium and Cellana became sparser with increasing water cover. Bembicium on both shores increased in density with increasing cover of the alga Peyssonelia, as did Cellana on the exposed shore. Cellana and Austrocochlea tended to increase in density with Nerita on the sheltered shore. Cellana tended to be sparse where Bembicium was dense on the exposed shore. The biological effects of substrata and other species on the densities of each species remain unexplained. Significant time effects were found for variations in the densities of Nerita and Cellana on the sheltered shore and of Austrocochlea and Cellana on the exposed shore. The seasonal change in density of Nerita was parabolic with a minimum in summer, and is consistent with the known period of recruitment of this species. The lack of significant time effects on density of Austrocochlea on the sheltered shore is consistent with the known continuous recruitment of this species. The declining density of Austrocochlea on the moderately exposed shore was found, by size-frequency analysis, to be due to reduced recruitment compared with surrounding shores. The density of Cellana on both shores showed seasonal changes, with a maximum in summer, which is consistent with the known period of recruitment in Cellana. The lack of seasonal changes in density of Bembicium is briefly discussed. This type of multifactor analysis identifies biological interactions affecting variation in density of these species. It also corroborates the results of other ecological investigations and determines the relative importance of a variety of physical and biological factors in the distribution of density of intertidal gastropods.

The rates of growth of two common intertidal gastropods, Nerita atramentosa Reeve and Bembicium nanum (Lamarck), were measured during ≈ 3 months in cages at four heights on a shore at Cape Banks, New South Wales. Replicate cages at each level were used to keep snails at constant densities throughout the experiments. Amounts of food on the substratum in each cage were measured as the concentration of chlorophyll in samples of the rock, which is known to be a reliable estimator of number of algal cells present. Rates of feeding were measured as the number of radular scrapes per min in snails removed to the laboratory, and the sizes of radulae were measured at the end of the experiments. Nerita did not increase in shell-length at the two higher levels on the shore, but grew at the two lower levels. Rate of growth was positively correlated with the amount of chlorophyll in the substratum, and was not apparently affected by the greater period of submersion (and concomitant time available for feeding) towards the lower levels on the shore. There was no difference in rates of feeding at the four heights. Radulae of snails from the lower levels were longer than those from higher on the shore, but this was attributable to the greater overall size of snails from lower levels (i.e. there was no significant difference in relative size of radula from one height on the shore to another). Bembicium increased in width of shell at all levels on the shore, but grew more quickly at all three lower levels than at the top of the shore. There was no difference in rate of feeding, nor in relative size of the radula from one level to another on the shore. Rate of growth was not significantly correlated with the amount of food available, nor with the period of submersion, nor the product of these two variables. The simplest explanation for these results was that sufficient food was available for maximal growth of Bembicium at the three lower levels, but not at the highest level. Thus, only where the amount of food was inadequate, at the highest level, was rate of growth reduced. These results are discussed with reference to the difficulties of determining the relative importance of influences on the growth of intertidal snails, because of inherent confounding of the period of submersion with different amounts of food at different heights on the shore.

The shell selection behaviour of Coenobita scaevola Forskal has been examined in a natural population by statistical methods and by selection experiments using shells whose weights had been artificially changed. In a natural population of C. scaevola there was a definite weight index (weightof the shell/weight of the animal) for the shells of each species of snail. The weight index for one population ranged between 0.7 and 13.0, with a maximum (46% of all values) between 1.3 and 1.9. The volume index (volume of the shell/weight of the animal) ranged from 0.7 to 4.0. The maximum (59%) for the shells of all snail species lay between 1.0–1.3. Small specimens of C. scaevola tolerate greater differences in the weight and volume indexes of their host shells than large specimens. A comparison between weight index and volume index shows that shell selection depends on volume. This is probably also true for a small hermit crab, Clibanarius signatus Heller, of the tidal zone. Selection experiments on the weight index showed maximum value for Galeodes of about 1, for Turbo at about 2, and for Nerita between 2 and 4. Selection experiments on the volume index showed that, independent of the species of the occupied snail shell, there was a maximum between 1.0 and 1.5. In selection experiments between shells of different snail species but of equal volumes the selection was made in regard to their weight indexes. At a weight index lower than 1.0 only Natica was preferred, between indexes 1.0 and 1.5 Natica and Galeodes were similar, while at a weight index of 2.0 or more Turbo was chosen. The shells of those snail species which are generally used by Coenobita scaevola are found in the same frequency in the snail fauna of a place as they are occupied by C. scaevola. With the exception of a few species of aquatic hermit crabs which are characterized by flat thoraxes and which live in shells with a long and narrow opening, all other water and land hermit crabs seem to have no relation to the shells of a given snail species. C. scaevola e.g., lives in the shells of 29 snail species. Broken shells or shells with a fouling of calcareous algae are better tolerated by water hermit crabs than by land hermit crabs. Land hermit crabs also live in fossil and subfossil shells.