P. Della Santina

Food availability and diet composition of three coexisting Mediterranean limpets (Patella spp.)

The gut contents of three intertidal patellid limpets were analysed by collecting foraging specimens on a breakwater on the Tyrrhenian coast (central Italy) between May 1988 and October 1989. The three species coexist there showing a different, but partially overlapping zonation: Patella aspera dominates the infralittoral fringe; the majority of P. caerulea inhabits the lower midlittoral, while P. rustica is most abundant in the upper midlittoral. The algae present on slivers of substratum over which each limpet collected was moving were identified. Moreover, floristic surveys were made along the shore in order to characterize the algal cover of the different zonal belts. The floristic study revealed that the basic elements of algal communities typical of western Mediterranean rocky shores are present in the study area. The algae found on the slivers under the foraging limpets were generally representative of the algal community typical of the same zone. There was a marked difference between the diets of P. rustica and P. aspera due to the fact that the first species forages on a few low lying epilithic and endolithic Cyanophyceae, while P. aspera feeds on a large number of species belonging to all the main algal classes and life forms considered, including frondose epilithics and epiphytics. The diet of P. caerulea resembles that of P. aspera in algal heterogeneity, but is dominated by Cyanophyceae as in P. rustica. A detailed analysis of the differences between gut contents of each limpet species and the relative slivers showed an obvious general correspondence, but revealed also that the diets of the three species do not completely reflect the availability of algae. These findings suggest that the basic diet segregation mechanism between the three populations is their zonal separation. However, the difference in gut contents of heterospecific limpets foraging in the same zone suggests the existence of supplementary morphy-functional or behavioural mechanisms for diet segregation between the three species.

Trail following in the chiton Acanthopleura gemmata: operational and ecological problems

The Indo-Pacific, intertidal chiton Acanthopleura gemmata (Blainville) is a central place forager which occupies definite, actively dug and defended scars, and migrates during nocturnal low tides up to 3 m away to feed on algal grounds. After each feeding excursion, most chitons home precisely to their scars. Photographic tracking, using lightemitting diodes (LED) glued to the chitons, and experiments were conducted throughout 45 d (August to September 1984) along the Nimu Peninsula (Somalia) in order to elucidate the importance of trail following in the homing and feeding of this species. LED tracking of the chitons throughout each activity phase and trail interruption experiments showed that following the personal outgoing trail is a basic homing mechanism in this species. Translocation of the chitons on a conspecific trail and homing performance analysis revealed that the trail-associated information involved in the homing is polymorphic in the population, thus minimizing the probability of following a conspecific trail despite the frequent trail crossings due to high population density. The LED tracking throughout successive activity phases showed a high coincidence between the paths traced by the same chiton night after night. In the few cases when the outgoing and return paths of the first night markedly diverge, the home-feeding ground route of the successive night overlaps the return trail of the previous night. A less pronounced inter-individual trail following also occurs during foodward excursions. These findings suggest that the trails released by A. gemmata are complex, including both quasi-individual and species-specific information. Moreover, this chiton may utilize its trail following capacity not only to home, but also for the retrieval of the feeding grounds night after night.

Route Selection in The Foraging of Patella Vulgata (Mollusca: Gastropoda)

The prosobranch limpet Patella vulgata is an intertidal grazer performing looped excursions centred on a home scar. Foraging within each excursion is mostly concentrated around the point of maximum distance from home. The orientation of the foraging excursions in a group of limpets was analysed on a vertical substrate on a sheltered shore in North Wales. The spatial relationship between different excursions of the same individual was also analysed. A total of 174 complete excursions from 47 adult limpets were obtained using the LED (light emitting diode) tracking technique. In particular, the leaving direction and the direction of the main foraging area of each excursion were computed. When considering the foraging strategy over five consecutive days, a substantially radial cropping pattern was evident at both the population and individual level, with no evident directional preference. However, when considering consecutive excursions of the same individual a concordance in leaving directions was evident in about 40% of cases. The high overlap between the outward branch of the trajectory of one night and the homing branch of the previous one suggests that the directional decision can be based on a trail-following mechanism. Moreover, our data suggest that directional decisions are taken at the beginning of each excursion when leaving home.

The role of trail following in the homing of intertidal chitons: A comparison between threeAcanthopleura spp.

Three species of intertidal tropical chitons observed between 1984 and 1986, exhibit different movement patterns:Acanthopleura gemmata (Indian and Pacific Ocean) is a strict homer;A. granulata (Caribbean Sea) is mostly a ranger but sometimes shows homing behaviour;A. brevispinosa (Western Indian Ocean) alternates homing and ranging for different excursions. The three species have been studied in the field using a motographic technique to assess the importance of intra-individual trail following in homing. The fraction of the path which overlaps with earlier segments of an individuals trail is different in the three species: when returning from feeding groundsA. gemmata follows almost the whole of the outgoing path; inA. granulata the path coincidence is minimal, whileA. brevispinosa shows intermediate trail following values. InA. gemmata andA. brevispinosa trail overlap rises rapidly just after feeding, and trail following is mostly backward along the outgoing path. On the other hand,A. granulata shows a linear increase in trail coincidence with increasing path covered and the proportion of forward trail following is relatively higher. Nevertheless, all specimens of the three species which show a high homing performance invariably show a large trail following also. These findings confirm that, despite the interspecific difference in ecology and the related variation in spatial organization of foraging excursions, trail following is the basic mechanism for homing in intertidal chitons.

A motographic analysis of foraging behaviour in intertidal chitons (Acanthopleura spp.)

The behaviour of two intertidal chitons (Mollusca: Polyplacophora) from the western Indian Ocean (Acanthopleura gemmata and A. brevispinosa) and one from the Caribbean Sea (A. granulata) were analyzed in order to assess the spatial and temporal organization of their foraging activity. Individual paths were recorded in the field using a motographic method based on LED-tracking and the speed variation during each excursion used to reveal the places where the animals slowed down or stopped for the purpose of grazing. Foraging segments of each trajectory were selected according to a threshold speed which was individually calibrated. The three species of Acanthopleura exhibited graded foraging strategies of different spatial complexity. Acanthopleura gemmata focused its foraging on a few large feeding sites well separated from its permanent home. Acanthopleura brevispinosa exploited more subdivided feeding grounds not distinctly separated from its rest area. Acanthopleura granulata spreads its grazing activity over many small segments, throughout each excursion. Due to different tidal and diel constraints on their activity the three species allocated different absolute time to foraging, but the fraction of time spent foraging relative to the total excursion was similar in each.