Guido Chelazzi

Behavioral Adaptation to Intertidal Life

The NATO Advanced Research Workshop on Behavioural Adaptation to Intertidal Life held in Castiglioncello, Italy (May, 1987) was attended by 50 participants, most of whom presented requested lectures. It was perhaps the first time that specialists of various animal groups, from cnidarians to birds, were able to meet and discuss the importance of behavioural adaptation to this peculiar, sometimes very harsh environment. But the taxonomic barrier is not the only one which the meeting attemped to over- come. Lately, the research on intertidal biology has spread from pure taxonomy and static analysis of community structure to such dynamic aspects as intra- and interspecific relationships, and physiological mechanisms aimed at avoiding stress and exploitation of limited-resources. This increasing interest stems not only from an inclination for this particular ecological system and some of its typical inhabitants, but also from the realization that rocky and sandy shore communities are suitable models for testing and improving some global theories of evolutionary biology, behavioural ecology and sociobiology. The number of eco-physiological and eco-ethological problems emerging from the study of intertidal animals is fascinatingly large and a complete understanding of this environment cannot be reached using a strictly reductionistic or a pure holistic approach.

A comparative study on the movement patterns of two sympatric tropical chitons (Mollusca: Polyplacophora)

The individual positions of 200 intertidal chitons [Acanthopleura brevispinosa (Sowerby) and A. gemmata (Blainville)] were recorded throughout 55 low tides on a Somalian shore, using a discrete sampling method. Both species feed during nocturnal low tide and rest during the day and high tide. Due to this activity rhythm, individual positions recorded during diurnal and nocturnal low tides give information on the rest habits of the chitons and on their movements, respectively. The resting and feeding fixes were analyzed with two new complementary methods. Both species show a distinct homing behaviour with periodical shift to new homes more frequent in A. brevispinosa than in A. gemmata. The latter species shows a stronger constancy to its feeding ground. The two species differ most in the orientation and length of their excursions. Due to their different movement pattern the two species minimize zonal overlapping during the night, which in turn reduces the interspecific competition for food.

Analysis of Movement Patterns and Orientation Mechanisms in Intertidal Chitons and Gastropods

Despite their different organization and biology, chitons (Mollusca, Polyplacophora) and gastropods (Mollusca, Gastropoda) share a large number of adaptations to intertidal life, including morpho-functional and behavioural traits. Communication, clustering, aggressiveness and even simple parental cares have been reported in both classes but, as in other animals, the basis of their behavioural adaptation to the intertidal environment is a proper organization of activity in space and time.

Competition for the home and aggressive behaviour in the chiton Acanthopleura gemmata (Blainville) (Mollusca: Polyplacophora)

SummaryComputerized screening of all the positions recorded during a synodic month on 120 individually marked chitons (Acanthopleura gemmata) pinpointed their preferential resting points. Unlike the majority of intertidal chitons so far studied, A. gemmata rests in well-defined homes actively dug in the rock. Homes proved to be not strictly individual and periodically interchangeable. A complex aggressive behaviour was recorded in the field when two animals came in contact at home. When competing for ownership of a resting site rival chitons may suppress their nocturnal feeding activity. Despite its strong home-related territoriality A. gemmata showed no mutual exclusion on the feeding area. The highly specialized resting habits clearly protects A. gemmata from its most important predator, the toad fish Arothron immaculatus. The behaviour of A. gemmata is compared to that of other chitons and gastropods, and the current hypotheses concerning the adaptive value of the homing behaviour in littoral molluscs are discussed.

How shore morphology and orientation mechanisms can affect the spatial organization of intertidal molluscs

Homing behaviour and clustering are very common in intertidal molluscs. A large number of observations stressed the importance of trial following in the orientation of each snail to its home and its cluster. The aim of this paper is to provide a mathematical model of the clustering and homing behaviours of Nerita textilis , adopting different hypotheses about the mechanisms employed by snails to find their way back home after each feeding excursion. Moreover, the model is able to simulate similar processes in other species on the basis of their specific behavioural ecology. The results given by the model confirm some hypotheses about the nature of the orientation cues and suggest some new aspects of the adaptive value of trail-following in molluscs.

Cooperative Interactions and Environmental Control in the Intertidal Clustering of Nerita Textilis (Gastropoda; Prosobranchia)

1. Somalian populations of the intertidal gastropod Nerita textilis Gmelin show a vertical separation between resting (mid-upper eulittoral) and feeding (lower eulittoral) zone. During high tides and mostly diurnal low tides some snails remain scattered (SF), while other form aggregations (AF) at lower levels of the rocky shore. During the mostly nocturnal low tides both SF and AF migrate downward to feed on microalgae. 2. Lower and upper aggregations are mostly frequented around neap (NT) and spring tides (ST), respectively. Clustering significantly reduces the exposure to waves during high tide, and probably overheating and dehydration during diurnal low tide. 3. About 90 percent of the total population enters into the cluster system throughout each NT-ST-NT cycle. Clustering begins to increase at NT and falls at ST, reaching a maximum (52 percent of the population) between NT and ST. The relative frequency of transition between SF and AF shows a spike after NT and falls after ST. The inverse transition (AF to SF) is negatively correlated to the amount of AF. 4. Return to the original cluster after one feeding excursion is high (76 percent) and decreases slightly at increasing intervals (about 40 percent after 14 feeding migrations), but it becomes significantly lower after one or more rest phases spent in SF. 5. The field study suggests that the periodical (NT) increase in density at the lower rest-zone (tide-pools belt), long-lasting chemical marking of collective rest sites and releasing mucus trails during feeding excursions cause the clustering. According to this hypothesis, a primer group stops at each aggregation site at NT, mostly returning to it by self-trailing after each feeding migration, while the progressive recruitment of snails from SF is caused by the interindividual trail-following during the return migration from the feeding ground. 8. The same model can explain the clustering behaviour of other intertidal gastropods, though the importance of the different parameters (periodical variation in zonation, marking of rest places, trail-following) probably vary among the different species.

Homing Mechanisms of Intertidal Chitons: Field Evidence and the Hypothesis of Trail-Polymorphism

The use of trail-following for homing in intertidal chitons and gastropods is well documented both in the laboratory and in the field (see Chelazzi et al., 1988 for a review). Recent findings suggest that this orientation mechanism may also play an important role in the retrieval of algal patches by these molluscs. This has been observed in A. gemmata where both intra- and inter-individual trail-following occurs during foodward migrations (Chelazzi et al., 1987b). Nevertheless the behavioural mechanisms and cues involved in trail-following are not perfectly clear, and among these the degree of trail-individuality is of particular interest.

The External Memory of Intertidal Molluscs: A Theoretical Study of Trail-Following

The secretion of a slimy mucus from the foot, besides being mechanically essential for the locomotion of many molluscs, seems to constitute a system of external storage of information used for orientation towards biologically significant goals such as hiding places, sexual partners or prey. Field observations and laboratory experiments have shown that many gastropod species are able to interpret chemical information contained in their mucous trails. In some cases the trail-following mechanism can involve the recognition of “personal” trails and the detection of trail polarization (Wells & Buckley, 1972; Cook & Cook, 1975).

Theoretical analysis of rhythmical clustering in an intertidal gastropod

Abstract As do many other molluscs inhabiting rocky shores, the intertidal snail Nerita textilis Gmelin exhibits very important stress-reducing behaviour, the collective homing to sheltered locations of the cliff via inter-individual trail-following. The aggregated population of snails presents, in this species, a semilunar rhythm reaching maximum around neap tide and minimum after spring tide. This paper proposes a mathematical model of this behaviour, which takes into account the trail-following mechanism, the effect of cliff morphology and the semilunar rhythm. The results show the effects that different population densities and tidal amplitudes have on the numbers of aggregated snails. They permit a discussion of the snails response to the environmental signal. The model reliability is also discussed, using field observations on a Somalian population of this species. Finally, the utility of the model in the estimation of parameter values is considered.

A Mathematical Framework for the Life-Cycle of Plankto-Benthonic Invertebrates

Organisms belonging to several distinct species among plankto-benthonic marine invertebrates tend to select — as location for their sessile adult life stages—suitable discrete habitable sites (‘units’), such as crevices or tide-pools. In this paper we present a brief description of the main behavioral aspects of the taxa that adopt this life-cycle, and we construct a fairly general mathematical framework for the study of their population dynamics.