Monitore Zoologico Italiano. Supplemento

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RESEARCHES ON THE COAST OF SOMALIA. THE SHORE AND THE DUNE OF SAR UANLE G. Chelazzi & M. Vannini To cite this article: G. Chelazzi & M. Vannini (1976) RESEARCHES ON THE COAST OF SOMALIA. THE SHORE AND THE DUNE OF SAR UANLE, Monitore Zoologico Italiano. Supplemento, 8:1, 161-178 To link to this article: http://dx.doi.org/10.1080/03749444.1976.10736833

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Date: 27 January 2017, At: 11:30

Monitore Zoologico Italiano ITALIAN JOURNAL OF ZOOLOGY PUBBLICATO DALLA UNIVERSITA. DEGLI STUD! DI FIRENZE CON IL CONTRIBUTO DEL CONSIGLIO NAZIONALE DELLE RICERCHE

30 . 10. 1976

N. S. SUPPLEMENTO VIII

NO.

4 : 161-178

RESEARCHES ON THE COAST OF SOMALIA. THE SHORE AND THE DUNE OF SAR UANLE. 9. COASTWARD ORIENTATION AFTER DISPLACEMENT IN NERITA TEXTILIS DILLWYN (GASTROPODA PROSOBRANCHIA) (PUBBLICAZIONI DEL CENTRO DI STUDIO PER LA FAUNISTICA ED ECOLOGIA TROPICAL! DEL C.N.R.: CXIV)

G.

CHELAZZI

and M. V ANNINI

Istituto di Zoologia dell'Universita di Firenze Received 11 June 1976

I. Introduction

page 161

II. Materials and methods

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III. Results

163

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A. Experiment A 1. Controls on the rocky platform 2. Controls on the cliff .

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B. Experiment B.

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Riassunto

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References

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IV. Conclusions and discussion Summary

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I. INTRODUCTION

The orientation mechanisms used by shore living animals to remain

in, or to return to specific zones have been the object of many studies. While Arthropods are undoubtedly the best known group in this respect

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CHELAZZI

and M. VANNINI

(see HERRNKIND, 1972, for a general review), Molluscs have also been given much attention. Several papers have recently appeared on tidal Gastropods, above all concerning the homing behaviour in limpets (for a general review see THORPE, 1956; FuNKE, 1968; CooK, 1969, 1971; THOMAS, 1973; BRANCH, 1975), but a satisfactory answer for many of the questions has to be found. NEWELL ( 1958a, 1958b) studied the dynamic maintenance of shore position in the rocky-shore gastropod Littorina littorea L. and also reviewed the preceding literature. Experiments on the orientation of specimens displaced from their optimal zone have been conducted on L. littorea (ALEXANDER, 1960) and L. punctata Gmelin (EVANS, 1961). More recently WARBURTON (1973) investigated orientation in Nerita plicata L. on Kenyan coasts. During the course of research on the Somalian coast, PARDI et al. (1974) and VANNINI (1975a, 1975b, 1976) studied behavioural aspects of some littoral Crustaceans and, simultaneously, we undertook an analogous examination of some intertidal Molluscs. At Sar Uanle, large colonies of Nerita textilis Dillwyn inhabit the zones which are more exposed to wave motion. The species demonstrates a well-defined, dynamic zonation. During their rest periods some animals are found scattered about 100 em above the mean spring tide high water level (MHWS) while others are found clustered below in various-sized groups at particular points on the rocky cliff. They fhow a pronounced fidelity to these points which are submerged by the rising tide. During low waters (expecially those occurring between about 12:00 and 24:00 hr) the animals conduct downwards feeding excursions and can even reach the base of the rocky cliff which is the lower limit of their feeding range (VANNINI & CHELAZZI, in preparation). However, during LW, some specimens were occasionally seen heading back to the cliff from various distance on the rocky platform. In our opinion, these animals were probably washed off the cliff during a preceding HW. If this hypothesis is correct, N. textilis must possess orientation mechanisms enabling the animals to return to a specific zone after being passively displaced. The study of this phenomenon and the mechanisms involved are the object of this paper. A second point of interest is the definition of zones and distances at which the homing capacity showed by N. textilis during feeding excursions on the cliff persists (VANNINI & CHELAZZI, in preparation). We wish to thank Prof. L. PARDI, Director of the Centro di Studio per la Faunistica ed Ecologia Tropicali of the Consiglio Nazionale delle Ricerche, for his invaluable criticism of this manuscript.

ORIENTATION IN NERITA TEXTILIS

163

II. MATERIALS AND METHODS

Locality. Field observations and experiments were conducted on the shore of the Indian Ocean at Sar Uanle in southern Somalia (0°29'48"S42025'30"E) in July and August 1975. Along this section of the coast,

Fig. 1. - Outline of Expt. A. Animals were collected from points C,." and released at Re, on the seaward edge of the rocky platform. The theoretical homing direction is indicated for each lot (black arrows). The arrow joining Re and C2 is perpendicular to the mean coast line between C, and C,. A cross-section of the coast at Re-C 2 shows the mean level of high spring tide (MHWS) and low neap tide (MLWN).

sandy stretches alternate with rocky cliffs. The cliffs are about 3.5 m high and drop directly to a rocky platform which extends for about 20 m seaward and is usually completely visible at LW (Fig. 1 ). VANNINI et al. (in preparation) provide environmental and meteorological data for this zone.

Animals. Specimens of Nerita textilis Dillwyn, longer than 1 em, were collected during LW, marked with water-resistant paint and immediately released. Apart from the size, the selection of the animals to be tested was made completely at random.

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Experiments. Two experiments were made, one measured the capacity of the animals to orient towards the coast after being passively displaced and verified whether this performance was due to directional or homing orientation (Expt. A), and the other sought to determine whether N. textilis oriented by direct or indirect cues (Expt. B). In Expt. A, 300 animals collected from three different sites were released from the same point and their position from the release point and direction of movement (i.e. azimuth of the body axis in active animals) subsequently registered. In Expt. B, photographs were taken of the prints left by 77 specimens on a sandy substratum. Data analysis. Tests were used to analyze the bivariate and circular distributions which are described in BATSCHELET ( 1965, 1972 ). The nonuniformity of the distributions was tested according to the Rayleigh (azimuth distributions) and Ajne's (non-unitary vectors distributions) tests. Orientation towards a definite direction was analyzed according to the V test. The circular distributions were compared by applying the Watson & Williams parametric test. Non-parametric tests were not used due to the numerous intersample ties caused by registering the directions to the nearest 5°. The linear distributions were analyzed according to Y.. 2 test.

III. RESULTS

A. Experiment A This experiment measured the capacity of N. textilis to return to the coast after being passively displaced and verified whether it uses directional or homing orientation. Three lots of 100 animals each were tested (Fig. 1 ). The first lot came from C in the direction of 254° at 46 m from the release point (Re), the second from C~ in the direction of 335° at 16 m from Re and the third from Ca in the direction of 57° at 50 m from Re. A stright line joining Re and C 2 is pependicular to the mean coastal line between C1 and Ca. Re is situated on the seaward edge of the rocky platform which extends from the base of the rocky cliff (see section Re-C in Fig. 1 ). The animals were collected, marked, taken to Re and released simultaneously at 11:32 hr on August 8, a few minutes before the turning tide (LWo) covered the platform. 1. Controls on the rocky platform. The first control was made during the following LW (LW 1 ) at about midnight, registering the direction and distance from Re of each of the 164 specimens located and the direction of the body axis of the 55 active individuals with foot and head out of

ORIENTATION IN NERITA TEXTILIS

Fig. 2. -

165

The average vectors of recovery for each lot of animals at LW, (white arrows) and LW, (black arrows).

the shell. The same data were registered at LW 2 (diurnal) where 107 animals were located of which 42 were definitely active. The average vectors of recovery were calculated for each lot and show a marked homogeneity in the selected direction of each lot (Fig. 2). Ajne's test given P