some observations on the biology and the ecology of

J. mar, bioI. Ass. U.K. (1985 ), 65, 889-900

889

Printed in Great Britain

SOME OBSERVATIONS ON THE BIOLOGY AND THE ECOLOGY OF A VENUS STRIATULA POPULATION IN THE BAY OF DOUARNENEZ, BRITTANY J. GUILLOU

AND

P . G. SAURIAU

Universite de Bretagne Occidentale, Institut d'Etudes Marines, Laboratoire d'Oceanographie Biologique, 29283 Brest, France

(Figs. 1-9)

An investigation was made on a subtidal population of Venus striatula (da Costa) in the Bay ofDouarnenezi Brittany, The main parameters of the biological cycle were evaluated, V. striatula showed a reproductive activity through the year, the spawning phases occurring in spring and autumn . They were correlated with twO periods of recruitment in summer and in late fall. Five plurimodal year classes were recognized from the length frequency histograms. Growth rate was high during the first two years, with an obvious lessening in winter. Sexual maturity was reached when the individuals were 15 mm long. The analysis of the annual rings indicated a maximal longevity of ten years, the maximal observed length being 35 mm. The growth curve was adjusted to the von Bertalanffy equation and comparison was made with other published data from the full species geographical range. Spatial distribution in Douarnenez Bay was found to be linked to the bathymetric gradient, and influenced by intra- and inter-specific relationships.

INTRODUCTION

Venus striatula (da Costa) (= Venus gallina L.) occurs in high densities in shallow water on a bottom of clean fine sand in the Bay of Douarnenez. The environmental conditions are characterized by an obvious sediment instability due to a high exposure to westerly swells (Guillou & Le Moal, 1978). This species is preferentially distributed in the fine sands (Petersen, 1913; Thorson, 1957), but is relatively tolerant towards certain ecological factors and so is sometimes found in very muddy sediments. It also shows an extended latitudinal range, since it is recorded from the Lofoten Islands to the Canaries, in the Mediterranean and the Black Sea (Tebble, 1966). Many studies have been made on its biology and ecology. Ansell (1961, 1962) in Scotland, has studied the larval and post-larval development, growth and reproduction. Lucas (1965) has described the main characteristics of its sexual cycle on a population from Morgat, Brittany. Masse (1971) in the French Mediterranean coast, and Warwick, George & Davies (1978) in Wales, have defined the principal aspects of its biological cycle. Studies of Mediterranean small-scale fisheries of V. striatula in Spain and the upper Adriatic have helped establish a rational management of the exploited stocks (Vives & Suau, 1962; Froglia, 1975). This paper gives results concerning the main biological parameters of V. striatula in the Bay of Douarnenez by way of a comparison with other populations at different levels of its geographical range, and some observations are made on its spatial distribution.

890

J. GUILLOU AND P. G. SAURIAU MATERIAL AND METHODS

Sampling was carried out at approximatively monthly intervals throughout the period 1976--9 along a transect localized in the eastern part of the Bay of Douamenez (Fig. 1) from a depth of zero to 18-20 m. Samples were collected using a Smith MacIntyre grab and a dredge, type' Rallier du Baty'. They were sieved on a 1 mm mesh screen, fixed in 4 % formalin, then sorted out and preserved in 70° alcohol. Size of individuals was taken as the length of the antero-posterior axis measured to the nearest 0 ' 1 nun.

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Fig. 1. Localization of the stations on a transect in the Bay of Douarnenez. 1. Fine sands. 2. Muddy gravels. 3. H eterogeneous muddy sands. 4· Coarse sands . 5. Dune sands. 6. Clean gravels. 7. Muddy gravels. 8. Heterogeneous gravels. 9. Sandy muds. to. Rocks.

State of gonad development

This was investigated over a one -year cycle from April 1977 to May 1978 on preserved material. Gonad smears from 26-28 mm reproductive specimens were examined under the microscope in order to define the stage of gonad maturation.

Reproductive activity index Anatomical distinction between gonad and hepato-pancreas being difficult, the evolution of the reproductive activity was observed through the dry flesh weight variations of a standard individual (Trevallion, 1971; Bodoy & Masse, 1978). This standard individual is selected to suppress the results of growth, so that accumulation or loss of organic matter associated with reproduction can be shown. Therefore, the three following size classes were selected from April 1977 to May 1978 : 26-27, 27-28,28-29 mm. Tissues were oven-dried at 80°C over 24 h.

Analysis of the size distributions A visual analysis of the size frequency histograms at each sampling station was first made in order to reveal eventual variations. The population structure was then determined from the monthly

POPULATION STUDY OF VENUS STRIATULA global distributions. Data processing was carried out using the modal analysis developed in Fortran by Tomlinson (in Abramson, 1970, pp. 11 (1 ) 2.1- 2.10) and adapted in Basic for the 9825 A HP computer by Conan & Ganzales (in Gros & Cochard, 1978). The program includes a smoothing-out method of histograms by a mobile averaging process and a discrimination of the overlapping gaussian components using the method of Bhattacharya (1967). Then, in the Normsep program, the principal parameters of each component (mean length with standard deviation and number) are calculated by the Hasselblad method (1966) with X2 test .

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RESUL T S

Reproductive cycle The purpose is to specify the spawning periods and the individual size at the first sexual maturity, in order to establish the relationship between spawning and recruitment. The state of gonad development was assessed by visual inspection of the genital gland, according to the practical scale defined by Lucas (1965). Stage A: the sex is not yet recognizable, the gonad has no proper coloration. Smears show only fragments of tissue not identifiable under the microscope. Stage B: the gland tends to become opaque, but the sexual products remain scarce. Nevertheless, the sex is recognizable by microscopic examination of sexual products obtained in gonad smears. Stage C: the genital gland is opaque, more or less swollen and coloured. The sex is recognizable in V. striatufa, the male gonad is beige and ramified, the female gland is duller and granulous. In all cases, the maturation was confirmed by smear examination under the microscope. During the sampling period, the condition of the V. striatufa gonad showed a great homogeneity in the 26-28 mm individuals. The gonads were swollen and coloured, and the sexual products were abundant. The sex was easily

]. GUILLOU AND P. G. SAURIAU

recognizable: stage C occurred in nearly all the individuals examined from April to November 1977 and from March to May 1978. Moreover, microscopical observations of smears showed that residual free oocytes were present throughout the year cycle, although their number was greatly reduced in November and March. This was confirmed by observations of female gonads in January 1979.

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Fig. 3. Relationship between the relative percentage of occurrence of each stage A. B and C of gonad development, and the size of V. striatula.

Observations of the state of development ofthe gonad did not, however, clearly indicate the spawning period. Consequently, the use of a condition index proved necessary. An analogy was established between the variations of the index in the three selected size classes, and confirmed the reliability of the results (Fig. 2). Two principal drops were observed from April to July and from August to October, suggesting two spawning phases during the year. The period from December 1977 to January 1978 was not sampled, and the decrease of the index was probably due to winter conditions and the use of reserves. Gonad observations made in January 1979 revealed that a winter spawning was unlikely. The increase of the condition index in summer and in autumn suggests a recovery of the gametogenic activity. The size of V. striatula at first sexual maturity is an important parameter in the understanding of the life cycle of the species . The reproductive activity being very strong in spring in Douarnenez Bay, an abundant sample allowing the determination of this parameter was collected in April 1977 (Fig. 3). Stages of gonad development were determined as indicated above. No sexual activity occurred in individuals smaller than 11 mm. From 15-16 mm length, most of the V. striatula were at stage C. All of them were ripe once they attained the size of 19-20 mm. Periods of recruitment Analysis of the size distributions (Fig. 4) suggests two main recruitments per year. The first extended from the end of spring throughout summer. The second one occurred in autumn. The size of the juveniles was variable according to the period they were recruited. This was because of a more rapid summer growth,

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