Population structure of Cerastoderma glaucum and Abra ovata in Vouliagmeni Lagoon (Attiki) E. Gontikaki, C. Antoniadou and C.C. Chintiroglou* Department of Zoology, School of Biology, University of Aristotle, PO Box 134, GR-540 06 Thessaloniki, Greece. *Corresponding author, e-mail: chintigl@bio.auth.gr Two species of bivalves, Cerastoderma glaucum and Abra ovata, typical inhabitants of brackish waters, were found in Vouliagmeni Lagoon. Seasonal qualitative and quantitative samples were extracted from the di¡erent types of the substratum during 1997^1998. Overall, 800 individuals of C. glaucum and 2700 individuals of A. ovata were collected and measured and their population structure studied. INTRODUCTION Two bivalve species, Cerastoderma glaucum and Abra ovata, recorded in Vouliagmeni Lagoon (surface area 4000 m 2 , Attiki, Greece). Both these species are typical inhabitants of brackish waters (Barnes, 1980). Bivalves can be useful indicators of the environmental condition of brackish water lagoons (Breber, 1996). In the present study, the population structure of both species was studied and the results discussed in relation to the tourist activity in the study area. MATERIALS AND METHODS Seasonal samples (07/1997, 10/1997, 01/1998 and 04/ 1998) were collected with SCUBA diving at di¡erent sites throughout the lagoon during 1997^1998. The maximum depth of the lagoon is 13m. Samples were collected from four di¡erent lagoon habitats (see Table 1). Samples from the soft substrate were taken with a corer (2015 10 cm¼3.180 cm 3 ). Nine and three random samples were taken from the upper (0.5 m) and lower parts (5^7 m) of the infralittoral zone, respectively. Samples from the hard substrate (4^8 m) were collected with a 2020 cm 2 sampler. Six random samples were taken seasonally from the hard substrate zone, which is restricted to the north side of the lagoon. In addition, three semi-quantitative random samples of seaweeds were collected from the deeper parts of the lagoon (3^5m). Each sample ¢lled a 5-l container. All samples were preserved in a 10% formalin solution. Samples were sieved through a 1-mm sieving net. Consequently dead individuals were separated from living ones, counted and all living individuals were measured using an electronic Vernier calliper (Mitutoyo, UK). The following morphological parameters were measured: (a) shell length (L, in mm); (b) shell height (H, in mm); and (c) total wet formalin weight (W, in g). The relationship between L and H was described using linear regression whereas that between W and L using the exponential regression (i.e. W¼ aL b ) after logar- ithmic transformation (LogW¼Loga þbLogL). Length^ frequency distributions were constructed, using a size-class step and analysed using Bhattacharya’s method. Mean abundance by habitat type was compared using one-way analysis of variance (ANOVA) after log transformation. RESULTS AND DISCUSSION Overall, 800 individuals of Cerastoderma glaucum were collected and measured. This species was found on all substrate types with the exception of LPI (Table 1). Shell length ranged between 3.00 and 30.78mm. The relationships between L and H and W and L were: H¼70.034 þ 0.7995L ( r 2 ¼0.99, N¼628, P50.05) and LogW¼73.996 þ 3.244LogL ( r 2 ¼0.99, N¼628, P50.05). Although abundance was generally higher in winter, seasonal di¡erences in abundance were not statistically signi¢cant (ANOVA, for all cases, P40.17) (Table 1). The size^frequency histograms clearly revealed that recruit- ment takes place mainly in autumn (Figure 1). Few recruits were also recorded in winter (Figure1), a fact indicating a rather prolonged reproductive activity (i.e. from spring to early summer). Separation of length frequencies into normal distribution components was possible only for the winter and spring samples, with two normal components identi¢ed for each season (Figure 1). The population almost disappeared in summer when only four individuals were caught (Table 1). Overall, 2700 individuals of Abra ovata were caught. Its distribution was limited to the UPI. A small number of indi- viduals collected from the soft substrate of the sublittoral zone were dead. Although abundance was higher in winter (Table 2), a fact that could be attributed to the limited number of bathers during that period, seasonal di¡erences in abundance were not signi¢cant (ANOVA, F¼1.54, P¼0.18). Shell length ranged between 2.00 and 8.86 mm. The size^frequency histograms showed that recruitment was generally observed throughout the year (Figure 2), a fact suggesting that reproductive activity is also continuous throughout the year. Separation of length frequencies into normal distribution components was possible only for the summer and spring samples, with one normal component identi¢ed for each season (Figure 2), suggesting longevity of one year. The absence of individuals with shell lengths larger than 8 mm in summer could be attributed mainly to natural mortality. The relationships between L and H and L and W were: H¼70.2906 þ 0.7833L ( r 2 ¼0.8817, N¼1835, P50.05) and LogW¼73.7953 þ2.9639 LogL ( r 2 ¼0.8691, N¼1835, P50.05). Cerastoderma glaucum and A. ovata both tolerate a wide range of salinities, ranging between 3 to 61 psu for the Journal of the Marine Biological Association of the United Kingdom (2003) J. Mar. Biol. Ass. U.K. (2003), 83, 1095^1097 Printed in the United Kingdom