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Seasonal growth variations of bivalve Modiolus barbatus: possible aquaculture species?

The bearded horse mussel Modiolus barbatus (Linnaeus, 1758) lives in Mediterranean Sea and from the British Isles south to Mauritania, West Africa. It dimensions are usually from 30 to 65 mm in length and it lives attached by a strong byssus on rocky bottoms from intertidal zone to 110 m deep. In the Adriatic Sea, it is one of the most important edible bivalves and is harvested by divers. This study was conducted to determine growth potential of this species and seasonal differences in growth and to examine its potential for aquaculture. A field experiment was undertaken in a period from May 2005 to May 2006 in Mali Ston Bay, south Adriatic Sea. Every three months about 50-60 M. barbatus were collected by SCUBA and marked with calcein seawater solution. Marked shells were placed in net and suspended from the pier just above the bottom. After three months shells were taken out. Right shell valves were embedded in epoxy resin, sectioned, ground, polished and analyzed through fluorescence microscope. Total of 202 shells were analyzed this way, out of these 21% did not have clear calcein mark. Shell length increase was determined with a micrometer, by measuring the distance between the bright green fluorescent calcein mark and the growing tip. These data were used for construction of a Gulland-Holt plot, where growth rates are plotted against the mean length. Growth parameters were estimated and modified von Bertalanffy equation was constructed. Results indicate that growth of M. barbatus is slow and seasonal. Values of growth constant (k) were highest for sample marked in May and collected in August (0.16), lower for animals kept from August to November (0.13), and lowest in November to February (0.06) and February to May (0.08) study periods. Results show that M. barbatus is slow growing species, taking over eight years to reach lengths of 3-4 cm (Fig. 1). However, calcein method is invasive, posing a shock to the animal that can be manifested in subsequent reduction of growth. Therefore these data should be interpreted with caution and results compared to results of studies employing different ageing methods such as length frequency analysis and analysis of growth lines in acetate peel replicas before M. barbatus potential for aquaculture is evaluated.

bivalve; growth; aquaculture; Modiolus barbatus

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