The Brachidontes pharaonis mussel bed in Birżebbuġa, nine years on

Abstract

The Indo-Pacific mytilid bivalve, Brachidontes pharaonis, has colonised many rocky shores around the Maltese Islands. As far as is known, it has only managed to successfully establish a mussel bed in Birżebbuġa; elsewhere it is found scattered as single individuals or small groups. In Malta, it is primarily a mediolittoral mussel however it can also colonise the uppermost infralittoral zone as well as extend to the upper reaches of the mediolittoral. A previous study made in 2010 (Bonnici, 2011) assessed aspects of the ecology and population biology of the B. pharaonis mussel bed at Birżebbuġa by comparing the biotic and abiotic factors at the bed with those of reference sites. The present study, carried out nine years later, was a follow up to this. The key research questions addressed are whether the ecology of the mussel bed has changed over nine years and what may be the underlying reasons why B. pharaonis only formed a bed in one area and not at the reference site. Similar methods to those of Bonnici (2011) were employed in the present study to ensure comparability of the results obtained. Mussels were collected from quadrats along transects on the shore and sex, size, and weight were determined in the laboratory. The denuded areas were then monitored at bimonthly intervals for signs of mussel recruitment. Changes in abundance over a period of eight months at the mussel bed site and two reference sites were monitored through analysis of photoquadrats. The overall abundance of mussels was found to have declined drastically compared to 2010. This could have been due to several factors, most likely physiological since no obvious changes in the physical environment have occurred in the interim period. Mussel sizes varied greatly between the two studies. In 2010, mussels greater than 20 mm in length were not observed at the mussel bed, but in the present study, these were relatively abundant. From this, it was evident that factors limiting the growth of mussels in 2010, were not operating in recent years. Mussels at the reference site, however, did not grow to these sizes. The sex ratio of mussels at both the mussel bed and the reference site was roughly equal to 1:1 and this did not differ from the 2010 study. Mussel abundance varied seasonally. The number of mussels declined during the autumn and winter months, showing a slight population recovery at the onset of spring. No recruitment was observed over the eight-month period in the denuded transects and these became densely populated by macroalgae. From the results of the present study, it seems likely that the mussel bed at Birżebbuġa will continue to thrive but the population ecology of the mussels here is dynamic and show marked interannual variations.