Settlement of the Tropical Box Mussel, Septifer bilocularis: Effects of Site, Position, and Substratum

The box tropical mussel Septifer bilocularis lives attached to a hard substratum, forming a dense aggregation or patch. The aggregation occurs at the time of larvae settlement in response to different cues. Larvae may have a positive response whereas larvae may settle and attach to the substratum, while in a negative response, postpone settling occurs. Byssus threads are produced and attached to the substratum, and for some reason, they are affected by many variables. We studied whether the attachment was affected by sites, position in the patch, and substratum. Large and small patches of mussel beds were selected arbitrarily, each in 3 replicates. Substrata of palm, coconut, polypropylene fibres, and mussel shells with byssus threads, each in 4 replicates, were distributed randomly into a PVC plate with 16 holes. The PVC plates with experimental substrata were placed along the edge and in the middle of large and small patches. A three-factorial ANOVA was then performed on samples of sites, position in patches, and substratum as main effects. A significant effect of sites, position within patches, and substratum as well were found. A significantly high number of new settlers in small patches than in other positions within patches occurred. The larger number of new settlers was found on coconut, palm, and polypropylene fibres, while low numbers on the mussel shells with byssus threads were found. The new settlers of S. bilocularis (<1 mm) with transparent shells were observed attached to coconut, palm, and polypropylene fibres. In contrast, the new settlers ranged from >1 mm to 3 mm with dark green shells attached to mussel shells with byssus threads. This study suggests that early settlement occurs when mussels’ larvae first settle and attach to coconut, palm, and polypropylene fibres before reattaching to adult shells with byssus threads.