Biometric differences between date mussels Lithophaga lithophaga colonizing artificial and natural structures

Abstract

          Shell width and body live weight related to shell length of the endolithic bivalve Lithophaga lithophaga (date mussel) colonizing a specific habitat (vaults under boulders) formed by artificial and natural structures were examined. Artificial structures consisted of limestone boulders of a breakwater (Marina Rovinj, northern Adriatic Sea, Croatia) constructed 19 years before sampling of the date mussel. Date mussels’ density (around 80 individuals per 0.1 m²) did not differ between the two types of structure. However, the length frequency distribution in artificial structures (25^th percentile = 3.20, median = 4.30 and 75^th percentile = 5.10 cm) differed from that in natural structures (25^th percentile = 3.66, median = 5.15 and 75^th percentile = 6.20 cm) leading to a substantial difference in total biomass (0.3 and 0.8 kg per 0.1 m² for artificial and natural structures, respectively). Parameter estimates of regression functions for width against length (linear function) and for live weight against length (allometric function) also significantly differed, indicating variations in date mussels’ morphometry between the two types of structure. Analyses of variance did not detect differences in width or weight for date mussels in the length range from 3 to 3.5 cm. However, width (average ± s.d., n = 18) of individuals in the range from 5.5 to 6 cm was significantly lower in artificial structures (1.46 ± 0.13 cm) than in natural structures (1.66 ± 0.10 cm). Consistent with this, live weight in artificial structures (8.36 ± 1.17 g) was significantly lower than that in natural structures (12.33 ± 1.48 g). It is suggested that these patterns reflect a growth rate of the date mussel that is higher in artificial than in natural structures. Information about date mussels’ biometric patterns in different habitats is important in planning studies assessing the resilience capability of natural populations after illegal destructive harvesting, particularly as,such studies are lacking.