Landmark-Based Geometric Morphometric Analysis of Brown Mussel, Perna perna (Linnaeus 1758), in South African Bay and Non-Bay Areas

The topography of the 3650 km South African coastline is not uniform but comprises bay and non-bay areas consisting of wave-sheltered and wave-exposed habitats. Site and habitat differences have been found to influence the size, shape, and behaviour of mussels along the South African coastline. Although quantifying methods for mussel size have been consistent, different methods have been employed to quantify mussel shape. We, therefore, investigated the influence of site and habitat environmental condition differences on P. perna size and morphometry. Mussels in sheltered and exposed habitats from bay and non-bay areas were measured for shell length, width and height before a landmark-based method was used to outline shell shape variations per mussel specimen. Corresponding environmental parameters were also recorded per site and habitat and overlaid with mussel size and shape data. Mussels in bays had larger shell dimensions than those in non-bays with shell length influencing both the width and height of mussels across sites and habitats. Mussels in bays were more obese than those in non-bays. Mussels in bays were found in deeper water than in non-bays with mussels in sheltered habitats found deeper than those in the exposed habitats. The depth at which mussels were found was negatively correlated with water chlorophyll, pH, conductivity, salinity, and oxygen. Overall, the environmental conditions; mussel size and shape were influenced by habitat type with significant differences between sheltered and exposed habitats. The landmark approach was effective in distinguishing mussels from sheltered versus exposed habitats in bay and non-bay areas. These findings are applicable to mussel populations beyond the South African waters.