Evolutionary traits in Perna viridis (Linnaeus) and Septifer virgatus (Wiegmann) (Bivalvia: Mytilidae)

Abstract

Perna viridis (Linnaeus) and Septifer virgatus (Wiegmann) are widely distributed mytilids but only rarely do their local distribution patterns overlap. Thus, whilst P. viridis occurs predominantly in sheltered environments in water which is often highly turbid and sometimes even heavily polluted, S. virgatus is more typically confined to cleaner wave-exposed habitats. S. virgatus possesses a broader more robust shell and stronger byssal attachment than P. viridis, characteristics well suited for life on high energy rocky shores; the shell of S. virgatus is also much flatter ventrally, a feature which in epibyssate taxa such as mussels is generally considered to enhance physical stability. Although weaker mechanically, P. viridis produces copious amounts of mucus and has unusually large labial palps with strong ciliary rejection tracts, features more appropriate for coping with the high sediment loadings often associated with sheltered, low energy habitats. P. viridis also possesses a larger more mobile foot, and crawls more readily than S. virgatus. In particular it exhibits a marked propensity to climb vertically upwards, a response which would effectively elevate this species above any accumulated sediment; S. virgatus, by contrast, does not exhibit this behavioural response. The production of byssus threads and their distribution around secured mussels held under calm and turbulent flow in laboratory aquaria were highly variable though significant interspecific differences are reported. These mytilids appear, therefore, to have evolved contrasting suites of structural and behavioural traits which correlate well with a presumed advantage for each species within its preferred habitat. Reciprocal transplantation of these mussels between shores of different wave exposure would help to establish whether or not these evolutionary traits do indeed have adaptive value.