First report of Phoronis ovalis from Africa and its effect on mussel hosts

Abstract

Phoronis ovalis is a cosmopolitan, shell-boring phoronid worm reported from 24 locations worldwide in temperate latitudes, but not previously from Africa. We identified a shell-boring phoronid in Namibia that is morphologically similar to P. ovalis and subsequently surveyed its latitudinal and tidal elevational range, host distribution, and evidence for long-term occupation of this shoreline. Phoronis ovalis in Namibia leaves characteristic burrows in its hosts (0.2 mm diameter), primarily the native brown mussel Perna perna. In all, eight additional host species were identified, including one barnacle, four gastropods and three bivalves. The distribution of P. ovalis was strictly subtidal, where it reached 99% prevalence in P. perna at some sites. Latitudinally, it occurs at least from the northern border of Namibia (17.4° S) to Walvis Bay (22.74° S). Its long-term presence was evident in subfossil shells. We hypothesised that extensive shell-boring could be energetically costly to the host due to the need for ongoing shell repair. Perna perna with higher phoronid infestation made thicker shells, which were less dense. In addition, colonised mussels had lower body condition (dry meat weight relative to internal shell volume), which implies a significant energetic cost to the host.