Calcified shells provide hard substrate which serves as habitat for a wide range of symbiotic marine organisms. A subset of these interactions are characterized by organisms that form burrows in the host shell matrix.  The effects of shell-boring organisms may be negative by weakening the shell, therefore making the host more susceptible to predation and catastrophic shell failure from impacts. An energetic cost to the host of continuously repairing shell is a logical outcome of these symbioses.

Phoronis ovalis (Wright 1856) is a cosmopolitan, shell boring Phoronid reported from 24 locations worldwide in temperate latitudes. It has not previously been reported from Africa. Our discovery of P. ovalis in Namibia prompted the following questions: What is its Namibian latitudinal range and elevation range (intertidal / subtidal)? What host species are colonized? Is it a new introduction, or simply a new report of a long-established species? What is its prevalence and density in host shells? What are its effects on the morphology and condition of the dominant host species in this range, the brown mussel Perna perna (Linnaeus, 1758)?

To address range, prevalence, density, and host species diversity, we sampled live (in-situ or beach-cast) mussels from 16 rocky intertidal sites between Luderitz (26.64°S), and the Kunene River mouth (17.26°S) and also recorded any other shells with characteristic phoronid boring. At two sites along this range, we found older mussel shells with similar boring patterns in upland sediments covered by 1-3m of silt, sand, and mature vegetation.

To assess impacts of P. ovalis on host morphology and condition, we collected 99 live P. perna. Shells were assessed for extent of phoronid infestation and processed to measure whole fresh weight, fresh shell weight, dry weight, and dry weight of meat. We then calculated investment in shell, porosity of the shell, and mussel condition.

Results/Conclusions

Evidence of P. ovalis was found at all sites, with prevalence reaching 99% infestation of P. perna  in subtidal mussels. Nine host species were identified, including one barnacle, four gastropods, and four bivalves. Phoronids caused mussels to increase shell thickness, but also reduced the density of  shell structure. In addition, there was a significant negative relationship between mussel condition and the level of phoronid infestation which implies a measurable energetic cost to the host. Given the extended time that P. ovalis has apparently been present in Namibia, it is remarkable that this ecologically-important and abundant species has previously gone unnoticed.