Here are some interesting facts about a horse mussel I picked up on Fish Hoek beach. It is 24 cm long and had a pea crab inside. This crab lives inside the mussel.
The horse mussel “lives partially buried vertically in sheltered mud or sand, with the wide posterior end projecting above the surface. The byssal threads of the mussel were reputedly woven into Jason’s golden fleece of Greek mythology.” From Two Oceans A guide to the marine life of southern Africa by GM Branch, CL Griffiths, ML Branch, LE Beckley
The following extra information is by George Branch of UCT:
“The mussel is the scaly horse-mussel Atrina squamifera. As it ages, it loses its shell-scales and becomes less recognisable; but its rounded end is characteristic. The other species has a square-cut end. The male pea crabs are minute and seldom seen, but they do move between shells to mate. Yes, you are right: this is a female (you can tell by the broad abdomen) and it is in berry.
I can confirm this is a pea crab, genus Pinnotheres. Distinguishing the different species is tricky, and it is likely that we have more than one species in S Africa. The common one is P. dofleini, which occurs in black mussels, but we have found pea crabs in oysters and in horse mussels, and their sizes are so different that we suspect there is more than one species in SA.”