Complex sentence: Horse clams often have a relationship with small commensal pea crabs, Pinnixa faba, often a mating pair, which enter through the large siphon and live within the mantle cavity of the horse clam.
Split into simpler sentences: Horse clams often have a relationship with small commensal pea crabs of the species "Pinnixa faba". These are often a mating pair which enter through the large siphon and live within the mantle cavity of the horse clam.