The « Sponge Finger » is a strange pteriid with a most interesting way of life of being embedded in sponges. This extraordinary sponge-bivalve relationship is obligatory and host-specific with the host being Spongia sp., an undescribed species, and is believed to be mutually beneficial to both animals. The bivalve settles on the sponge, and then remarkably loses the byssus to rely on the sponge for life, becoming endozoic. A cluster of Vulsella serves as an endoskeleton for the sponge to grow, while the sponge covers them and protect them from potential predators. Recent studies further revealed that the sponge builds an internal canal system to take advantage of the bivalve's exhalent flow and increase its own filtering rate -- the two filter-feeding species work together to gather food more efficiently. This phenomenon is commonly observed across the very wide range of V. vulsella across the Indo-West Pacific, southwards from Honshu, Japan; in shallow water around -2~30m in depth. Its shape and pattern is very variable but larger individuals tend to grow long shells resembling a finger, hence its common name. Typical shell length around 70mm., very large specimens may exceed even 115mm. Though Vulsella's true systematic position has always been a topic of debate (some even gave it its own family, Vulsellidae), it was traditionally placed in Malleidae. Recent phylogenetic studies of the superfamily Pterioidea, however, revealed it to be distant from the true Malleidae but closer related to a mixed Pteriidae-lsognomonidae clade. Isognomonidae was thus synonymised with Pteriidae, and Vulsella was moved to Pteriidae.

Vulsella vulsella (Linnaeus, 1758)
18-29 mm. Ovoid, resilifer a small sunken crescent, beaks not widely separated. Anterior shoulder never rising above top of ligament. Light beige, with or without fine radial line. Inhabits a coarse-grained, spiky-surfaced sponge. (25 mm) Distribution: Very rare, northern Gulf of Suez.
Notes; The resilifer is much less conspicuous in V. vulsella than in V. fornicata. It is easier to distinguish the Gulf of Suez specimens of these two species on the basis of the size and shape of the resilifer than by other features.

Bohol Island, Central Visayas, Philippines