Members of the family Lasaeidae, commonly known as kellyclams, are small thin-shelled bivalves that often live in close association with larger invertebrates such as crustaceans, worms or cnidarians. The clam may be directly attached to its host or share a burrow with it; one genus, Entovalva, includes associates of sea cucumbers that live within their host's esophagus (you can find some more details of this particular relationship here). Though the clam's presence may not be entirely without physical effect on its host, such effects are usually minor and kellyclams are generally regarded as commensals rather than parasites. Other species live independently and may live nestled among rocks or buried in sediment; these free-living forms may possess a relatively large muscular foot for mobility. Like most bivalves, kellyclams are filter feeders; the invertebrate-commensal species are believed to take advantage of water currents created by the host to increase the effectiveness of their own feeding currents.
The family-level classification of the kellyclams has been fairly turbulent. The family has variously been known as the Lasaeidae, Erycinidae or Leptonidae, owing to confusion over which of these names has priority, while some authors have regarded them as separate families and/or recognised further segregate families Kelliidae or Montacutidae. With their small size, kellyclams have simplified a number of the characters used in classifying other bivalves, and a number of the commensal species have become modified in order to co-exist with their host. The Peregrinamor species, for instance, live attached longitudinally underneath the thorax of the ghost shrimp Upogebia; they have accordingly become low and elongate, and were until recently misclassified as mussels of the Mytilidae (note that the Lasaeidae and Mytilidae represent evolutionary lineages that first diverged some time in the Ordovician). In the broad sense, the Lasaeidae have been separated from the closely related family Galeommatidae by the fact that members of the latter have the soft body enlarged so that the shell becomes internal. However, Goto et al. (2012) established that even this distinction is not reliable, with the Galeommatidae nested and probably polyphyletic within the Lasaeidae. Also, while Goto et al. did not support recognition of the Lasaeidae as a holophyletic group, nor did they support any of the segregate families. Commensal species are scattered phylogenetically among free-living species, and host-switching has apparently happened numerous times within commensal lineages.
The Lasaeidae are hermaphrodites, and often brooders. Rather than being released as eggs, young are retained within their parent until they are released as veligers (later-stage larvae that have begun to develop a shell) or as miniature versions of the adults.
Goto, R., A. Kawakita, H. Ishikawa, Y. Hamamura & M. Kato. 2012. Molecular phylogeny of the bivalve superfamily Galeommatoidea (Heterodonta, Veneroida) reveals dynamic evolution of symbiotic lifestyle and interphylum host switching. BMC Evolutionary Biology 12: 172.