Marine symbiotic Palaemonidae, comprising over 600 species, live in association with marine invertebrates of different phyla, like Cnidaria, Echinodermata, Mollusca, Porifera, and Tunicata. A phylogenetic study is performed on a clade of bivalve- and ascidian-associated endosymbiotic shrimp species (Caridea: Palaemonidae), using morphological and molecular data. A Total Evidence approach is used in order to include all currently known ingroup species in an evolutionary framework. Ancestral state reconstruction analyses are performed to identify host-switching events and ancestral ranges. The clade, including Ascidonia, Conchodytes, Dactylonia, Odontonia, and Pontonia, and various smaller genera, is recovered as monophyletic, with an ascidian-associated ancestral host state. At least six interphylum host switches are tentatively identified, with members of Odontonia and Notopontonia switching back to an ascidian host affiliation after the ancestral host switch of the clade including Conchodytes, Odontonia and related genera, from an ascidian- to a bivalve host. The clade including Ascidonia and Pontonia was recovered to have an ancestor with an East Pacific/Atlantic distribution. The other studied genera remained in the original ancestral Indo-West Pacific range. We hypothesize that similar internal environments of shrimp hosts from different phyla will function as hot spots for interphylum host switching in various lineages of symbionts.

A new species of bivalve mollusk dwelling palaemonid shrimp of the genus Odontonia is described from the Berau Islands, East Kalimantan, Indonesia. This is the only Odontonia species recorded as a symbiont of a bivalve mollusk, all other congeners are known to associate with solitary ascidians. The new species belongs to the group wherein the dactylus of the ambulatory pereiopods has an accessory tooth. It differs from these species in the absence of a forward directed proximal tooth on the flexor margin of the corpus of the ambulatory dactyli. It also lacks the small denticles posterior to the distoventral accessory tooth on the dactylar corpus.