Global bottlenecks in the distribution of marine Crustacea: temperature constraints in the family Lithodidae

Members of the crustacean family Lithodidae share preferences for cold-water environments; however, the specific role of temperature in governing lithodid biogeography has not been examined to date. In the present study this relationship was quantified through the analysis of habitat data, and the results were interpreted in the light of previous physiological studies. It was hypothesized that lineage-specific temperature thresholds underlie differences in the distribution of the two lithodid subfamilies. The family Lithodidae is divided into the subfamilies Hapalogastrinae and Lithodinae. The Hapalogastrinae inhabit depths of between 0 and 200 m in the North Pacific. The Lithodinae are distributed globally in the deep sea, with a few genera occurring intertidally at high latitudes. Descriptions of 86 species of lithodids, sampled at 627 locations worldwide, were obtained from a wide range of published and original sources. For each specimen, the water temperature at the time and locality of collection was recorded. Molecular sequence data for the 16S, cytochrome c oxidase subunit I (COI) and 28S genes were analysed to construct a phylogenetic tree for the major lithodid genera, using a maximum likelihood method in the program paup *. Further analyses examined the link between the habitat temperature range and the position of taxa within the lithodid phylogeny. Phylogenetic evidence indicated that the deep-water lithodid lineages had ancestors that inhabited the coastal waters of the North Pacific. Adults of North Pacific lithodid taxa were found in regions where water temperatures ranged from 0 to 25°C; however, deep-water lineages of the Lithodinae were excluded from waters exceeding temperatures of 13°C. Despite the higher temperatures tolerated by adults, North Pacific intertidal/subtidal genera were restricted to regions that had water temperatures lower than 16°C during periods of larval development. Temperature has restricted the range of most shallow-water genera of Lithodidae to the coastline of the North Pacific since the early history of the family. Distribution in these groups remains constrained by the detrimental effects of temperature extremes on early life-history stages. Deep-water lineages moved away from seasonal temperature fluctuations, and underwent at least three radiations into water bodies outside the North Pacific. Species from within the deep-water lineages currently live close to the threshold of their temperature tolerance in the Southern Ocean, and their future distribution may be affected by increases in ocean temperature.