The Evolutionary History and Phylogeny of the Lithodinae (Decapoda: Anomura: Lithodidae)

Abstract

The anomuran sub-family Lithodinae comprises a great diversity of morphological and
ecological forms, whose global radiation has not been specifically addressed since the
modern syntheses of plate tectonics, oceanography, species theory and cladistic
systematics. The focus of this thesis was to investigate the origin and radiations of the
deep-sea Lithodinae as a case study for interchanges between deep and shallow oceans
in mobile benthic fauna. Molecular sequences were obtained from six genes (for 47
species belonging to 10 genera of Lithodidae) and different aspects of morphology were
examined in order to identify nested monophyletic groups based on shared, derived
characteristics. The hypothesis that lineage-specific temperature tolerances influence
the distribution of deep- and shallow-water groups was tested by examining habitat
alongside phylogeny.
Lithodid ancestors are likely to have had a north Pacific, shallow-water distribution
and planktotrophic larvae. Some shallow-water populations of Lithodidae are tied to
locations north of 30°N because of the restricted thermal tolerance of pelagic larval
stages; however, life-history changes allowed the subfamily Lithodinae to expand
through the global deep sea, where they are now living at the frontier of their lower
temperature threshold in the Southern Ocean. Phylogenies indicate the importance of
large-scale dispersals within deep-sea groups, linked to the cold deep-water currents
that connect the major oceans. The subfamily Lithodinae includes examples of at least
two genera in which diverse morphologies have arisen within the deep ocean in the
absence of discernable barriers to gene flow. Adult migration and larval dispersal
partially explain the widespread occurrence of the Lithodidae, but this does not indicate
that lithodids roam the ocean depths unconstrained by physical or chemical conditions.
Climate change throughout the Cenozoic has substantially altered the marine
environment and shaped the distribution and radiation of the extant Lithodidae. In the
forthcoming years, measurable changes in ocean temperature, ocean currents and
benthic habitat will affect the distribution of the lithodids and the communities they live
in, as they have in the past.

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