Speciation in modern marine bivalves (Mollusca: Bivalvia): Insights from the published record

What can living marine bivalves tell us about speciation in the marine environment? Three sets of literature data on Recent marine bivalves are analyzed for insight into the mechanisms behind bivalve speciation processes. (1) A dataset of all marine bivalves described as new to science during the years 2000-2009 (381 species in 135 published papers) reveals that malacologists are still describing undiscovered biodiversity, based largely upon newly collected expedition material. New species include those of both large and small body size (0.86-500 mm, mean 28 mm), from 51 bivalve families, all oceanic basins, and a wide range of water depths (intertidal to 7,333 m, mean 444 m). External shell characters dominate the diagnoses but are increasingly supplemented by anatomical, molecular, and phylogenetic evidence. Endemism is low (2.6%) when stated as such although another 57% of species were described as (thus far) restricted to a particular geographic region, habitat, or both. High percentages of deep-water and otherwise (geographically or ecologically) restricted species, plus several case studies, suggest that physiological specialization, in the form of bathymetric limits, unique dietary adaptations, or host/symbiont associations, plays an important role in setting up barriers to gene flow in marine bivalves. (2) Bivalve species complexes [i.e., closely related, cryptic (possibly sibling) species with obscure morphological boundaries, or highly variable single species] also imply factors involved in ongoing speciation. Seven recently studied marine taxa are presented in which species complexes are either revealed or resolved by molecular data. Apparent barriers to gene flow are in most cases physiological (sympatric), or (in two cases) physical (allopatric), and in one case can be readily broken down by anthropogenic transport. (3) Two recent published phylogenetic analyses are discussed that show (a) disparately sized sister taxa with their synapomorphies (glochidia in Unionoidea; chemosymbiotic bacteria and mucus-tube feeding in Lucinoidea; cruciform muscle and long siphons in Tellinoidea; aortic bulb in Veneroidea) as recognized innovations that facilitated radiation of the more species-rich sister, and (b) polytomies in Lucinoidea that suggest rapid ongoing evolutionary change in several clades. Together these three sets of published data defeat the concept of a Marine Speciation Paradox in bivalves speciation clues are merely subtler in marine bivalves and most often act at the physiological, rather than physical, level.