Morphology and molecules conflict to confound species boundaries in salt lake ostracodes of the genus Mytilocypris (Crustacea: Ostracoda)

The ostracode genus Mytilocypris comprises eight species, and has a widespread distribution in saline habitats of semi-arid Australia. Fifteen populations, representing all eight species in the genus, were analysed at 19 enzyme loci to identify markers to delineate species boundaries. Only five distinct genetic groups were found: M. mytiloides, M. ambiguosa, M. splendida, M. praenuncia, and M. henricae. Three presumptive species, M. mytiloides, M. tasmanica chapmani, and M. minuta have sympatric distributions in Western Australia. The three show similar internal morphology, lack any diagnostic features of colour, pattern or setation of the shell, but differ in the size and shape of the shell. They also show microhabitat divisions along a salinity gradient. There were no genetic markers to distinguish the three Western Australian species from one another, and they showed high genetic similarity to a population of M. mytiloides from near its type locality in South Australia. Furthermore, seven populations containing more than one species in the mytiloides complex failed to show evidence of reproductive isolation between species, when tested for genetic differentiation. In contrast, a multivariate analysis, used to quantify and evaluate patterns of variation within and among the three species in the mytiloides complex, revealed differences in size, shape and allometry among species, but also showed overlap of body shapes and sizes of individuals. Two explanations are plausible for the lack of congruence between morphological and allozyme data: the three may be closely related species with undetected genetic differentiation, or the three taxa may be conspecific, and shell size, shape and allometry may be plastic characters. The role of fluctuating environmental conditions on crustacean morphology and life history variation is discussed.