Males of pelagic copepods are consistently smaller than the females. A new study from Centre for Ocean Life describes the patterns for 400 species and suggests a simple explanation.
Males and females of most species differ in size, with either the males or the females being the largest, and researchers have long sought general patterns in intersex size differences, and evolutionary explanations. In 400 species of copepods, mm-sized pelagic crustaceans, females are - with few exceptions - consistently about 10 % larger than the males and thus differ from the patterns observed in most other animal taxa. The consistent size difference is inconsistent with the implications of sexual selection but may simply arise when the reproductive advantage of delayed maturation at a larger size is larger for the females than for the males.
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Reference
Andrew G. Hirst and Thomas Kiørboe (2014) Macroevolutionary patterns of sexual size dimorphism in copepods. Proc. R. Soc. B, doi:10.1098/rspb.2014.0739