Egg strings in Euchirella pseudopulchra (Aetideidae) and comments on constraints on egg brooding in planktonic marine copepods

Adult female Euchirella pseudopulchra Park, 1976 from the California Current System bear a pair of unusual linear ovisacs or “egg strings.” Each membrane-bound, single file row of eggs contains 11–14 relatively large ova. Calculations suggest that the geometric arrangement of single file rows of eggs could facilitate oxygen diffusion in O2-deficient waters. The presence of ventrally carried egg masses in E. pseudopulchra and other members of the Calanoida appears to be associated with evolutionary loss of the fifth swimming legs (P5) in the adult female. We hypothesize that loss of the female P5 would improve hydrodynamic thrust during escape responses and reduce the probability of loss of ventrally brooded ova. We re-examine the relationship between egg size and body size for planktonic marine calanoid and cyclopoid copepods from the mesopelagic and epipelagic zones and compare the advantages of brooding versus broadcast-spawning life histories. The size distribution of adult females of 43 egg-brooding copepod species is bimodal, comprising a number of small-bodied species and large-bodied species, with only one intermediate-sized species (between 10–100 μg C). The size distribution of 75 broadcast spawners includes a large number (41) of intermediate-sized species. The interrupted size distribution of the egg-brooding species probably reflects enhanced predation risk to intermediate-sized copepods of carrying attached egg masses in the epipelagic zone.