Sexual Dimorphism and Niche Divergence in a Mid-Water Octopod (Cephalopoda: Bolitaenidae).

In the translucent mid-water octopod Eledonella pygmaea, the posterior salivary glands that release proteolytic enzymes into the esophageal crop grow five times faster in males than in females. I suggest that the sexes vertically partition the water column and that large glands have evolved in males as a result of their deepwater habitat. Members of the species undergo ontogenetic vertical descent and are suggested to mate at the lower end of the adult depth range where receptive females signal males with light organs. Selection for increased fitness is inferred to result in females increasing their fecundity by feeding at the upper limit of the adult range and in mature males increasing their encounters with mates by living at depths where mating occurs. To further increase their fitness, mature males--despite occurring in a prey-limited habitat--must expend energy to visually detect potential mates, to travel over wide areas, and to attempt to copulate. To increase the energy available to them, males at depth may exploit bioluminescent prey. The large glands protect the translucent males from increased predation by physically blocking light emitted by bioluminescent prey in their crops, and by speeding digestion.