THOMPSON, J.T.*; VOIGHT, J.R.; University of North Carolina; The Field Museum of Natural History: First account of erectile tissue in an invertebrate animal: the Octopus copulatory organ

The most familiar examples of erectile tissue are the genitalia of mammals, notably the penis and clitoris. Among the soft-bodied invertebrates, erectile tissue is virtually unknown, even in sex organs. Here it is reported that the ligula, the copulatory organ that is a modification of an arm tip of male octopuses, is erectile in Octopus bimaculoides. The normally minute ligula was observed during an unsuccessful mating attempt to be engorged. Histological sections of the ligula reveal striking structural convergences with mammalian erectile tissue: large, well-vascularized internal cavities subdivided by networks of collagen fibers and enclosed by an array of collagen fibers. This internal structure differs markedly from the dense, three-dimensional array of muscles and connective tissues in other octopodid ligulae. Erectile tissue may represent an evolutionary compromise between opposing selective forces. Small ligulae may be advantageous because O. bimaculoides hunts in daylight and the white-faced ligula may be conspicuous to predators. Large ligulae, however, may be advantageous if the intense sexual selection thought to occur among octopodids selects for large ligulae that can transfer more sperm at copulation. An erectile ligula may minimize the impact of these opposing selective forces.