Male pregnancy and the formation of seahorse species Male pregnancy and the formation of seahorse species © Photo Scott Tuason/ imagequestmarine.com Adam G. Jones Texas A&M University, USA Seahorses have long been part of the pantheon of imagination-inspiring creatures. The external appearance of this strange fish probably accounts for the widespread appeal of the seahorse. Reproduction in seahorses is even more unusual, as the male rather than the female, becomes pregnant. Recent research on seahorse mating behaviour reveals that male pregnancy in seahorses and their relatives has had profound effects on the evolutionary legacy of these organisms. The seahorse is one of the most famous fish, appearing in movies, television shows, greeting cards, and children’s toys, just to name a few. The fact that relatively little research has been done on seahorses thus comes as a bit of a surprise. Naturalists have long appreciated that the males of this group carry the young, but it has only been in the last two decades that we have begun to appreciate the evolutionary implications of male pregnancy. The pregnant male and his devoted wife Mating in seahorses starts with an elaborate courtship rit- ual that in some species lasts several hours. Courtship has been described in few seahorse species, but we can grasp the flavour of the event from descriptions of mating inter- actions in an Australian species known as Hippocampus whitei (Vincent and Sadler, 1995). Upon meeting, the male and female brighten in colour and then begin a dance, twisting their tails together and swimming in circles. After the prolonged courtship dance, the female inserts her ovipositor through a small opening into a pouch located on the male’s tail (Figures 1 and 2). She transfers unfertilised eggs into the pouch of the male, who releases sperm into the pouch to fertilise the eggs. The actual transfer of eggs only takes a few seconds. At this point, the female’s investment in the offspring is complete, but the male is left with a pouch full of growing offspring. The male then carries the developing brood for a period of several weeks, and his pouch bulges larger and larger until he finally gives birth. The pouch appears to be a serious burden for pregnant males and they move around very little when pregnant (Kvarnemo et al, 2000). Even though he is solely responsible for care of the devel- oping embryos, the life of a pregnant male seahorse is not entirely lonely. The female, although she provides no parental care, comes by each morning to greet the male (Vincent and Sadler, 1995). These greetings are essentially shortened versions of the courtship ritual, but obviously without a transfer of eggs, since the male is pregnant already. These greetings are not without purpose. Indeed, the female seems to be checking on the progress of the male’s pregnancy so she can be ready to re-impregnate him as soon as possible after he gives birth. Biologist (2004) 51 (4) 1 Title image. Thorny seahorse.