Syngnathid Sex Roles

By Jessica Cruz

 

The male seahorse spots a female. He gracefully swims to her as she awaits her future egg bearer. The pair circles one another rhythmically.
The mating dance begins.

The seahorses rub torsos, intertwine their tails, and change colour. The female plants a string of eggs into the male’s brood; a pouch on the male’s posterior.

Male pregnancy suggests a sex role reversal in syngnathids, a fish family of seahorses and pipefish. The males nurse the eggs in their pouches, which seal after mating.

Does this mean that females compete for male attention? Does it depend on the species of syngnathid? Dr. Amanda Vincent, a Marine Biologist and Conservationist at Cambridge University, explains that different syngnathidae species vary in dominant gender. In her study, “Pipefishes and Seahorses: Are they all Sex Role Reversed?” published in 1992, Vincent and her team studied mating patterns and sex roles in different syngnathid species. Their work reviewed previous studies and experiments on polygamous and monogamous relationships in this unique species of seahorses.

Reproductive ecology of Nerophis ophideon and Syngnathus
typhle pipefishes (Category 1)

Vincent and her team discovered that in the two species from Sweden, females acted as the dominant sex. Larger females caused less egg production by males. While neither sex dominated the population, a female “produced eggs for about two males within the timespan of one male pregnancy.” Either way, both sexes remain polygamous:

  1. Syngnathus typhle seahorses were simultaneously polygamous, spreading their eggs amongst different partners.
  2. Nerophis ophideon seahorses were sequentially polygamous, giving their entire egg batch to one male.

Females are larger, more colourful, and grow skin folds during mating. These features allow males a greater selection in their mates.

Reproductive ecology of Hippocamus Seahorses and
Corythochtyhs Pipefishes

Why are some syngnathid populations more male-dominant? The second category proposes that seahorses and corythoichthys pipefishes form monogamous relationships. The most distinct feature of these syngnathids is the pouch, which implies a possible sex role reversal. Lab experiments conducted by Cambridge University show that:

  1. Males become more aggressive when another male is present upon pursuing a female.
  2. Males show unique behaviours such as snapping and wrestling during the whole period of courtship.

The male seahorses also demonstrated unique qualities, such as snapping with their snouts and wrestling. Females remained neutral.

Importance of daily greetings in maintaining seahorse pair bonds

Monogamous male and female seahorses equally regulate mating. Females wait until after the pregnancy to mate again, even if other males court her. The mating processes happen through “daily greetings” of a “seahorse dance.” The mating occurs in the morning, lasting around six minutes, and the seahorses tend to change colour throughout. The male gives birth on an evening roughly six months later. The pair mates again the following morning for nine consecutive hours. This is essential to the maintenance of seahorse pair bonds.

In her study, Amanda Vincent sought to determine whether females mate more often with an “ex-seahorse” partner or with a seahorse who greets her often. The seahorses were kept in a tank at 28 degrees Celsius, mimicking natural conditions. The female was placed with a short-term mating partner, then a regularly greeted seahorse. Vincent and her team determined that regular greetings with male and female partners reinforce pair bonds in monogamous relationships.

Conclusion

All male syngnathid species give birth to offspring, but the dominant role in the mate selection process varies. Category 1 pipefishes showed a “higher production rate” in the larger females. Category 2 showed more males without partners.

Sources

Vincent, Amanda et al. (1994). A role for daily greeting in maintaining seahorse pair bonds. Animal Behaviour 49, p. 258-260.
Vincent, Amanda et al. (1992). Pipefishes and seahorses: are they all sex role reversed? Trends Ecol. Evol.7, p. 237-241.