Reef Science: Development Highlights
Katrien J. W. Van Look, Borys Dzyuba, Alex Cliffe, Heather J. Koldewey and William V. Holt. Dimorphic sperm and the unlikely route to fertilisation in the yellow seahorse. 2007. J Exp Biol. 2007 Feb 1; 210 (Pt 3):432-7.
Uniquely among vertebrates, seahorses and pipefishes (Family Syngnathidae) incubate their eggs within a male brood pouch. This has contributed to a widespread, but poorly founded belief, that the eggs are fertilised using spermatozoa that are deposited directly into the brood pouch via an internal sperm duct. Anatomical dissections showed, however, not only that direct sperm deposition into the pouch is physically impossible, but that spermatozoa must somehow travel a significant distance (>4 mm) outside the body of the male, to reach and fertilise eggs in the pouch. Observations of courtship and mating behaviour also revealed that the pouch closes immediately after mating, and that sperm transfer must occur within a time window of no more than 6 s. In addition to this, the yellow seahorse produces extraordinarily low quantities of dimorphic spermatozoa, but is nevertheless highly fertile and can produce broods that exceed 100 embryos. The entire fertilisation process in seahorses is therefore uniquely efficient among vertebrates, yet paradoxically involves several steps that would seem to complicate, and even appear to prevent, the interaction of the gametes. Although we are still unable to describe the exact fertilisation mechanism, we speculate that spermatozoa are ejaculated into a mixture of ovarian fluid and eggs, while the male and female are in close contact. Thereafter, this mixture must enter the pouch, whereupon the spermatozoa encounter seawater. These observations also support the view, indirectly inferred in previous publications, that sperm competition in seahorses is not only non-existent but impossible.
The Syngnathidae family (seahorses and pipefish) is best known for its role reversal in which the male internally broods the eggs. In most species of the family, the male has a pouch into which the eggs from the female are deposited, leading to a long-standing belief that the eggs’ fertilization also occurs inside the male’s pouch. This paper takes a closer look at the Yellow seahorse (Hippocampus kuda).
With the use of anatomical dissections these researchers have shown that “in pouch” deposition of sperm is impossible. Not only is there no sperm duct into the pouch, but the sperm would have to travel over 4 mm outside of the male’s body in order to reach the pouch. The Yellow seahorse produces very low quantities of spermatozoa, yet is very fertile and capable of producing large broods. Despite the exact mechanism of fertilization not having been determined yet, the Yellow seahorse is obviously very efficient at fertilizing eggs.