Tubesnout fish lays eggs in sea squirts

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Japanese scientists have observed and described the unusual breeding behaviour of the Japanese tubesnout (Aulichthys japonicus), a species that conceals its eggs in ascidians (sea squirts) after copulation.

The tubesnout is a gasterosteiform fish related to sticklebacks, and although its unusual spawning method has been recorded, the exact behaviour during spawning remains unknown.

In a study published in the most recent issue of the journal Ichthyological Research, Izumi Akagawa, Masako Hara and Toshitaka Iwamoto record the spawning behaviour of the Japanese tubesnout in aquaria for the first time.

The authors caught on video the spawning behaviour of wild-caught fish that were maintained in aquaria and record them here for the first time.

Gravid females were considerably darker in colour, with numerous white spots (nongravid females were lighter brown without the spots), which changed to a bright gold several minutes prior to spawning.

The female fish began paying much closer attention to the sea squirts at this point. They then began to repeatedly put the head into the incurrent and excurrent apertures of the sea squirts.

The females occasionally physically contacted the sea squirts with their mouths; this caused the sea squirts to contract the sphincter of the apertures but they reopened it after a short time.

The female tubesnout deposited an egg mass above the ascidian, held it in her mouth briefly, and then placed it in the excurrent aperture.

After egg deposition, the females did not exhibit any maternal care to the egg mass. As soon as, or even before the female tubesnout pulled her head from the sea squirt, male tubesnouts began to attempt copulation.

Several competing males swam parallel with the female in many cases, with one of the males quickly turning upside down under the female s abdomen, projecting his urogenital papillae (usually recessed into the body) into the genital pore of the female.

After copulation, the sperm reach the micropyle of ovarian eggs and then wait for spawning and fertilization.

It is thought that the males do not attempt copulation until after the fertilised eggs from the previous spawn have been deposited in the sea squirt.

The authors also offer a hypothesis of how this unusual spawning behaviour evolved: (1) during severe competition for external fertilization, some males may release sperm (via the urogenital papilla) not to egg masses but to the genital pore of females; (2) an immobile papilla (seen in close relatives) evolves into the mobile copulatory organ of A. japonicus; (3) as males adopt copulation, they will not guard eggs that are fertilized by other males but begin to eat the deposited eggs; and eventually, (4) females may begin to hide their eggs against male cannibalism, initially in cracks of rocks or in thickets of seaweed, but shifting the deposit sites to sea squirts.

For more information, see the paper: Akagawa, I, M Hara and T Iwamoto (2008) Egg concealment in ascidians by females of the Japanese tubesnout, Aulichthys japonicus (Gasterosteiformes), and its subsequent copulation. Ichthyological Research 55, pp. 85"89.