An explanation for sexual skew in the priapiumfish

(writing in progress)

The position and the proportional size and complexity of the intromittent organ of the priapiumfish (https://www.inaturalist.org/taxa/188061-Phenacostethus-smithi) and some of its relatives are remarkable.

The priapiumfish (Order Atheriniformes: family Phallostethidae: Phenacostethus smithi; the family name is derived from the ancient Greek for ‘phallic erection’) is among the smallest of fishes, at only 1.5 centimetres long (https://www.inaturalist.org/observations/100386712 and https://www.inaturalist.org/observations/68324746).

This species occurs in southeast Asia, from Thailand to Sumatra and the Philippines.

This oviparous (as opposed to viviparous or ovoviviparous) fish is so small that it would be expected to reproduce most efficiently by means of external fertilization. The anomalous evolution of the intromittent organ of the priapiumfish is puzzling.

The genital undercarriage of the priapiumfish - which is located under its head - consists of components as varied as

• an ejaculatory tube (i.e. genital papilla),
• a false phallus (the toxactinium),
• a hook (the ctenactinium), and
• a sucker-like disc (the pulvinulus).

These phallic accessories, all presumably participating in copulatory anchoring and penetration, are supported by their own bony skeleton despite their tiny size.

Even odder, their front-to-back and side-to-side arrangement makes these genitals an unusual example of bilateral asymmetry in vertebrates. In its complexity, the copulatory complex of the priapiumfish shows parallels with e.g. the basking shark (which, in contrast to the priapiumfish, is among the largest of all fishes): in both cases, males are equipped to compete indirectly by removing the sperm of any previous copulation.

Although the finger-like phallus of the priapiumfish lacks any ejaculatory passage, we postulate that it may nevertheless be inserted into the vagina, not to inseminate but rather to remove sperm packages deposited by other males.

The habitat of the priapiumfish is rivers or canals near the coast, where the water is fresh but rises and falls subject to tidal influences and - on a large scale - the seasonal ebbs and flows of the monsoons.

It therefore makes sense that the priapiumfish would have an extreme delay between mating and egg-laying, with insemination taking place during the dry season, when its populations are concentrated in reduced bodies of fresh water, and eggs being laid in the extensive waters of the wet season where competition for food is minimal.

Genital complexity in priapiumfishes may have evolved because females must – despite their diminutive size – store sperm for several months and males cannot deposit their own sperm without first emptying the female tract.

(writing in progress)

Posted on June 13, 2022 10:17 AM by milewski