Bigger mandarinfish produce more sperm

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Bigger male mandarinfish (Synchiropus splendidus) produce more sperm, according to a study published in the most recent issue of the Journal of Zoology.

Maria Rasotto, Yvonne Sadovy and Gearge Mitcheson test the hypothesis that the pelagic spawning female mandarinfish choose larger males, and that they derive fecundity benefits in doing so, i.e. that larger males produce more sperm. 

The authors carried out a series of mate choice experiments in which they allowed female fish to choose between a large and a small male (differing in size by 1.0–1.5 cm in total length) in the aquarium. 

They also collected freshly released eggs and sperm (using a plastic bag) of 67 pair spawns involving focal individuals off Malakal Island, Palau, visually estimating the sizes of the males (3.1–5.8 cm total length) and females (2.3–3.8 cm total length) involved in each spawn. 

The numbers of the collected eggs and sperm were then estimated, and the fertilisation rate of each spawn was recorded. 

In the mate choice experiments, the researchers found that females showed a preference for the larger male, attempting to pair exclusively with them. 

This was corroborated by the field observations made of spawning fish, in which larger females (bigger than 3.0 cm total length) spawned 94% of the time only with males larger than 4.0 cm total length and always with males larger than themselves. 

The authors found the male mandarinfish to release anywhere from 50,000 to 230,000 sperm per ejaculate, with a strong positive correlation between both the mean number of eggs and the mean number of sperm released during a single spawning and body size. 

Although the body size of the males does not predict fertilisation success, the researchers found a positive association between the number of sperm released and fertilization rate. This implied that females might suffer a fecundity risk when mating with smaller males.

The results of this study have implications for the conservation of this species. The mandarinfish is intensively collected for the aquarium trade, and large males are preferentially targeted. The authors contend that this could negatively affect female mate choice and fertility, possibly limiting the reproductive potential of this species.

For more information, see the paper: Rasotto, MB, Y Sadovy de Mitcheson and G Mitcheson (2010) Male body size predicts sperm number in the mandarinfish. Journal of Zoology 281, pp. 161–167.