Gender's Just a State of Gonads

It didn't take long for Adam Yates to recognise this animal:


Juvenile Pagrus auratus. Photo by Richard Ling.

This is the fish that goes by the name of 'snapper' in New Zealand, though that name is used for different kinds of fish elsewhere. In older references, you'll find this species under the name of Chrysophrys auratus, but the genera Chrysophrys has since been synonymised with Pagrus (Paulin, 1990). However, the molecular phylogenetic analysis of Chiba et al. (2009) failed to recover monophyly for Pagrus, so we may yet see Chrysophrys make a comeback some day.


Mature individuals of Pagrus major, a north-west Pacific species regarded by some authors as a synonym of P. auratus. These two are probably engaging in courtship behaviour. Photo from here.

As this young snapper gets older, its body will change in numerous ways. One is that the blue spots along its side will fade away and it'll become more evenly pink. Its head will become deeper, and if it may develop a large supraorbital boss on its forehead. And one other significant change that it may go through is a reassignment of gender. Members of the marine fish family Sparidae, to which Pagrus belongs, show a bewildering range of sexual development, including forms which show protandrous hermaphroditism (they start life as males before developing into females), protogynous hermaphroditism (starting as females, developing into males) and gonochorism (completely separate males and females, as we have ourselves). Other species start life with the rudiments of both male and female gonads but have only one or the other develop to maturity, without any subsequent sex changes, while a single species has been recorded as possessing simultaneously functional gonads of both sexes (Buxton & Garratt, 1990).

Different species of sparids feed on a variety of different diets, from predators of other fish such as the Dentex species to herbivores on algae such as Sarpa salpa. This variation in diet is reflected in a variety of dental morphologies. Predators such as Dentex possess pointed caniniform teeth while invertebrate feeders such as Pagrus auratus have a combination of pointed teeth in the front and round molariform teeth in the back. Algal feeders have flat-topped incisiform dentition, leading to occassional reports on fish with human teeth:


Teeth of sheepshead, Archosargus probatocephalus. Photo from Nathan Thurston.

In the past, dentition has been used as the basis for dividing sparids into a number of subfamilies, but both molecular (Chiba et al., 2009) and morphological (Day, 2002) analyses indicate multiple polyphyletic origins of the various dentition types. Contrast that to the situation in the possibly related* family Lethrinidae where trophic type and phylogeny show a much closer fit.

*A relationship between the two has been suggested on morphological grounds; molecular analyses have so far not supported such a relationship, but nor have they produced any strong relationships for either family.

REFERENCES

Buxton, C. D., & P. A. Garratt. 1990. Alternative reproductive styles in seabreams (Pisces: Sparidae). Environmental Biology of Fishes 28: 113-124.

Chiba, S. N., Y. Iwatsuki, T. Yoshino & N. Hanzawa. 2009. Comprehensive phylogeny of the family Sparidae (Perciformes: Teleostei) inferred from mitochondrial gene analyses. Genes and Genetic Systems 84 (2): 153-170.

Day, J. J. 2002. Phylogenetic relationships of the Sparidae (Teleostei: Percoidei) and implications for convergent trophic evolution. Biological Journal of the Linnean Society 76 (2): 269-301.

Paulin, C. D. 1990. Pagrus auratus, a new combination for the species known as "snapper" in Australasian waters (Pisces: Sparidae). New Zealand Journal of Marine and Freshwater Research 24: 259-265.