Field of Science

Taxon of the Week: Give Us a Kiss!

The fish genus Lethrinus is found in tropical waters of the Indian and western Pacific Oceans, with a single species making an incursion into the eastern Atlantic. The group is commonly known as emperors, though I have heard people here in Australia refer to them as snappers, a confusing piece of terminology for me because they are quite different fish from the one I knew in New Zealand as snapper*. The prominent lips in Lethrinus adults, often a different colour from the surrounding face, have given at least one species the memorable name of "sweetlips" (image above of Lethrinus harak, from Wikimedia). Carpenter & Allen (1989) listed 26 described species and two unnamed species in the genus. One of these undescribed species was named Lethrinus ravus by Carpenter & Randall (2003) (image below of Lethrinus nebulosus from here).

*If you excuse me, I'm just going to have a little rant about the common names of Southern Hemisphere fishes. As with other animals and plants, British settlers in New Zealand and Australia labelled the fish they found in their new country with the names of fish they had been familiar with back in the Old Country. However, when it came to fish the new immigrants seem to have gone to extraordinary lengths to find Northern Hemisphere analogues, with the result that it becomes difficult to see how they ever found a connection. The New Zealand grayling (now unfortunately extinct) was no relation to the Northern Hemisphere grayling. The New Zealand cod is even less like the original. And as I've already indicated, the confusion surrounding the name "snapper" is beyond anyone's ability to sort out. Rant over - please resume normal service.

Emperors are all predators, but are divisible into three ecological groups (Lo Galbo et al., 2002) - low-bodied stalkers with conical teeth that are active hunters of high-speed invertebrates and small fish, high-bodied benthic feeders with molariform teeth that can feed on shellfish and other hard-shelled invertebrates, and high-bodied species with conical teeth that feed on softer-bodied slow-moving invertebrates. The molecular phylogeny of Lo Galdo et al. (2002) recovered a good correlation between trophic type and phylogeny. The high-bodied conical-tooth form appears to be ancestral, with one species (Lethrinus minatus) sister to all the other species, and one species each low down in the two major clades that the other species fell into. Low body-form and molariform teeth both appeared twice, in each case with one clade containing most of the species showing the novel feature, and a single species appearing to have developed it independently.

Of course, what discussion of tropical reef fishes would be complete without a mention of transexuality? Many species of Lethrinus have been shown to be protogynous hermaphrodites - that is, they reach maturity as females before changing sex at a later date to males (Young & Martin, 1982). The mechanism inducing this change in emperors remains unknown. In other protogynous reef fish species, males may maintain harems of females, the largest of which switches sex if the male is removed for whatever reason, but whether emperors have a similar system has not yet been established.


Carpenter, K. E., & G. R. Allen. 1989. FAO Species Catalogue vol. 9. Emperor Fishes and Large-eye Breams of the World (Family Lethrinidae): An annotated and illustrated catalogue of lethrinid species known to date. Food and Agriculture Organization of the United Nations.

Carpenter, K. E., & J. E. Randall. 2003. Lethrinus ravus, a new species of emperor fish (Perciformes: Lethrinidae) from the western Pacific and eastern Indian oceans. Zootaxa 240: 1-8.

Lo Galbo, A. M., K. E. Carpenter & D. L. Reed. 2002. Evolution of trophic types in emperor fishes (Lethrinus, Lethrinidae, Percoidei) based on cytochrome b gene sequence variation. Journal of Molecular Evolution 54 (6): 754-762.

Young, P. C., & R. B. Martin. 1982. Evidence for protogynous hermaphroditism in some lethrinid fishes. Journal of Fish Biology 21 (4): 475-484.

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