In an earlier post on this site, I referred to a fish of the family Lethrinidae being known by the name of "sweetlips". However, as is usually the way with fish vernacular names, there is more than one family of fishes to which this name can be applied. 'Sweetlips' is also the vernacular name for fishes in the Plectorhinchinae.
The Plectorhinchinae is most commonly treated as a subfamily within the family Haemulidae, the grunts (some sources will place 'Plectorhynchidae' as a separate family, and no, that wasn't a typo: read on). Plectorhinchines are distinguished from other subfamily of haemulids, the Haemulinae, by characters including a longer dorsal fin and the presence of at least four prominent lateral line pores under the chin (Johnson 1980). The name 'sweetlips' refers to the prominent lips of mature individuals of two of the genera of plectorhinchines, Plectorhinchus and Diagramma, which are often a distinct colour from the rest of the head. Members of the third genus, Parapristipoma, have the lips not quite so prominent, and are commonly referred to as 'grunts' like the remaining haemulids.
The plectorhinchines are found around tropical reefs in the Indo-West Pacific and East Atlantic, with a single species, the rubberlip grunt Plectorhinchus mediterraneus, being found in the in the Mediterranean and Black Seas. No plectorhinchines are found on either side of the Americas. They are nocturnal predators of benthic invertebrates, emerging at night from the secluded crevices and overhangs where they spend the day. Most are medium-sized fish, though the painted sweetlips Diagramma pictum can get up to 90 cm. They are popular with fishers; Smith (1962) referred to them as "among the best if not the best eating fishes of the reef-haunting species". Many species can go through significant changes in coloration as they mature: spotted juveniles may become unicoloured adults, or blotchy babies may mature into stripes. The differences are great enough that juveniles and adults have often been mistaken for separate species.
But failure to associate parents with their children is not the only way in which this group has been dogged by confusing taxonomy. The name of the type genus has been variously spelled Plectorhinchus or Plectorhynchus, with the family name varying accordingly (it seems that 'Plectorhinchus' is the correct spelling). A surprising number of sources (e.g. Tavera et al. 2012) seem to have it both ways, with the genus being called Plectorhinchus but the higher taxon being called Plectorhynchinae (R. van der Laan et al. confirm the correct family-name spelling). Meanwhile, Smith (1962) argued for the use of the name Gaterin in place of Plectorhinchus, and called the family Gaterinidae. And if you have any interest in the vagaries of taxonomy, settle in: this is going to be a whole thing.
The name 'Gaterin' dates from what is usually known as Forsskål's (1775) Descriptiones animalium, which Fricke (2008) argued should be attributed to Niebuhr (see, right from the first sentence it's confusing). Peter Simon Forsskål and Carsten Niebuhr were members of a Danish scientific expedition in 1761 to 1763 to the Red Sea (though Forsskål himself was Swedish, but that's another story). Forsskål was the expedition's naturalist, while Niebuhr was there as a geographer. The history of the expedition, and of the composition of Descriptiones animalium, has been summarised by Fricke (2008). The expedition was particularly ill-fated; of six original members, Niebuhr was the only one to make it back to Denmark alive. After returning to Denmark, Niebuhr started preparing Forsskål's notes for publication. However, he found this no easy task. Forsskål had not prepared a single manuscript, but made notes on various scraps of paper; in the end, Niebuhr suspected that many of these scraps had gone missing. As an engineer, Niebuhr knew little Latin and even less biology, so he obtained the services of an academic adviser. The identity of this adviser was not divulged in the final publication by Niebuhr himself, but he has since been identified as the Danish naturalist Johann Christian Fabricius. The relationship between Niebuhr and Fabricius was not entirely positive (Niebuhr later stated that his adviser on Descriptiones animalium had been a 'strange fellow'), and Fabricius does not seem to have spent any more time on the Forsskål notes than he absolutely had to. As a result, the final publication that emerged was partly Forsskål, partly Niebuhr, partly Fabricius, and all dog's breakfast.
The name 'Gaterin' is listed by Forsskål/Niebuhr/Fabricius as one of the sub-divisions of the genus Sciaena, and Smith's (1962) revival of the name was based on the assumption that Forsskål intended these subdivisions to represent what we would now call subgenera. As such, Gaterin published in 1775 would clearly be an earlier name than Plectorhinchus published in 1802. Smith further supported this interpretation by pointing out that two names listed by 'Forsskål' as subdivisions of Chaetodon, Acanthurus and Abudefduf, had since been widely accepted as names for separate fish genera. There were no grounds, he claimed, for taking Abudefduf as valid but refusing Gaterin.
As it happens, Forsskål probably never intended either Gaterin or Abudefduf to represent generic names of any kind. It seems that his notes had used local Arabic names to refer to taxa to which he had not yet supplied formal Latin names. When Fabricius compiled these notes, he simply used the Arabic names as formal names, probably because he just didn't care. When 'Forsskål' referred to 'Gaterin' in his introductory paragraph for Sciaena, he was probably referring to the individual species known in Arabia as gaterin rather than any formal group. 'Abudefduf' may have been similarly inadvertent, but long usage as a genus name means that it should probably be retained whatever its original status. No such argument can be marshalled in favour of 'Gaterin', whose usage in place of Plectorhinchus has been minimal.
Fricke, R. 2008. Authorship, availability and validity of fish names described by Peter (Pehr) Simon Forsskål and Johann Christian Fabricius in the ‘Descriptiones animalium’ by Carsten Niebuhr in 1775 (Pisces). Stuttgarter Beiträge zur Naturkunde A, Neue Serie 1: 1–76.
Johnson, G. D. 1980. The limits and relationships of the Lutjanidae and associated families. Bulletin of the Scripps Institution of Oceanography 24: 1–114.
Smith, J. L. B. 1962. Fishes of the family Gaterinidae of the western Indian Ocean and the Red Sea with a resume of all known Indo Pacific species. Ichthyological Bulletin 25: 469-502.
Tavera, J. J., A. Acero P., E. B. Balart & G. Bernardi. 2012. Molecular phylogeny of grunts (Teleostei, Haemulidae), with an emphasis on the ecology, evolution, and speciation history of New World species. BMC Evolutionary Biology 12: 57. http://www.biomedcentral.com/1471-2148/12/57.