The Serranidae are a group of marine fish that go by vernacular names such as sea bass, rock bass or rock cod. They are carnivores, and are found mostly around reefs in tropical and subtropical waters around the world. In appearance, they are fairly generalised (these are fish that look like fish) with a body shape that is longer than high, but not too long, and relatively big jaws with the lower jaw often jutting forward a bit beyond the upper. Some of them are quite colourful (as befits a tropical reef fish) and some of the smaller ones turn up in marine aquaria as a result. As used in the past, the Serranidae has been quite a broad grouping of fish united by having three spines on the margin of the opercle (the gill cover) and the maxilla in the upper jaw not hidden by the cheekbone when the mouth is closed. Members of this broad Serranidae were commonly divided between three subfamilies: the Serraninae (including the sea basses), Epinephelinae (including the groupers) and Anthiinae (basslets and goldies), though some authors further subdivided the Epinephelinae. However, recent molecular studies have indicated the polyphyly of this grouping, with the Serraninae and Epinephelinae occupying distinct positions within the clade known as the Serraniformes or Perciformes sensu stricto (see this old post), and so have cut the latter out of the Serranidae. As for the Anthiinae, their position remains uncertain, with some analyses placing them with the Serraninae and others with the Epinephelinae (Lautredou et al. 2013). As a result, a monophyletic Serranidae is probably to be restricted to the old 'Serraninae'.
There are over eighty species listed for this restricted Serranidae on FishBase, but new ones continue to be described. As is common among reef fishes, it can be hard to determine exactly what counts as a species (whatever your preferred definition). A prime example of this is the genus Hypoplectrus, small serranids known as hamlets (no, I don't know why either) found in the Caribbean and the Gulf of Mexico. Hamlets come in a range of different colours and patterns, but structurally speaking the various forms are otherwise indistinguishable. As a result, some authors have regarded them as all colour morphs of a single species. Others have recognised close to twenty different species. Domeier (1994), conducting field observations on hamlets together with breeding experiments in the laboratory, found that different colour morphs would usually only mate with partners sharing their own colour pattern, though hybrid matings could be produced if no more suitable mate was provided. These hybrid matings produced offspring bearing intermediate colour patterns, and the rarity of such intermediates in the field led Domeier to infer that the different morphs were mostly acting as good species.
Most sea basses are simultaneous hermaphrodites: they have both male and female reproductive organs functional at the same time. Though they are capable of fertilising their own eggs, they still usually breed in pairs with each individual alternating the release of male and female gametes. Not all serranids follow this reproductive template: members of the genera Chelidoperca and Centropristis are protogynous, starting their mature lives as females before switching over to males. Two species of Serranus, the lantern bass Serranus baldwini and the barred serrano S. psittacinus, are mostly simultaneous hermaphrodites like other species in the genus, but the largest individuals resorb their female organs and become exclusively males. Finally, many species of the genus Paralabrax have entirely separate males and females. Phylogenetic analysis suggests that protogyny may be the original mode of sexual development in the serranids, with separate sublineages developing simultaneous hermaphroditism vs separate sexes (Erisman & Hastings 2011). In correlation with this, individuals of Paralabrax that are functionally single-sexed have been found to retain non-functional remnants of the other sex's organs.
Domeier, M. L. 1994. Speciation in the serranid fish Hypoplectrus. Bulletin of Marine Science 54 (1): 103–141.
Erisman, B. E., & P. A. Hastings. 2011. Evolutionary transitions in the sexual patterns of fishes: insights from a phylogenetic analysis of the seabasses (Teleostei: Serranidae). Copeia 2011 (3): 357-364.
Lautredou, A.-C., H. Motomura, C. Gallut, C. Ozouf-Costaz, C. Cruaud, G. Lecointre & A. Dettai. 2013. New nuclear markers and exploration of the relationships among Serraniformes (Acanthomorpha, Teleostei): the importance of working at multiple scales. Molecular Phylogenetics and Evolution 67: 140–155.