Of Hawks and Marble

The acanthomorph fishes (a major clade of fishes mostly characterised by the presence of spines at the front of the dorsal fin) have long been recognised as a particularly thorny problem for higher-level systematics. Morphological relationships between many of the large number of families recognised in this clade have been almost impossible to unravel, and it is only in recent years that molecular analyses have been able to start making sense of the rapid divergences. Nevertheless, there are some subgroups of the acanthomorphs that have been recognised for a long time and which recent analyses have continued to support. One such group is the cirrhitoids.

Spottedtail morwong Goniistius zonatus, copyright Joi Ito.

Variously referred to in recent sources as the Cirrhitoidea, the Cirrhitoidei, or the Cirrhitiformes, the cirrhitoids include about eighty known species usually divided between five families. These are the hawkfishes of the Cirrhitidae, the trumpeters and morwongs of the Latridae, the Cheilodactylus fingerfins, the Chironemus kelpfishes and the Aplodactylus marblefishes (the morwongs were historically placed with the fingerfins in the Cheilodactylidae but have recently been transferred based on molecular data—Ludt et al. 2019). The largest cirrhitoid is the dusky morwong Dactylophora nigricans of western and southern Australia, growing to 1.2 metres in length, but most species are only a fraction of this size. Some of the largest species are of note to fisheries. Cirrhitoids are generally inhabitants of reefs, mostly feeding on benthic invertebrates such as crustaceans. They have long been recognised as a coherent group owing to their distinctive fin structure. The lower rays of the pectoral fins are not branched, and in a number of species they are thickened and protrude past the fin membrane (observant readers of this post may have already noticed a theme in many of the genus names given to cirrhitoids, relating to this feature). The pelvic fins are set well behind the pectoral fins. Other notable features of the clade include a relatively high number of vertebrae, a relatively low number of rays in the caudal fin, and the presence in juveniles of a fatty sac running along the fish's underside (Greenwood 1995).

Coral hawkfish Cirrhitichthys oxycephalus, copyright Aquaimages.

Both morphological and molecular studies have agreed that the hawkfishes of the Cirrhitidae represent the sister clade to the remaining cirrhitoids. Hawkfishes are brightly coloured inhabitants of the tropics, usually well under a foot in length. They are distinguished by bundles of trailing filaments emerging from the ends of the spines on the dorsal fin. Perhaps the most familiar member of the group is the longnose hawkfish Oxycirrhites typus, a regular in marine aquaria. However, this is also perhaps the most atypical member of the family as other species do not have the elongate snout. Hawkfishes commonly perch atop corals on the uppermost part of the reef, protected by the coral's sting and able to maintain a clear view of their surrounds. Wikipedia suggests that this behaviour is the inspiration for the name of 'hawkfish', but I'm not sure I buy this. I mean, it sounds plausible, but it also sounds like the sort of thing you would have to be diving below the reef to see. Vernacular names for fish tend to more often refer to things you might observe while hauling them onto a boat.

Marblefish Aplodactylus arctidens, copyright Peter Southwood.

The remaining cirrhitoids are all found in cooler waters, mostly in the Southern Hemisphere. Two species of Latridae, the redlip morwong Goniistius zebra and the spottedtail morwing G. zonatus, are found in the northern Pacific off the coast of eastern Asia (the kind of distribution shown by the genus Goniistius, where species are found in northern and southern temperate waters but not in the intervening tropics, is known as 'anti-tropical' and it's an interesting question how such a distribution would come to be). They are mostly found among rocky reefs, with the kelpfishes Chironemus and marblefishes Aplodactylus being particularly associated with patches of seaweed. The marblefishes feed on algae (particularly reds) as well as on some invertebrates and are characterised by a transverse mouth that is little or not protractible (Regan 1911). As noted above, the family Latridae has been inflated recently by the inclusion of most of the species previously included in the Cheilodactylidae. Cheilodactylus itself is now restricted to two species found around southern Africa. They differ from the remaining species in the latrids by the absence of a gas bladder as well as by elements of the skeleton. Many of the latrids are favourites of anglers, being well regarded as eating fish. By contrast, the herbivorous marblefishes are maligned as very poor fare and avoided. There's something to be said for eating your greens.

REFERENCES

Greenwood, P. H. 1995. A revised familial classification for certain cirrhitoid genera (Teleostei, Percoidei Cirrhitoidea), with comments on the group's monophyly and taxonomic ranking. Bulletin of the Natural History Museum of London (Zoology) 61 (1): 1–10.

Ludt, W. B., C. P. Burridge & P. Chakrabarty. 2019. A taxonomic revision of Cheilodactylidae and Latridae (Centrarchiformes: Cirrhitoidei) using morphological and genomic characters. Zootaxa 4585 (1): 121–141.

Nelson, J. S., T. C. Grande & M. V. H. Wilson. 2016. Fishes of the World 5^th ed. Wiley.

Regan, C. T. 1911. On the cirrhitiform percoids. Journal of Natural History, series 8, 7: 259–262.