Body depth less than head length, depth contained 2.6 to 3.1 times in standard length (for fish 15 to 62 cm standard length). Head length contained 2.3 to 2.5 times in standard length; interorbital area convex; preopercle rounded, finely serrate, the serrae at “angle” slightly enlarged; subopercle and interopercle smooth: eye diameter greater than or subequal to interorbital width in fish 10 to 30 cm standard length, less than interorbital in fish over 40 cm standard length; nostrils subequal or rear nostril slightly larger; maxilla naked, reaching to or slightly past vertical at rear edge of eye; midlateral part of lower jaw with 2 to 4 rows of subequal teeth. Gill rakers 7 to 10 on upper limb, 14 to 16 on lower limb, total 22 to 25. Dorsal fin with XI spines and 14 to 16 rays, third or fourth spine longest, longer than longest dorsal-fin ray and contained 2.3 to 2.8 times in head length, the interspinous membrane distinctly incised; anal fin with III spines and 8 rays (1 of 87 fish counted with 9 rays); pectoral-fin rays 17 to 19, the fin length contained 1.6 to 2.0 times in head length: pelvic fins distinctly shorter than pectoral fins, not reaching anus (except in some fish less than 20 cm standard length), pelvic-fin length contained 1.8 to 2.4 times in head length; pelvic-fin origin below or slightly posterior to base of pectoral fins; caudal fin rounded (in juveniles) or truncate with rounded corners (large adults). Midlateral-body scales ctenoid (at least in area covered by pectoral fins); adults with numerous auxiliary scales; lateral-line scales 62 to 73; lateral-scale series 98 to 116. Pyloric caeca 26 to 50.
Colour: Head and body dark reddish brown or greyish dorsally, usually yellowish gold ventrally; irregular white, pale greenish yellow or silvery grey blotches usually visible on body and head and mostly arranged in vertical series: black maxillary streak more or less distinct: median fins dark brown: distal edge of anal fin, caudal fin, and often the pectoral fins narrowly white; pelvic fins blackish distally; pectoral fins dark reddish brown or grey; margin of spinous dorsal fin and basal part of paired fins often golden yellow.

E. marginatus occurs on both sides of the Atlantic Ocean, throughout the Mediterranean Sea and round the southern tip of Africa to southern Mozambique and Madagascar. Heemstra and Randall (1993) examined specimens from the Azores, Spain, France, Italy, Greece, Lebanon, Israel, Algeria, Madeira, Canary Islands, Cape Verde Islands, Angola, South Africa, Mozambique, and Brazil. Based on identifiable records as “Epinephelus guaza”, the species is also known from Egypt, Tunisia, Morocco, Mauritania, Senegal, Côte-d’lvoire, and the Congo. According to Wheeler (1969), "Epinephelus guaza” is rare in British Seas. Reported from India by Reddy (1984; as “Epinephelus guaza”), but Heemstra and Randall (1993) have not examined any Indian Ocean specimens from north of 24ºS. In the western Atlantic, E. marginatus is known from southern Brazil, and it has also been reported (as “Epinephelus guaza”) from Uruguay and Argentina by Ringuelet and Aramburu (1960).

E. marginatus prefers rocky bottoms. It is the most common species of grouper in South African waters, where it is well known as the “yellowbelly rockcod” (“Epinephelus guaza”). It occurs from shallow water out to depths of 50 m, and is readily taken by anglers. Smale (1986) found that crabs and octopus were the principal prey and that larger specimens fed on a greater proportion of fishes, of which the majority were reef-associated species.
Considerable information has been published on the age, growth, and reproduction of “E. guaza” in the Mediterranean, but many authors who have published on the Mediterranean species of Epinephelus (e.g., Boulenger, 1895; Tortonese, 1970; Bauchot and Pras, 1980; Bauchot, 1987) may have confused E. marginatus (which they called “E. guaza”) with the similar species E. haifensis. Consequently, it is uncertain that all of the published information on the biology of ‘E. guaza” in the Mediterranean applies only to the species here recognized as E. marginatus.
In Tunisian waters, Bouain et al. (1983) found that females were mature at 44 to 53 cm total length (estimated age 6 to 8 years); and the smallest mature male was 85 cm total length (estimated to be 16 years old). Chauvet (1988) calculated the following growth equation for the Tunisian population: LTt = 114.49( 1-e-0.093(t+0.075)) where LTt is total length in millimetre. Extrapolating from the graph of this equation, he estimated a theoretical maximum age of 35 years for a fish of 118 cm total length. Chauvet (1988) also determined that females become mature at 5 years of age (38 to 58 cm total length) and change sex between ages 9 and 16 (68 to 90 cm total length).

Maximum size 120 cm total length and 35 kg (for Tunisian fish, according to Bouain et al., 1983). In Brazilian waters, E. marginatus is reported to attain 60kg (Figueiredo and Menezes, 1980).

See Taxonomy below.

A detailed explanation of the nomenclatural confusion involving E. marginatus is given by Heemstra (1991). A summary of this exposition is given here in order to justify the change in name of this well known and commercially important species. Jordan and Evermann (1896), in their influential and comprehensive work, The Fishes of North and Middle America, were the first to use the Linnaean name E. guaza [sic] for the species that currently bears this name. Previously, the species had been identified as Serranus gigas (Brünnich, 1768) by Valenciennes (1828), Gunther (1859) and Steindachner (1877) or Cerna gigas by Doderlein, 1882 or Epinephelus gigas by Jordan and Swain (1885), Jordan and Eigenmann (1890), Boulenger (1895) etc.; or it was described as a new species (Serranus marginatus Lowe, 1834 and Epinephelus brachysoma Cope, 1871). After Jordan and Evermann’s (1896) publication, E. marginatus and E. haifensis were confused under the names E. guaza or E. gigas.

Unfortunately, the species name “Epinephelus guaza” (originally Labrus Gvuza Linnaeus, 1758) cannot be used for this well-known species, because the original description clearly applies to a species of the genus Mycteroperca from the coast of Venezuela. Linnaeus’s description of Labrus Gvaza (1758:285) was taken verbatim from the travel diary of his student Pehr Löfling (spelt “Loefling” on the title page). This diary was published in 1758, two years after the death of Löfling and in the same year as the tenth edition of Linnaeus’s Systema Naturae. Löfling spent two years in Spain waiting for the Spanish to organize the expedition to South America in which he was to participate (Wheeler, 1980). While he was in Spain, Löfling collected plants and animals, recording descriptions of the various species in his travel diary. In South America, Löfling added descriptions of more plants and animals to his journal, but he died not long after his arrival. In the published version of this diary (Loefling, 1758) the page with the description of Labrus guaza is headed with the rubric “CUMANA,” which is the name of a port on the Caribbean coast of Venezuela; and all of the animals described on this page are from this locality. Although most of the species descriptions by Löfling that were incorporated in the Systema Nature (indicated by the reference “Loefl. epist.“) are of plants and animals that he observed in Spain, that of Labrus guaza is clearly not from Spain. For some reason or perhaps as an oversight, Linnaeus gave as the type-locality of this species “in pelago,” rather than the more explicit mention of Cumana or South America or the Caribbean.

The original description of Labrus Gvaza, as given by Linnaeus (1758) is typically brief:

"L. [Labrus ] fuscus, cauda rotundata, radiis caudatus membranam superantibus. Loefl. epist. D.11/27. P.16. V.6. A.13. C.15. Habitat in Pelago.” (“Dusky Labrus, caudal fin rounded, the rays projecting past the membrane. Dorsal fin with 27 rays, of which the first 11 are spines and the last 16 soft-rays; pectoral-fin rays 16; pelvic-fin rays 6 [i.e., I,5]; anal fin with 13 rays [= 3 spines + 10 soft-rays]. Habitat: in the open ocean.“)

This description does not fit the well-known amphi-Atlantic/Mediterranean dusky grouper that is commonly identified as Epinephelus guaza. In fact, it cannot apply to any species of Epinephelus, as they all have 7 to 9 anal-fin rays (one specimen of 29 E. morio that were counted has 10 anal-fin rays), and no Epinephelus species has the caudal-fin rays projecting beyond the membrane. The description does, however, fit Mycteroperca cidi Cervigón, 1966, M. interstitialis (Poey, 1860), and M. phenax Jordan and Swain, 1885; and these three species are common in the vicinity of Cumana (the type locality given by Löfling for Labrus guaza). Since the description could apply to any one of these three species of Mycteroperca and there is no extant type-specimen, the name Labrus gvaza Linnaeus, must be considered a doubtful name (nomen dubium) and is thus not available as the valid name of any species.

In the literature on Mediterranean and eastern Atlantic groupers, two similar species (the dusky grouper, E. marginatus, and the Haifa grouper, E. haifensis ) have been confused under the names Epinephelus (or Serranus ) guaza or gigas. E. marginatus differs in having 8 anal-fin rays (9 in E. haifensis), more elongate body (depth 2.6 to 3.1 versus 2.4 to 2.8 times in standard length), pelvic fins distinctly shorter than pectoral fins and not reaching the anus (pelvic fins subequal to pectoral fins and reaching to or beyond anus in E. haifensis less than 30 cm standard length), 17 to 19 pectoral-fin rays (18 to 21 in E. haifensis), and the head and body usually showing irregular pale blotches (no pale blotches in E. haifensis).

Of the eastern Atlantic groupers, E. marginatus is most similar to E. goreensis and E. haifensis. See the Key to Eastern Atlantic Groupers (above) for differences that will distinguish these species. In the western Indian Ocean, E. marginatus is most likely to be confused with E. chabaudi, which has 9 anal-fin rays and does not show the irregular pale blotches that are usually visible on E. marginatus; also, E. chabaudi is usually pinkish grey ventrally (rather than yellowish, the usual colour for E. marginatus).