Cortez damselfish

Stegastes rectifraenum is a small, territorial damselfish of shallow rocky reefs, described by Gill in 1862 ^[1]. The sources that describe its range place it in the Mexican Pacific, and in the Gulf of California it is one of the two most common damselfishes ^[2],[3].

It tends a territory of algae, and its diet is omnivorous, mixing filamentous algae, small invertebrates and plankton ^[4],[3]. The male guards the clutches and sometimes eats part of them ^[5]. The IUCN classifies it globally as Least Concern ^[6].

The genus name Stegastes comes from the Greek stegastos, meaning covered ^[7]. The species was described by Gill in 1862 and its currently accepted name is Stegastes rectifraenum ^[1].

It is a small damselfish of the genus Stegastes. In an anatomical description based on 9 adult museum specimens, each jaw carries a single row of long, sharp-edged teeth that shrink away from the midline; the dorsal fin has 12 spines and 16 rays, the anal fin 2 spines and 14 rays, and the pectoral fins 19 to 21 rays ^[2].

The first gill arch has 11 to 12 gill rakers, and the third of the five suborbital bones is the largest, with a serrated edge ^[2]. The pharyngeal teeth are central to its diet: the lower pharyngeal jaw is Y-shaped, with mostly single-cusped teeth in the middle and hooked ones at the edges ^[2].

It is small: the 22 adults caught near La Paz, in the Gulf of California, averaged about 78 mm in standard length ^[3]. FishBase gives a maximum of 13.5 cm total length for males or unsexed fish ^[7].

Similar species

• Stegastes flavilatus: In the Gulf of California it is the other common damselfish and looks very similar; Stegastes flavilatus has a rounder body, a taller supraoccipital crest and more gill rakers than Stegastes rectifraenum ^[3].

According to an anatomical description, it is endemic to the Mexican Pacific: it lives on rocky reefs at roughly 1 to 10 m depth, from the west coast of the Baja California peninsula, north of Magdalena Bay, through the Gulf of California and the Revillagigedo Islands down to Acapulco, in Guerrero ^[2].

An underwater census study in the Gulf of California recorded it at every site but one, that is, in 81 % of the censuses ^[3]. Its density rose slightly towards the north, and in the censuses where it co-occurred with Stegastes flavilatus there were about 25 individuals per 100 m² of Stegastes rectifraenum against about 4 of the other species ^[3].

Its abundance was higher in shallow water and fell where macroalgae and sand cover increased ^[3]. The authors note that the niche of herbivorous damselfishes is partly shaped by their algal crop, and that this link deserves further study ^[3].

It is an omnivore with a preference for algae, and is regarded as a grazer. Algae make up about half of its diet, and invertebrates and other animals the rest ^[2]. Its intestine is of medium length, and its coils resemble those of damselfishes that eat zooplankton ^[2].

On the rocky reef at Los Frailes, in Baja California Sur, the stomachs of 50 individuals were analysed. The most important components were benthic copepods and the algae Bryopsis and Ectocarpus, and snails of the genera Cerithium and Olivella also appeared; there were no differences between sexes or between seasons ^[4].

The male guards the clutches. In a population in the central Gulf of California, males ate 28 % of the clutches they received, preferentially those smaller than average and those at early stages of development ^[5]. Females tended to lay with males already guarding young eggs and to avoid those with eggs at late stages ^[5].

It actively defends a territory and spends most of the day in it, leaving only briefly to chase intruders or catch food ^[2]. It is regarded as a farming species: it maintains in its territory a crop of algae that alters the algal community and the productivity of the reef, and the authors propose that the territory may also collect detritus and attract small invertebrates that serve as food ^[2].

Using stable isotopes on 22 individuals from the Gulf of California, plankton supplied about 47 % of its food (credibility interval 38 to 55 %), filamentous algae about 30 % (22 to 38 %) and zoobenthos about 24 % (10 to 37 %) ^[3]. The authors therefore describe it as omnivorous and stress that plankton matters even though it is a bottom-bound species ^[3].

Its isotopic niche overlapped strongly with that of the fish Thalassoma lucasanum, but not with that of Abudefduf troschelii, clearly a zooplankton feeder ^[3]. According to the same authors, an earlier study saw this damselfish eating the faeces of Abudefduf troschelii in the water column ^[3].

The IUCN classifies Stegastes rectifraenum globally as Least Concern ^[6]. It is abundant throughout the Gulf of California, where it is regarded as endemic ^[3]. The sources do not fully agree on its extent: one describes it as endemic to the Mexican Pacific and another as endemic to the Gulf of California ^[2],[3].