Eugorgia daniana

Eugorgia daniana Verrill, 1868 is an octocoral of the family Gorgoniidae ^[1]. Its colonies form branched fans, bright red or streaked with yellow; the examined specimens reach 25 cm in height and 36 cm in width ^[2].

It is a species of the tropical eastern Pacific. For Peru, the revision of the genus records only a museum specimen from Cabo Blanco, in Piura, collected in 1941 ^[2].

Colonies grow in several planes and are heavily branched: they form a broad fan with feather-like branching. The main stem is 2 to 4 mm in diameter and is very short (up to 1 cm) or missing, so the branches arise almost directly from the holdfast. The pinnae, about 1 mm in diameter, are crowded together and laid out without a regular order, and several branch again ^[2].

The polyps are colourless and are spread in irregular lateral bands without crowding; each one rises from a prominent mound with a small two-lipped opening, sometimes with yellow spots. The colony is a uniform bright red or streaked with yellow on the branches or on the mounds, and it keeps its colour alive, dried or in alcohol. In 2009 a white variety was also mentioned, seen beside the red one in Costa Rica and Mexico ^[2]; in 2013 that white form was described as a separate species, Eugorgia mutabilis ^[3].

The sclerites of the tissue covering the axis are red and yellow. Double discs predominate, up to 80 µm long and 60 µm wide, and spindles up to 130 µm are abundant. The dominant double discs are what separates Eugorgia from Leptogorgia, a genus of very similar outward appearance, and in this species they are sharper than in its two closest relatives ^[2].

Similar species

• Eugorgia aurantiaca: Its branches and branchlets are more widely spaced, thicker and more compressed, its polyp mounds are larger with yellow rings, and its polyps are arranged in 2 to 4 rows ^[2].
• Eugorgia mutabilis: It resembles Eugorgia daniana in maximum number of branches, spacing of the branchlets and distribution of the polyps, but its colonies are white, not red or yellow, and its sclerites are very different ^[3].
• Eugorgia multifida: Its branchlets are compact, thick and short, and unlike the other two, in the examined samples it has sclerites at the base of the tentacles ^[2].

The genus Eugorgia is restricted to the eastern Pacific, from southern California to Peru ^[2]. Eugorgia daniana was described from material from the Pearl Islands (Panama) and the Gulf of Nicoya (Costa Rica); it is also reported from Corinto (Nicaragua), El Salvador, Gorgona Island, the Galápagos Islands, the Gulf of Guayaquil, Mexico (from Baja California and the Gulf of California to Oaxaca) and, with a single specimen, San Diego ^[2].

The depths of the examined specimens range from 3 to 30 m in Panama and Costa Rica; the original series, from the Pearl Islands, was taken between 11 and 15 m. In the Galápagos it was collected at 25–29 m, in the Gulf of Guayaquil at 20 m, and in Oaxaca one specimen from 40 to 70 m came up in a fishing net ^[2].

The genus lives from shallow water down to about 65 m, and its colonies attach to hard substrate, debris, or coarse sand or mud ^[3]. Eugorgia mutabilis lives on rocky bottom beside other gorgonians, including Eugorgia daniana ^[3]. In a 2012 survey of rocky reefs of the north Pacific of Costa Rica, at less than 35 m, the species is listed at a depth of 15 m ^[4].

The polyps of Eugorgia retract completely into the tissue covering the axis, and the axis is horny, with a cross-chambered core and a network of organic filaments that is often mineralised. According to the revision, the branches of these gorgonians do not anastomose, that is, they do not fuse with one another ^[2]; later work notes, however, that pseudo-anastomosis is frequent in the genus ^[3].

According to the revision, in Eugorgia daniana and its two closest relatives the feather-like branching goes up to 9 times, and all share red and yellow sclerites in the tissue covering the axis. The study does not describe its feeding or its reproduction ^[2].

In a 2012 survey of rocky reefs of the north Pacific of Costa Rica, Eugorgia colonies were very scarce and scattered, and formed no aggregations. By contrast, Leptogorgia cuspidata and Leptogorgia rigida did form them between 5 and 7 m depth ^[4].

For octocorals in general, the authors note that their populations depend on currents, waves, connectivity between sites and the availability of substrate. They name mechanical damage and some fishing gear, which tear off or weaken colonies, as the main causes of mortality ^[4].

The revision of the genus records for Peru a single specimen of Eugorgia daniana, from Cabo Blanco, in Piura, collected on 12 May 1941; the specimen label gives no depth, and the revision cites no other Peruvian locality ^[2].

The authors warn that their list does not reflect the true geographic range of the species, because surveys and a review of museum material are lacking. The absence of further Peruvian records therefore does not prove that the species is missing from other sites ^[2].