Sertularella affinicostata Calder and Faucci 2021

Sertularella affinicostata Calder and Faucci, 2021

(Figure 6e, f)

Sertularella affinicostata Calder and Faucci, 2021: 23, figs 7a–e, 8

Type locality

Ecuador: Galápagos Islands, Isla Darwin (Calder and Faucci 2021).

Material examined

Chatham Bay, 5.56126, −87.04516, 1 colony, 2 mm high, without gonothecae, coll. I. Keith, #253534. –Chatham Bay, 5.56216, −87.04516, 9 colony fragments, to 6 mm high, without gonothecae, coll. I. Keith, #240612. –Wafer Bay, 5.5456, −87.06235, 9 colony fragments, to 2 mm high, without gonothecae, coll. G. Ashton, #240597. –Wafer Bay, 5.5456, −87.06235, 2 colonies, on two barnacles, to 2 mm high, without gonothecae, coll. G. Ashton, #240591. –Wafer Bay, 5.5456, −87.06235, 6 colony fragments, to 3 mm high, without gonothecae, coll. G. Ashton, #240592. –Wafer Bay, 5.54535, −87.06185, 4 colony fragments, to 2.5 mm high, without gonothecae, coll. G. Ashton, #240636. –Wafer Bay, 5.54535, −87.06185, 1 colony, on a barnacle, 4 mm high, without gonothecae, coll. G. Ashton, #240629. –Wafer Bay, 5.54535, −87.06185, 1 colony fragment, 4 mm high, without gonothecae, coll. G. Ashton, #240630. –Chatham Bay, 5.56126, −87.04516, 2 colony fragments, to 5 mm high, without gonothecae, coll. I. Keith, #307712. –Chatham Bay, 5.56126, −87.04516, 1 colony fragment, 3 mm high, without gonothecae, coll. I. Keith, #307711. –Chatham Bay, dock 004, no coordinates, 1 colony fragment, 2 mm high, without gonothecae, coll. G. Ashton, #266336.

Remarks

Sertularella affinicostata Calder and Faucci, 2021 was first reported, as S. costata Leloup, 1940, from the two northernmost and warmest of the Galápagos Islands, Wolf and Darwin (Calder et al. 2003). It was discovered a second time in a collection from French Frigate Shoals in the Northwestern Hawaiian Islands (Calder and Faucci 2021) and recognised therein as an undescribed species. Calder and Faucci (2021) also provide records of previously unreported material from Cousin Rock and Marchena Island in the Galápagos. The present record from Cocos Island constitutes the third record of this tiny but morphologically striking hydroid species.

Hydroids of S. affinicostata are noteworthy in having a series of sharp-edged horizontal ridges that ring walls of its hydrothecae. Trophosomes of the species differ in part from those of S. costata, originally described from Sagami Bay, Japan, in having fewer (10–14) ridges instead of about 20 (Leloup 1940; Hirohito 1983, 1995). Hydrothecae also differ in being barrel-shaped rather than distinctly tapered distally, and a neck region lacking ridges below the rim is noticeably longer. The proximal end of the hydrocaulus is typically short rather than extending as a long, slender peduncle as in S. costata, and internodes of the hydrocaulus are shorter and thicker.

The original account of S. affinicostata in Calder and Faucci (2021) was based on specimens from both the Galápagos Islands and the Northwestern Hawaiian Island. The holotype, from Darwin Island in the Galápagos archipelago, was selected as being the only fertile colony in the two collections. The species was well represented in our Cocos samples. It was present in 11 of the 42 samples (26%) of hydroids in the collection, although all of the specimens were sterile.

Hydroids of S. affinicostata are minute, with colonies from Cocos Island ranging between 2 and 6 mm in height. Indeed, the holotype was merely 1.5 mm high. A substrate generalist, the species has been reported from barnacles, a sponge, calcareous rubble, algae and a hydroid stem (Calder and Faucci 2021). Specimens examined here were removed from fouling panels exposed at Chatham Bay and Wafer Bay. While S. affinicostata is certainly a species of shallow waters, its overall bathymetric range is not yet well known. Specimens from the Galápagos were collected at depths of 6 m off Wolf Island (Calder et al. 2003), and from 10 m off Cousin Rock and 8 m off Marchena Island (Calder and Faucci 2021). Collections from Cocos Island were on panels exposed at depths of 0.5– 3 m.

A description and additional comments on S. affinicostata are provided by Calder and Faucci (2021).

We treat S. affinicostata as cryptogenic in the Cocos, Galapagos and Hawaiian Islands. While potentially a member of a naturally transpacific fauna, its occurrence in shallow water, and particularly on biofouling panels in Cocos, do not exclude it from shipmediated transport. As we discuss below, an increasing number of invasions are recognised from open-ocean environments (such as in the Galapagos and the French Frigate Shoals), thus not excluding this species from being potentially introduced, even if its home port remains unknown at this time.

Reported distribution

Cocos Island: first record.

Elsewhere: Galápagos Islands (Calder et al. 2003, as Sertularella costata); Northwestern Hawaiian Islands (Calder and Faucci 2021).