Biofouling hydroids (Cnidaria: Hydrozoa) from a Tropical Eastern Pacific island, with remarks on their biogeography Author Calder, Dale R. Author Carlton, James T. Author Keith, Inti Author Ashton, Gail V. Author Larson, Kristen Author Ruiz, Gregory M. Author Herrera, Esteban Author Golfin, Geiner text Journal of Natural History 2022 2022-07-11 56 9 - 12 565 606 http://dx.doi.org/10.1080/00222933.2022.2068387 journal article 126956 10.1080/00222933.2022.2068387 5ca061c2-0cbb-4434-a28a-fc6c60e7ed7c 1464-5262 7012486 Clytia linearis ( Thornely, 1904 ) ( Figure 5e, f ) Obelia linearis Thornely, 1904: 453 , pl. 44, fig. 6. Type locality Papua New Guinea : New Britain , Blanche Bay ( Thornely 1904 , as Obelia linearis ) . Material examined Chatham Bay, 5.55271 , −87.03826 , 1 colony, 6 mm high, without gonothecae, coll. I. Keith, #240566. –Wafer Bay, 5.54556 , −87.06221 , 25 colony fragments, to 1.1 cm high, with gonothecae, coll. G. Ashton, #240607. –Chatham Bay, 5.55271 , −87.03826 , 3 colony fragments, to 1.1 cm high, with gonothecae, coll. I. Keith, #307693. –Chatham Bay, 5.55236 , −87.04173 , 8 colony fragments, to 1 cm high, without gonothecae, coll. I. Keith, #307703. –Chatham Bay, dock 004, no coordinates, 1 colony, on Macrorhynchia philippina , 7 mm high, without gonothecae, coll. G. Ashton, #266335. –Chatham Bay, dock 004, no coordinates, 4 colony fragments, to 1.3 cm high, without gonothecae, coll. G. Ashton, #266338. Remarks Hydroids of Clytia linearis are distinctive in having inward-folding pleats extending from the tip of each hydrothecal cusp to the distal wall of the hydrotheca ( Lindner and Migotto 2002 ). Colonies of the species are also usually erect and sympodially branched, and hydrothecae are particularly large and deep ( Cunha et al. 2020 ). Molecular studies indicate that C. linearis has a close relationship to such well-known species as C. hemisphaerica ( Linnaeus, 1767 ) and C. gracilis (M. Sars, 1851 ) ( Govindarajan et al. 2006 ; Leclere et al. 2009 ; Maronna et al. 2016 ; Cunha et al. 2020 ). Subjective synonyms of C. linearis in the Tropical Eastern Pacific include Obelia striata Clarke, 1907 from Panama ( Rees and Vervoort 1987 ; Calder 1991 ), as well as Clytia acutidentata Fraser, 1938a from the Galápagos Islands and Mexico , C. carinadentata Fraser, 1938a from the Galápagos, and Gonothyraea serialis Fraser, 1938a from Colombia and Panama ( Calder 1991 ; Calder et al. 2009 ). While all of these were originally described from small colonies, they share the most distinctive character of the species in having a pleat running from the apex of each marginal cusp to the distal wall of the hydrotheca. Lindner and Migotto (2002) followed the complete life cycle of C. linearis in laboratory cultures. Although medusae liberated from hydroids were reared to maturity, adults could not be linked with certainty to a known medusa species. Newly liberated medusae were similar in morphology to those described in other species of the genus Clytia Lamouroux, 1812 . Schuchert and Collins (2021) have now identified and figured mature medusae of C. linearis , identified by DNA barcoding, from the Gulf Stream off Florida . In the tropical and subtropical eastern Pacific, C. linearis has been reported from Rocas Alijos, west of Baja California ( Calder 1996 ), and the Gulf of California (MendozaBecerril et al. 2020; Estrada-González et al. 2022 ) to Salinas, Ecuador (Calder et al. 2021) and the Galápagos Islands offshore ( Calder et al. 2003 , 2019, 2021). The species has been considered essentially circumglobal in distribution ( Medel and Vervoort 2000 ; Lindner and Migotto 2002 ), and phylogenetic analyses have confirmed that it is widely distributed ( Lindner et al. 2011 ; Cunha et al. 2017 , 2020). Particular notice has been taken of its occurrence on certain species of shelled pteropods (e.g. Clarke 1907 , as O. striata ; Kramp, 1921 , as Laomedea striata ; Stechow 1925 , as Clytia striata ; Vervoort 1946 , as L. striata ; Millard 1975 , as C. gravieri ( Billard, 1904 ; Cornelius 1982 )), which may contribute to long-range transport of the species. This said, the presence of C. linearis in port and bay fouling communities here, as well as on the Ecuador mainland and in the Galapagos Islands (Calder et al. 2021), leads to its interpretation as an entirely naturally distributed complex, and we thus regard it as cryptogenic in Cocos, in line with our previous categorisation for the Galapagos (Calder et al. 2021). The genetic similarity among its populations around the world further suggests relatively fluid gene flow, more likely to be achieved by ships moving between oceans than by the far more limited interoceanic movement of pteropods. Reported distribution Cocos Island: first record. Elsewhere: taken to be circumglobal in tropical and warm-temperate waters ( Medel and Vervoort 2000 ; Lindner and Migotto 2002 ; Calder et al. 2019 , 2021).