Tridentata borneensis Billard, 1925a

Calder, Dale R., Carlton, James T., Keith, Inti, Ashton, Gail V., Larson, Kristen, Ruiz, Gregory M., Herrera, Esteban & Golfin, Geiner, 2022, Biofouling hydroids (Cnidaria: Hydrozoa) from a Tropical Eastern Pacific island, with remarks on their biogeography, Journal of Natural History 56 (9 - 12), pp. 565-606 : 590-592

Tridentata borneensis Billard, 1925a

( Figure 6g –i )

Sertularia borneensis Billard, 1925a: 649 , fig. 1D.

Type locality

Indonesia: Borneo, Borneo Bank, 02°25 ʹ S, 117°43 ʹ E, 34 m, on fine coral sand ( Billard 1925 a, 1925b; Van Soest 1976; Vervoort and Vasseur 1977).

Material examined

Wafer Bay , 5.54535, −87.06185, 5 colony fragments, to 7 mm high, without gonothecae, coll . G . Ashton , #240634 . – Wafer Bay , 5.54535, −87.06185, 1 colony, with 1 gonotheca, coll . G . Ashton , #240636 . – Wafer Bay , 5.54535, −87.06185, 1 colony, on a barnacle, 3 mm high, without gonothecae, coll . G . Ashton , #240629 . – Wafer Bay , 5.54535, −87.06185, 3 colony fragments, to 5 mm high, without gonothecae, coll . G . Ashton , #240637 . – Wafer Bay , 5.54535, −87.06185, 6 colony fragments, to 7 mm high, without gonothecae, coll . G . Ashton , #240630 . – Chatham Bay , 5.55318, −87.03996, 1 colony fragment, 6 mm high, without gonothecae, coll . I . Keith , #240556 . – Chatham Bay , 5.55318, −87.03996, 2 colony fragments, to 5 mm high, without gonothecae, coll . G . Ashton , #179806 . – Chatham Bay , dock 004, no coordinates, 1 colony fragment, 1 cm high, with gonothecae, coll . G . Ashton , #266336 . – Chatham Bay , dock 004, no coordinates, 3 colony fragments, to 9 mm high, without gonothecae, coll . G . Ashton , #266335 .

Remarks

These specimens from Cocos Island conform with accounts of the trophosomes and gonosomes of Sertularia borneensis Billard, 1925a by Billard (1925 a, 1925b), Gibbons and Ryland (1989), Schuchert (2003), and Preker and Lawn (2010, 2010), and they are referred to that species here. The nomenclature of the species is nevertheless unsettled, as the relationship between hydroids assigned the names S. tumida Allman, 1877 , S. maldivensis Borradaile, 1905 , S. tongensis Stechow, 1919 , and S. borneensis is unclear. They all appear much alike in terms of trophosomal morphology; gonosomes have been described to date only in S. borneensis . All of these binomena predate that of S. borneensis , and the names of any found conspecific with it would have priority. In several works, S. borneensis has already been included in the synonymy of S. tumida ( Leloup 1960; Calder 1991; Vervoort and Watson 2003; Zhenzu et al. 2014; Song 2019).

Characters of both trophosome and gonosome of S. borneensis are very much as in the type species of the genus Tridentata Stechow, 1920 , T. perpusilla ( Stechow, 1919) , and quite unlike those of S. argentea Linnaeus, 1758 , type species of Sertularia Linnaeus, 1758 . Other species now assigned to Tridentata besides its type include T. turbinata , T. marginata ( Kirchenpauer, 1864) , T. tumida ( Allman, 1877) , T. rugosissima ( Thornely, 1904) and T. orthogonalis ( Gibbons and Ryland, 1989) .

Included in the synonymy of T. borneensis by Schuchert (2003, as S. borneensis ) were the records of S. westindica Stechow, 1920 from India by Mammen (1965) and from Eniwetak Atoll by Cooke (1975). As for the Atlantic species S. westindica , it has been considered conspecific with T. tumida by Leloup (1960), Calder (1991), Zhenzu et al. (2014), and Song (2019). However, records of it from the Indo-Pacific are taken here to have been based on T. borneensis .

Vervoort and Vasseur examined and illustrated the holotype of S. borneensis , referring it in error to the synonymy of S. turbinata ( Lamouroux, 1816) (= Tridentata turbinata ). Most importantly, the species always lacks the distinctive intrathecal ridge of perisarc extending around the abcauline wall of the hydrotheca in T. turbinata . The two are therefore recognised as distinct species.

The trophosome of T. borneensis resembles that of S. longa Millard, 1958 , but gonothecae of the two species are fundamentally different in shape. Those of T. borneensis are horizontally ribbed, with two horns flanking a wide distal aperture ( Figure 6g ), while those of S. longa are smooth-walled, with a distal collar and narrower aperture ( Millard 1975). The report of S. longa from the Galápagos Islands by Calder et al. (2003, as T. longa ), based on sterile specimens, is believed here to have been based on T. borneensis instead. Given the morphology of its gonothecae, as described by Millard, S. longa cannot be assigned to either Tridentata or Sertularia . Its generic affinities are presently unclear.

In accounts on hydroids obtained during the Allan Hancock Pacific Expedition, Fraser (1938a, 1938b, 1938c, 1948) did not report T. borneensis from the eastern Pacific. The closest of his species to it appears to be S. dispar Fraser, 1938a . However, hydrothecal walls of that species are marked by very fine striations ( Calder et al. 2009) rather than being smooth as in T. borneensis , and the two are considered specifically distinct.

In a study of stylasterids, Cairns (1991) considered that species of both Cocos Island and the Galápagos Islands had been derived from the western Pacific, and that they had no affinity with the eastern Pacific shelf and slope fauna of continental North and South America. In being known in the eastern Pacific only from those same oceanic islands, T. borneensis appears to have had a similar geographic origin. While possibly hailing from the Indo-West Pacific, we conservatively treat T. borneensis as cryptogenic in both the Eastern and Central Pacific.

Reported distribution

Cocos Island: first record.

Elsewhere: shallow tropical waters from the east coast of India to the Galápagos Islands ( Billard 1925a; 1925b, as Sertularia borneensis ; Mammen 1965, as S. west-indica ; Pennycuik 1959, as S. borneensis ; Cooke 1975, as S. westindica ; Van Soest 1976, as S. borneensis ; Vervoort and Vasseur 1977, as S. turbinata and S. borneensis ; Gibbons and Ryland 1989, as S. borneensis ; Schuchert 2003, as S. borneensis ; Tang, 1991, as S. westindica ; Calder et al. 2003, as Tridentata longa ; Kirkendale and Calder 2003; Vervoort and Watson 2003, as S. tumida ; Preker and Lawn 2008, 2010, as S. borneensis ).

Figures

Figure 6. Leptothecata: Obeliidae, Haleciidae, Sertularellidae and Sertulariidae. (a) Obelia microtheca: hydrocaulus with two pedicels and hydrothecae, #240607. (b) Obelia microtheca: part of hydrocaulus with long pedicel and a hydrotheca, #240607. (c) Obelia microtheca: part of hydrocaulus and base of a pedicel with a gonotheca, #240607. (d) Halecium sp.: part of hydrocaulus with hydrothecae and base of a branch, #266335. (e) Sertularella affinicostata: stolon and stem with five hydrothecae, #240592. (f) Sertularella affinicostata: part of stem with two hydrothecae, #240592. (g) Tridentata borneensis: base of a stem with two pairs of hydrothecae, #240634. (h) Tridentata borneensis: distal end of a stem with two pairs of hydrothecae, #240634. (i) Tridentata borneensis: gonotheca, #266336. Scale bars: a, i = 0.2 mm; b–d, f–h = 0.1 mm; e = 0.5 mm.