Bioacoustics of Tridentata borneensis

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 ).

Remarks . For contemporary descriptions of Tr. borneensis , refer to Gibbons & Ryland (1989) and Schuchert (2003) (both as Sertularia ). The species appears genetically divergent from all its congeners, with which it nevertheless forms a monophyletic clade, exclusive of Tr. loculosa ( Fig. 28 ). Morphologically, it should be added that its hydrothecal walls are distinctively chiseled, especially when observed laterally ( Figs 15I , 18C ) or apically ( Figs 15J , 18B ), with a broad, transverse constriction in middle part and an arching ridge running parallel to the adnate adaxial wall ( Figs 15K–L , 18A ). These features are easily overlooked in specimens retaining their coenosarc, unless the stems are carefully examined in lateral view (and at a high magnification of the stereomicroscope). Ideal observations are made on material cleared with a 1‒2% solution of domestic bleach. Sole Pennycuik (1959: 197 , pl. 6 fig. 5, left-hand side drawing) was able to notice “[o]ne detail which [Billard] failed to observe”, namely “the well developed tooth to which the abcauline caecum of the hydranth is attached” (see Figs 15K , 18A herein). The best illustration of a fully-formed gonotheca so far is that of Gibbons & Ryland (1989 : fig. 34D). Calder et al . (2022 : fig. 6I) also provided a good figure, and a new illustration is now available in Fig. 15F herein. Mammen (1965 : fig. 71, as S. west-indica ) and Schuchert (2003 : fig. 43C, as S. borneensis ) obviously had specimens with immature gonothecae whose distal horns were not yet formed. It should be noted that the latter author misinterpreted the outer ornamentation of the gonothecal wall (“wall with sharp, projecting spiral structure in 7‒8 loops”), the ridges being irrefutably transverse in this species. The specific name became available in 1925, not 1924, as mentioned in several earlier accounts ( e.g. Billard 1925b ; Van Soest 1976 ; Vervoort & Vasseur 1977 ; Van Praët 1979 ; Gibbons & Ryland 1989 ; Preker 2001 ; Preker & Lawn 2010 ). Indeed, Billard’s report was read at the December 23 th , 1924 meeting of the French Zoological Society, and published subsequently on April 10 th , 1925 in numbers 8‒10 of its Bulletin. As noted by Calder (1991: 110) , “[t]he hydroid identified by Vannucci (1949) as Sertularia borneensis f. parvula appears to have had an intrathecal septum, and it is referred […] to T[ridentata] turbinata ( Lamouroux, 1816 ) ”, and we agree. 8 The other one bears the registration number H.L.710. 9 To allow an easy distinction between the genera Thuiaria and Tridentata , abundantly mentioned below, the former is abbreviated as “ Th .” and the latter as “ Tr .”. ......Figure legend found on the next page We also concur with Gibbons & Ryland’s (1989: 419) , Calder’s (1991: 110 , 2020: 221 ) and Schuchert’s (2003: 190) arguments that Vervoort & Vasseur’s (1977) specimens assigned to Tr. turbinata ( Lamouroux, 1816 ) belong to the present species instead. The scanning electron microscope images provided by Migotto & Vervoort (1998: figs 2‒3) of their S. vervoorti leave no doubt about its conspecificity with the present hydroid. Their material obviously bore immature gonothecae. The Guam record by Kirkendale & Calder (2003: 176) is unreliable in the absence of a formal description and/or illustrations. Their colonies are reportedly said “[r]igid, linear […], with upright cormoids perpendicular to the substratum and usually not more than ⁓ 3 cm in height”, a size not reported so far in Tr. borneensis [literature data indicate stems ranging commonly from “about 1 cm ” ( Billard 1925b: 55 ; present study), to 1.3 cm ( Gibbons & Ryland 1989: 418 ), to 1.5 cm ( Preker & Lawn 2010: 131 ), to maximum 2 cm ( Schuchert 2003: 189 )]. In addition, for the synonymy of this species, the authors referred to Calder (1991) , who considered it as a junior synonym of Tr. tumida ( Allman, 1877 ) , which further complicates the taxonomic assessment of their material. Calder et al . (2022: 592) , however, listed this record in the reported distribution of Tr. borneensis . The material from Guadeloupe assigned to S. tumida by Galea (2008: 36) may belong to the present species, as well. Unfortunately, the specimen used to draw fig. 7A in that report is now lost, but photomicrographs of it are still available, though of limited value, as the hydrothecae are only seen in frontal view. Calder et al . (2022: 591) noted that the “report of S[ertularia] longa from the Galápagos Islands by Calder et al . (2003 , as T[r]. longa ), based on sterile specimens, is believed […] to have been based on T[r]. borneensis instead”, and we concur. One of the two slides deposited at the MNHN of Paris, belonging to the syntype of Tr. borneensis , was reexamined. Although the mounted stems could only be seen frontally, the outer, arched hydrothecal ridge (running parallel to the adnate portion of the adaxial wall) is well visible in some hydrothecae ( Fig. 15G, K ), as is the internal perisarc projection for the attachment of the blind sac ( Figs 15K , 18A ). There is no doubt that the type material of Tr. borneensis exhibits the same morphological features as those met with in the new material from Bali . The taxonomic status of Tr. borneensis was a matter of debate for nearly a century. Many authors [ e.g. Billard (1925b: 172‒173) ; Mammen (1965: 41) ; Cooke (1975: 100) ; Calder (1991: 109‒110 ; 2020: 220‒221 ); Schuchert (2003: 189‒190) ; Calder et al . (2022: 591) ] noted morphological similarities with one or several nominal species, namely: S. tumida Allman, 1877 , Thuiaria maldivensis Borradaile, 1905 , S. tongensis Stechow, 1919b , Tr. westindica Stechow, 1919a , S. malayensis var. sorongensis Leloup, 1930 . According to Calder (1991: 110) , type material of S. tumida could not be located, but he subjectively assigned a Bermudan hydroid to that species on the account of some apparent morphological similarities 10 . The original account on S. tumida is, unfortunately, little informative (succinct description, sketchy illustrations) to allow a reliable identification, and Allman’s nominal species may possibly have been based on other hydroids known to occur in the (tropical) western Atlantic, e.g. Amphisbetia distans ( Lamouroux, 1816 ) [ Calder (1991) , as Tr. distans ; Migotto (1996) , as S. distans ; Galea & Ferry (2015) , as S. distans ], Tr. turbinata ( Lamouroux, 1816 ) 11 [ Migotto (1996) , as S. turbinata ; Galea & Ferry (2015) , as S. turbinata ], and Dynamena dalmasi ( Versluys, 1899 ) [ Vervoort (1959) , as S. dalmasi ; Calder (2013) ; Galea et al . (2021)]. The illustration provided by Allman (1877 : pl. 16 fig. 4) suggests, at least, that S. tumida has longer and slenderer internodes compared to Tr. borneensis , and it is unlikely to be a senior synonym of it. 10 Unlike in Allman’s (1877 : pl. 16 fig. 4) species, the hydrothecae of Calder’s (1991 : fig. 59A) hydroid have an obviously shorter portion adnate to the internode, suggesting specific (and also possibly generic) differences. 11 Prior to the accounts of Billard (1925c : fig. 1F, G; 1925b: fig. 34M), who revealed their fine anatomy, A. distans [roughly illustrated by Lamouroux (1816 : pl. 5 fig. 1B)] and Tr. turbinata (not illustrated in the original account), respectively, were virtually unknown morphologically to Allman. It is not impossible that he established his Sertularia tumida based on either one of these or D. dalmasi , unless Calder’s (1991) material is, indeed, demonstrated unequivocally to belong to that species. Calder’s (1991: 110) assumption that Allman’s “illustration shows the back of a hydrothecal pair instead of the front” is unconvincing, given the expertise of the British author in the field. Type material of Th. maldivensis was reexamined ( Figs 16A–B , 17A , 18D–E ) and, as originally reported, both stems mounted on slides are unfortunately sterile. Specimen NHM 23.2.15.46 is complete ( Fig. 16A , left), and it is here designated as lectotype ; it originates from a branched stolon and is composed proximally of a smooth, 390 µm long, athecate part, ending in an oblique hinge joint, and a 5.15 mm long, distal part divided into a regular sequence of thecate internodes; the nodes are faint and transverse, 70‒85 µm wide, and the internodes 540‒675 µm long, rather slender, though gradually expanding from their proximal to distal ends, and there merging smoothly into a pair of contiguous hydrothecae; the latter are slightly more than half adnate (adnate part 180‒235 µm long, free part 150‒180 µm long), almost tubular (maximum width 135‒150 µm), though curving sharply away from the internode, more markedly on their abcauline side (proximally on stem, the angle is of 70°, but decreases distally to 50°; the abcauline wall is 195‒240 µm long). The aperture, whose diameter is 80‒90 µm, comprises 2 large, lateroabcauline, triangular cusps with rounded tips, in addition to a minute adcauline cusp. The hydranths (all retracted into their corresponding thecae) are not satisfactorily preserved, and do not seem to possess an abcauline caecum. Though difficult to observe, the opercular apparatus is composed of a large abcauline flap, and what appears now to be 2 latero-adcauline, triangular flaps, the latter possibly resulting from the shredding of an original, pleated flap (upon the chemical treatment used to prepare the mounts). Gravier-Bonnet & Bourmaud (2012: 110 , as S. maldivensis ) included a putative new record of this species from the Maldives but did not provide a description and/or illustrations of this poorly-documented species, simply stating that “[f]ertile specimen ( sic ) is needed to go further in identification”. The record from Hawaii by Calder (2020: 220 , fig. 6C–E, as Tr. maldivensis ), is clearly a misidentification, as the hydrothecae do not curve rapidly away from their internodes. The available, scant information on Th. maldivensis appears, at first, potentially problematic. However, the apparent lack of an abcauline caecum would place it into the genus Dynamena Lamouroux, 1812 , where it comes close to D. heterodonta ( Jarvis, 1922 ) . Type material of the latter ( Fig. 18F, G ) was reexamined by Vervoort & Vasseur (1977: 39) 12 , and the specimens from the Marquesas studied by Galea (2016: 6 , fig. 1L, N) appear also very similar ( Fig. 18H ). Hydrothecae of Th. maldivensis , through their distinctive abcauline bent, recall from afar those of Tr. malayensis ( Billard, 1925a ) , but their free part in that species is comparatively longer, and the marginal cusps are much sharper [ Vervoort & Vasseur (1977 : fig. 25C‒D), type material]. The quite unusual specimens of Amphisbetia distans ( Lamouroux, 1816 ) depicted by Calder (1991 : fig. 55A‒B, as Tr. distans ) have similar hydrothecae, but the division of the stem into internodes is typical of Lamouroux’ hydroid [“Occasionally two thecate internodes separated by a short athecate internode marked by an oblique node at proximal end and an oblique hinge-joint at distal end” ( Calder 1991: 106 ); see also below the note on the taxonomic status of Tr. occulta Stechow, 1926 ]. No additional nominal species of sertulariids are known to have hydrothecae resembling those of Th. maldivensis . Therefore, Th. maldivensis and D. heterodonta are very likely coterminous. Consequently, Borradaile’ (1905) species has priority over that created by Jarvis (1922) , and it is here referred to as D. maldivensis ( Borradaile, 1905 ) , comb. nov. , due to the absence of an abcauline caecum in the polyps. It should also be noted that the type localities of the two nominal species are both situated in the western part of the Indian Ocean [Huvadhu Atoll, the Republic of Maldives for Th. maldivensis , and Saint Brandon ( Cargados Carajos Shoals ), Mauritius , for D. heterodonta ], being distant of 2400 km . Type materials of three nominal species created by Stechow, namely S. tongensis ( Fig. 16D ), Tr. westindica ( Fig. 16F , 17D ), as well as the rather enigmatic taxon (not illustrated so far) Tr. occulta ( Fig. 16C ), were also examined. The hydrothecae of S. tongensis show the same shape, submarginal intrathecal perisarc projections (two latero-adcauline, one abcauline), and horseshoe-shaped internal ridge ( Figs 16E , 17C , 18K ) as S. orthogonalis Gibbons & Ryland, 1989 , the latter evidently becoming a junior synonym of Stechow’ species (N.B.: the type locality of the former, the Kingdom of Tonga , is distant of only about 800 km from that of the latter, the Fiji Islands ). Although originally described based on infertile stems, a description of its gonothecae should be based on Gibbons & Ryland’s account. The paired condition of its hydrothecae, their three-cusped margin and the transversely-ridged gonothecae justify the assignment of Stechow’s hydroid to the genus Tridentata Stechow, 1919a , under the binomen Tr. tongensis ( Stechow, 1919b ) , a combination introduced earlier by Calder (1991: 110) . In an earlier paper, one of us ( Galea 2010: 18–20 ) considered Sertularella tongensis Stechow, 1919 as belonging to the genus Sertularia Linnaeus, 1758 , on the account of the morphology of its closing apparatus. In doing so, it became a secondary homonym of the present hydroid, for which the replacement name S. ephemera Galea, 2010 was introduced. However, molecular evidence led Song et al . (2019) create a new genus, Bicaularia , for it. Consequently, S. ephemera should be now dropped. The geographical distribution of S. tongensis is restricted, to date, to the Kingdom of Tonga ( Stechow 1919b , as S. tongensis ; Gibbons & Ryland 1989 , as S. orthogonalis ) and the Hawaiian Islands ( Calder & Faucci 2021 , as Tr. orthogonalis ). 12 The length of the abcauline side (230–255 µm) and of the hydrothecal aperture (80–95 µm) given by them fit our measurements, but it should be noted that they measured as the adnate adcauline portion only the adjacent part of the thecae forming a pair, otherwise their measurements would contrast with their drawings (free portion much longer than the adnate part). ......Figure legend found on the next page Tridentata westindica has hydrothecae provided with a transverse, horseshoe-shaped, internal ridge in their middle part ( Figs 16G , 17E , 18L ), and are indistinguishable from those of the type of Tr. turbinata ( Lamouroux, 1816 ) depicted by Billard (1925b : fig. 34M, as S. turbinata ). Tridentata westindica is thus a junior synonym of Lamouroux’ species. The occurrence of Tr. turbinata in Martinique , its type locality, is confidently-documented ( Galea & Ferry 2015 , as S. turbinata ). Based on specimens from Les Saintes (HRG-0342), whose hydrothecae have been perfectly cleared with domestic bleach, it is realized that there are many attachment sites of the hydranths to the inner walls of their corresponding hydrothecae, as illustrated in Fig. 18M . The shape of the hydrothecae of Tr. occulta , together with the characteristic presence of internal, perisarcal projections ( Figs 17B , 18I ), as well as the division of the stem into moderately-long, hydrothecate internodes occasionally alternating with comparatively-shorter, athecate internodes (the latter with a proximal, transverse node and a distal, oblique node; Figs 16C , 18I ), are features of Amphisbetia distans ( Lamouroux, 1816 ) ( Billard 1925c : fig. 1, as S. distans ; N.B.: his figs 1F‒G illustrate type material). Consequently, T. occulta becomes a junior synonym of Lamouroux’ species. In conclusion, Tr. borneensis has only two junior synonyms: an objective one, namely S. vervoorti , and a subjective one, S. malayensis var. sorongensis (specimens of the latter should be examined to confirm the synonymy). As demonstrated above, none of the nominal species whose basionyms are S. tumida , Th. maldivensis , S. tongensis and Tr. westindica belong to its synonymy.