Ophisternon berlini, Arroyave & Angulo & Mar-Silva & Stiassny, 2024

Ophisternon berlini , new species urn:lsid:zoobank.org:act:FE496557-A0F3-4C22-8BE1-C3AAF3D7B077

Berlin’s Bloodworm Eel

Figures 2–11 View FIG View FIG View FIG View FIG View FIG View FIG View FIG View FIG View FIG View FIG , Table 1

Holotype.— AMNH 281169, 163 mm TL, Costa Rica, Limón, Siquirres, Reventazón-Parismina drainage basin, Destierro River sub-basin, swampy area within the premises of Las Brisas Nature Reserve , semidry muddy soil near artificial permanent ponds, 1087 0 34.16 00 N, 83836 0 18.97 00 W, 247 masl, Freddy Perez , Juan Chávez , and Erick Berlin , 25 April 2022.

Paratypes.— AMNH 281170 About AMNH (1 tissue voucher [ UCR-CTP 5731 ], 1 alc, 167 þ mm TL [terminal elements of tail missing]), female, Costa Rica, Limón, Siquirres, Reventazón-Parismina drainage basin, Destierro River sub-basin, swampy area within the premises of Las Brisas Nature Reserve , 1087 0 53.22 00 N, 83836 0 11.95 00 W, 235 masl, Freddy Perez , Juan Chávez , and Erick Berlin , 18 June 2021 ; AMNH 281171 About AMNH (2 tissue vouchers [JA2005, JA2006], 2 alc, 144–162 mm TL), Costa Rica, Limón, Siquirres, Reventazón-Parismina drainage basin, Destierro River sub-basin, swampy area within the premises of Las Brisas Nature Reserve , 1087 0 38.53 00 N, 83836 0 14.76 00 W, Freddy Perez , Juan Chávez , and Erick Berlin , 6 February 2023; UCR 3321- 001 (2 alc, 134–159 mm TL), Costa Rica, Limón, Siquirres , Reventazón-Parismina drainage basin, Destierro River sub-basin, swampy area within the premises of Las Brisas Nature Reserve , 1087 0 53.22 00 N, 83836 0 11.95 00 W, 235 masl, Freddy Perez, Juan Chávez, and Erick Berlin, 18 June 2021; UCR 3322- 001 (1 alc, 142 mm TL), same data as holotype .

Diagnosis.— Ophisternon berlini is assigned to the genus Ophisternon by having a pectoral girdle connected to the skull by a forked posttemporal joining the supracleithrum at the level of the third vertebra vs. posteriorly displaced to the level of the fifth vertebra with posttemporal reduced to a simple rod and disconnected from the supracleithrum in Synbranchus ( Fig. 4 View FIG ) and a branchiostegal membrane opening slit-like vs. small pore-like in Synbranchus ( Fig. 5 View FIG ). Ophisternon berlini is unique among its Neotropical congeners in having: 1) an elevated number of precaudal vertebrae: 100 vs. 65 in O. infernale and 75 in O. aenigmaticum ; as the number of total vertebrae is similar among Neotropical species of Ophisternon (140), the precaudal/caudal vertebrae ratio is significantly higher in the new species ( Fig. 6 View FIG ); 2) a distinctive oral dentition characterized by longer and larger (and therefore less numerous), conical premaxillary (,50), dentary (,30), palatine (,10), and ectopterygoid (,20) teeth ( Fig. 7 View FIG ); 3) palatine and ectopterygoid teeth in a single row vs. two or three irregular rows becoming a single row posteriorly on ectopterygoid ( Fig. 7 View FIG ); and 4) a small and narrow (acutely angled) crescent-shaped gill membrane opening, occupying about 20% of the head’s ventral surface vs. wider and larger ( Fig. 5 View FIG ). Its troglobitic phenotype, characterized by depigmented skin (pink when alive and cream when preserved) and extremely reduced, subcutaneous eyes ( Fig. 8 View FIG ), further distinguishes O. berlini from other Neotropical synbranchids except for O. infernale , a species geographically restricted to the cenotes and submerged caves of the Yucatán Peninsula in Mexico,. 1,200 km northwest in linear distance from the collection sites of O. berlini . Ophisternon berlini is further distinguished from O. infernale in lacking a neurocranial crest ( Fig. 9 View FIG ) and in having a single tooth row on the palatine (vs. three rows; Fig. 7 View FIG ), elongate (vs. shorter) gill rakers, and stouter (vs. slenderer) fifth ceratobranchials and toothplates ( Fig. 10 View FIG ).

Description.— For general appearance (including overall body shape and coloration) see Figure 8 View FIG . Morphometric data derived from the type series are presented in Table 1. Body worm-like, small (maximum size c. 170 mm TL), and extremely slender (maximum body width 2.3% of TL). Head pointed but slightly rounded and blunt anteriorly, with small nuchal hump resulting from expansion of the adductor mandibulae musculature over the neurocranial roof, most developed in (presumed) males (e.g., Fig. 8A View FIG ). Anterior nostril at snout tip; posterior nostril situated anterior to eye. Eyes very small and subcutaneous, covered with thick skin. Mouth subterminal. Premaxilla with several rows of large, conical, and slightly recurved teeth at expanded anterior tip, and a single row of teeth along most of ventral margin. Maxilla edentate. Dentary teeth large and arranged in a single row. Total number of vertebrae 142 (99–101 precaudal, 41–43 caudal). Six fully ossified branchiostegal rays extending posteriorly and reaching vertical through cleithrum. Branchiostegal membrane forming folds between branchiostegal rays. Gill opening ventral, small (occupying about 20% of head width), and lunate (acutely angled). Lateral line absent. Laterosensory system greatly reduced, externally visible as 2–3 tiny pores on the lower jaw and the preopercle.

Coloration.— In life, uniformly light/translucent pink, with epaxial and hypaxial musculature visible. Reduced and subcutaneous eyes visible as tiny dark spots. In alcohol, uniformly whiteish/pale cream ( Fig. 8 View FIG ); body darker at branchial basket and around some sections of the digestive tract (possibly corresponding to clotted blood and undigested food items visible due to body translucency, respectively). Coloration like that of other troglomorphic synbranchids such as the congenerics O. infernale and O. candidum .

Distribution and habitat.— Ophisternon berlini is only known from specimens from three point localities in a swampy area within the premises of Las Brisas Nature Reserve, a tropical rainforest estate primarily devoted to biodiversity conservation and research, located near the town of La Alegría in Siquirres, Limón, Costa Rica, on the northeastern slopes of the Turrialba Volcano ( Figs. 1 View FIG , 2 View FIG ). These point localities are located within 200–600 m of each other, no further than 400 m from the Río Destierro, the main lotic system in the area (Reventazón-Parismina drainage basin), and in proximity to a few artificial ponds. These ponds were created by the landowner to recreate lentic aquatic environments within his property to encourage their use by native fauna and flora, thus contributing to their preservation. The reserve offers a confined area of secondary but highly protected and conserved forest surrounded by a highly modified landscape dominated by crop fields ( Fig. 1D View FIG ). Ophisternon berlini appears to be an endogean species restricted to living in soil. All specimens from the type series were found buried in the mud at around 50–75 cm under the surface, in the subsoil layer. Some specimens were found together with Caecilia volcani , a relatively common caecilian in the reserve, and a species previously thought to be endemic to Panama but since 2017 confirmed to also occur in Costa Rica ( Kubicki and Arias, 2017).

Etymology.— The specific epithet (a noun in the genitive case) is a patronym honoring Mr. Erick Berlin, a strong supporter of conservation and scientific research of Costa Rican biodiversity, discoverer of the population of swamp eel herein formally described, and owner of La Brisas, a private nature reserve that contains the type locality of the new species.

Remarks.— With a size range of just 114–c. 170 mm TL, specimens in the type series are considerably smaller than sexually mature individuals of both congeners ( Rosen and Greenwood, 1976). However, dissection of one of the paratypes of O. berlini (AMNH 281170) revealed that a single ovary, containing about 15 large eggs, filled the visibly distended abdominal cavity ( Fig. 8C View FIG ), indicating that O. berlini is mature at a very small size. Completion of maturation at a small size (predisplacement) is evident also in a comparison of the neurocrania of O. berlini and O. aenigmaticum in individuals of similar total lengths. As indicated in Figure 9 View FIG , the neurocranium of O. berlini ( Fig. 9A View FIG ) is fully ossified and “adult-like,” while that of O. aenigmaticum at the same size ( Fig. 9D View FIG ) is clearly still developing with open sutures and a domed neurocranial roof, contrasting with the neurocranial morphology of mature specimens of that species ( Fig. 9C View FIG ). Taken together, these data indicate that O. berlini may correctly be a true dwarf species. Besides the abovementioned diagnosis, O. berlini could be easily recognized by the fact that, being sympatric only with S. marmoratus , it is the only swamp eel from the region that is endogean (i.e., soil-dwelling), dwarf, and troglomorphic.

Newly generated synbranchid mitochondrial genomes.— The complete mitochondrial genomes of O. berlini (GenBank Accession PP975746; AMNH 281171, paratype) and O. aenigmaticum (GenBank Accession PP975747; CNPE-IBUNAM 24368) are 16,997 and 16,834 bp long, respectively. Both mitogenomes display the typical length, composition, and arrangement of synbranchiform mitochondrial genomes, consisting of 37 genes, namely, 13 PCGs (12 on the H-strand and only ND6 on the L-strand), 2 rRNAs (both on the H-strand), 22

tRNAs (14 on the H-strand and eight on the L-strand), and one Control Region (D-loop). Notably, incomplete stop codons were detected in six and seven PCGs for O. berlini and O. aenigmaticum , respectively. The non-coding region D-loop is longest in O. berlini (1,270 bp) and flanked by tRNAPro and tRNAPhe in both species. Detailed information on the structure and organization of these newly generated mitogenomes is presented in Table 2.