Hydrophis platurus xanthos, Bessesen, Brooke L. & Galbreath, Gary J., 2017

Bessesen, Brooke L. & Galbreath, Gary J., 2017, A new subspecies of sea snake, Hydrophisplaturusxanthos, from Golfo Dulce, Costa Rica, ZooKeys 686, pp. 109-123 : 113-115

Hydrophis platurus xanthos subsp. n.

Type specimens.

Holotype female from COSTA RICA: Golfo Dulce: inner basin, 08°35.76'N; 083°13.25'W; collected 13 February 2009 by A. Solórzano, and deposited in the Zoological Museum of University of Costa Rica, San Jose, Costa Rica (MZUCR:HERP:20614); body yellow with 4 black specks (<1 mm) and 5 black dots (2-4 mm) along the dorsum, no tail markings; TL 52 cm, tail length 6 cm, paddle height 11 mm, weight 55 g. Paratype female from COSTA RICA: Golfo Dulce: inner basin, 08°35.76'N; 083°13.25'W; collected 2 September 2009 by A. Solórzano, and deposited in the Zoological Museum of University of Costa Rica, San Jose, Costa Rica (MZUCR:HERP:20836); body yellow with 4 dark-brown dorsal blotches (2-3 mm) on or just caudal to the head, 2 black specks (<1 mm) and 2 dots (3-4 mm) farther down the dorsum, no tail markings; TL 51 cm, tail length 6 cm, paddle height 10 mm, weight 55 cm.

Diagnosis.

Here we describe a new, allopatric subspecies, Hydrophis platurus xanthos subsp. n., or yellow sea snake, from the inner basin of Costa Rica’s Golfo Dulce. The new subspecies is diagnosed based on a dramatic color character state, as well as by a marked difference in body size. Aspects of behavior also appear to be unique.

Hydrophis platurus xanthos is diagnosed as differing from other H. platurus by its predominantly yellow coloration and smaller size. Apparent additional behavioral diagnostic traits include a sinusoidal ambush posture, and a preference for surfacing in rough rather than smooth waters, lacking an association with drift lines. No specimens unambiguously assignable to this taxon have shown full lateral striation on head and body or prominent spots or bands on the tail. The appearance of H. p. xanthos starkly contrasts with the coloration of most conspecifics found in the broader Eastern Pacific, or even in the adjacent mouth of the Golfo Dulce, which evince nearly solid black pigmentation along the dorsum, breaking into spots or bands at the caudal end (Fig. 2A). H. platurus xanthos is also shorter in length, with an average adult TL of 49 cm, compared to about 60-75 cm for other H. platurus populations. Comparing our TL data for H. p. xanthos with those of Rubinoff et al. (1986) for H. platurus in the Gulf of Panama, note that the two samples do not even overlap, despite good sample sizes. The difference between the underlying distributions is significant with P <.01 (Wilcoxin Rank Sum Test, N1 = 15, N2 = 69, T1 = 1155). Behavioral differences are equally pronounced between H. platurus xanthos and its yellow-bellied conspecifics. Drift lines, which play a critical role in the natural history of the species, supporting aggregations for feeding, reproduction and transport, are not used by H. platurus xanthos . Furthermore, while H. platurus feeds diurnally, stretched out in smooth water, H. platurus xanthos feeds at night, in turbulent water, assuming a sinusoidal ambush posture never previously reported for the species.

Coloration detail.

Head, body and tail are bright canary-yellow. Although appearing entirely yellow at a glance, most individuals possess at least one dark, black or brown, dorsal speck or dot (Fig. 2C); such markings lack any fixed pattern, but frequently present caudal-medial to the supraocular scales. A few specimens retain larger black spots along the dorsal ridge; of the yellow sea snakes photographed by an author (BLB; N = 120), less than 13% (N = 15) exhibited a black mark longer than 2 cm.

Etymology.

From Greek xanthos, “yellow,” to highlight a diagnostic feature of this subspecies.

Geographic distribution.

The breeding population of Hydrophis platurus xanthos appears confined to approximately 320 km2 in the northern half of the Golfo Dulce, Costa Rica, from 08°32'N to 08°44'N and 083°11'W to 083°28'W. A spatial gap up to 22 km separates the yellow sea snakes from the usually bi- or tricolored oceanic population, and appears to restrict genetic exchange ( Bessesen 2012, Fig. 1).