Tuesday, July 25, 2017

[Herpetology • 2017] Hydrophis platurus xanthos • A New Subspecies of Sea Snake from Golfo Dulce, Costa Rica


Hydrophis platurus xanthos 
Bessesen & Galbreath, 2017

ambush posture; floating at the sea surface at night in a sinusoidal shape, head below, mouth agape. 
Abstract
We describe a distinctive new subspecies of sea snake from the occasionally anoxic inner-basin waters of Golfo Dulce, Costa Rica, based on combined data garnered between 2010 and 2017 for 154 specimens, 123 free-ranging and 31 museum-held. The yellow sea snake, Hydrophis platurus xanthos Bessesen & Galbreath, subsp. n., is diagnosed by a notably smaller body size and nearly uniform yellow coloration, which contrasts with the black and yellow striae and tail spots or bands typical of the species. Within the modest geographic range (circa 320 km2), nearly all specimens possess both diagnostic character states. Bathymetrics appear to restrict genetic flow between this allopatric population and conspecifics in the broader Eastern Pacific. In perspicuous contrast to typical H. platurus, H. p. xanthos shows no association with drift lines, and feeds at night in turbulent waters, assuming a sinusoidal ambush posture never previously reported for the species. This evolutionarily significant unit (ESU) warrants taxonomic recognition and active protection.

Keywords: Golfo Dulce, Costa Rica, sea snake, yellow color morph, platurus, type specimens, taxonomy



Figure 2: Hydrophis platurus xanthos sharply contrasts typical H. platurus in color, body size and behavior.
A Scaled size comparison of yellow sea snake, TL 43 cm (left), and yellow-bellied sea snake, TL 69 cm (right; note the use of Vetrap as a calming mask and sickly condition of the specimen) B ambush posture of H. p. xanthos; floating at the sea surface at night in a sinusoidal shape, head below, mouth agape C although predominantly yellow, xanthic individuals often possess black spots along the dorsum.
  
Hydrophis platurus xanthos subsp. n.

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.

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 sq.km. in the northern half of the Golfo Dulce, Costa Rica. 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).



Conclusions
Hydrophis platurus xanthos is a well-defined evolutionary subspecies inhabiting a small area of unusual geography. Given unique aspects of its behavioral ecology, it could well represent an intrinsically genetically isolated taxon of recent origin, in which case a species designation would be appropriate. We have been appropriately conservative here, in defining it at a subspecific level. This provides footing for protective strategies, while allowing future research to refine its taxonomic rank.


 Brooke L. Bessesen and Gary J. Galbreath. 2017. A New Subspecies of Sea Snake, Hydrophis platurus xanthos, from Golfo Dulce, Costa Rica.
 ZooKeys. 686: 109-123.  DOI:  10.3897/zookeys.686.12682

  

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