Revista de Biología Tropical, ISSN: 2215-2075, Vol. 72: e57498, enero-diciembre 2024 (Publicado Set. 18, 2024)

Notes on the natural history and identity of

the pelagic sea snake Hydrophis platurus (Serpentes: Elapidae)

from Golfo Dulce, Puntarenas, Costa Rica

Alejandro Solórzano1*

Mahmood Sasa1,2; http://orcid.org/0000-0003-0118-5142

1. Museo de Zoología, Centro de Investigaciones en Biodiversidad y Ecología Tropical, Universidad de Costa Rica, Ciudad

Universitaria Rodrigo Facio, San Pedro de Montes de Oca, Costa Rica; solorzano29@gmail.com (*Correspondence)

2. Instituto Clodomiro Picado, Escuela de Biología, Universidad de Costa Rica, Ciudad Universitaria Rodrigo Facio, San

Pedro de Montes de Oca, Costa Rica; msasamarin@gmail.com

Received 02-XI-2023. Corrected 08-IV-2024. Accepted 16-IX-2024.

ABSTRACT

Introduction: The pelagic sea snake Hydrophis platurus has the broadest distributional range of all the snakes

and is the only sea snake of tropical waters off the west coast of the Americas. Within the variation in the color

pattern of this species, completely yellow specimens were considered very rare or occasional. However, a yel-

low pattern dominates a population established in the interior of the Golfo Dulce in the Southern Pacific of

Costa Rica.

Objective: We studied the abundance, activity patterns, morphometry, and feeding behavior of the yellow H.

platurus within the Golfo Dulce.

Methods: Between February 2009 and July 2018, we conducted surveys for sea snakes from a boat within the

perimeter of the Golfo Dulce.

Results: Adults from the gulf population are smaller than the bicolor oceanic population, with females having

larger bodies. Sea snakes within the gulf float and move with surface currents without being linked to drift lines

and debris. These snakes are mainly found in clean, calm waters with little turbulence. They feed on a wide

diversity of small fish, which they capture using passive stalking predatory behaviors floating on the surface. Prey

are quickly captured and swallowed alive. This species has an almost total absence of predation, a pattern that

extends to this yellow population.

Conclusions: The characteristics of the gulf and the direction and strength of the surface currents at its entrance

might restrict the transit of this yellow population to the outside and the entry of the bicolored snakes of the

oceanic population. We do not consider making taxonomic changes in this Golfo Dulce population prudent. This

yellow population is relatively common within the basin. However, the high tourist traffic and other activities are

risk factors for its conservation. Protection policies must be created to preserve and protect this small population

of yellow sea snakes unique within this species’ distribution range.

Key words: Elapidae; Hydrophis platurus; distribution; taxonomy; natural history; Golfo Dulce.

RESUMEN

Notas sobre la historia natural e identidad de la serpiente marina pelágica Hydrophis platurus

(Serpentes: Elapidae) del Golfo Dulce, Puntarenas, Costa Rica

Introducción: La serpiente marina pelágica Hydrophis platurus tiene el rango de distribución más amplio de

todas las serpientes y es la única serpiente marina de aguas tropicales del oeste de América. Dentro de la varia-

ción en el patrón de color de esta especie, los ejemplares completamente amarillos se consideraban muy raros u

https://doi.org/10.15517/rev.biol.trop..v72i1.57498

VERTEBRATE BIOLOGY

2Revista de Biología Tropical, ISSN: 2215-2075 Vol. 72: e57498, enero-diciembre 2024 (Publicado Set. 18, 2024)

INTRODUCTION

The pelagic sea snake Hydrophis platurus

(Serpentes: Elapidae) has the broadest distribu-

tional range of all the snake species (Boundy,

2020; Greene, 1997; Heatwole, 1999; Wallach

et al., 2014), and is the only occurring in tropi-

cal waters off the west coast of the Americas

(Campbell et al., 2004; Kropach, 1975). In

this region, H. platurus ranges in tropical and

subtropical waters from the Gulf of Califor-

nia southward through Ecuador, including the

Galapagos and Easter islands, to Northern Peru

(Campbell et al., 2004). It is common along the

Pacific coast of Costa Rica, especially on gulfs

and bays, usually between 1 and 20 km from

the coast (Lillywhite et al., 2015; Savage, 2002;

Solórzano, 2004; Solórzano, 2022).

The color pattern of H. platurus varies from

the extremely rare entirely or almost complete-

ly yellow specimens recorded in the Eastern

Pacific (Campbell et al., 2004; Kropach, 1975)

to the more common bicolored or tricolored

individuals: a black dorsum with cream, brown

or yellowish flanks in the belly (Campbell et al.,

2004). The yellowish morphs, once considered

extremely rare in early population assessments

of the species, were often interpreted as the

result of rare recessive alleles or the adverse

effect of natural selection (Campbell et al.,

2004; Kropach, 1975; Savage, 2002; Tu, 1976;

Voris, 1983). However, a recent discovery has

challenged this notion. A population composed

entirely of yellow specimens was reported for

Golfo Dulce in the Southern Pacific of Costa

Rica (Solórzano, 2011), suggesting that infre-

quent alleles may not be sufficient to explain

the prevalence of that phenotype there.

In fact, the restricted geographical area

where this population is located suggests that

it may experience selective and environmental

pressures different from those found in other

populations throughout the distribution of the

species. Evaluating ecological aspects in this

yellow-morph population could help clarify

its enigmatic origin. Since 2009, we began a

series of studies on these unusual yellow pat-

tern snakes to better characterize this popula-

tion’s morphological and ecological patterns

and compared them with those of the bicolored

population inhabiting the Pacific coast out-

side the gulf (from now on referred to as the

oceanic population). Here, we present some

observations on the natural history, density,

ocasionales. Sin embargo, los patrones amarillos dominan una población establecida en el interior del Golfo Dulce

en el Pacífico sur de Costa Rica.

Objetivo: Monitoreamos y estudiamos la abundancia, patrones de actividad, morfometría y comportamiento

alimentario del H. platurus amarillo dentro del Golfo Dulce.

Métodos: Entre febrero 2009 y julio 2018, realizamos búsquedas mensuales de serpientes marinas desde una

embarcación dentro del perímetro del Golfo Dulce.

Resultados: Los adultos de la población de golfo son más pequeños que la población oceánica bicolor, con las

hembras de cuerpos más grandes. Las serpientes del Golfo Dulce flotan y se mueven con las corrientes super-

ficiales sin asociarse con líneas de deriva y escombros. Las serpientes se encuentran principalmente en aguas

limpias y tranquilas con poca turbulencia. La dieta se compone de una amplia diversidad de peces pequeños, que

capturan con un comportamiento depredador de acecho pasivo, flotando en la superficie. La presa es rápidamente

capturada y tragada viva. Esta especie tiene una ausencia casi total de depredación, patrón que se extiende a esta

población amarilla.

Conclusiones: Las características del golfo y la dirección y fuerza de las corrientes superficiales en su entrada

podrían restringir el tránsito de esta población amarilla hacia el exterior y el ingreso de las serpientes bicolores de

la población oceánica. No consideramos prudente realizar cambios taxonómicos en esta población. La población

amarilla es relativamente abundante dentro de la cuenca. Sin embargo, el alto tráfico turístico y otras actividades

son factores de riesgo para su conservación. Políticas de protección deben crearse para preservar y proteger esta

pequeña población de serpientes marinas amarillas, única dentro del rango de distribución de esta especie.

Palabras clave: Elapidae; Hydrophis platurus; distribución; taxonomía; historia natural; Golfo Dulce.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 72: e57498, enero-diciembre 2024 (Publicado Set. 18, 2024)

and morphometrics of H. platurus within the

Golfo Dulce region in Costa Rica.

MATERIAL AND METHODS

Study site: The Golfo Dulce is a deep

tropical estuary like a fjord (Wolff et al., 1996),

surrounded by steep mountains, located on the

South Pacific coast of Costa Rica (8°27’-8°45’

N & 83°07’-83°30’ W, Fig. 1). This ecosystem

represents the only anoxic basin on the Pacific

coast of the Western Hemisphere and is one of

the four systems with these characteristics that

exist in the tropics (Quesada-Alpízar & Cortés,

2006; Quesada-Alpízar & Morales-Ramírez,

2004; Svendsen et al., 2006). The gulf is about

50 km long and has an approximate surface

area of 680 km2, with a width of 10 to 15 km

(Quesada-Alpízar & Cortés, 2006). The maxi-

mum depth reaches 215 m in the inner part

of the gulf, with a shallow entrance of about

60 m deep (Fig. 2). These characteristics tend

to limit water circulation in the internal basin

of this gulf, and most of the circulation results

from the entry of fresh water from rivers,

effect of local winds, and its interaction with

the topography (Svendsen et al., 2006). There

are two layers of water in the Inner Gulf: one

superficial with high temperatures, low salinity,

and moved mainly by the winds, and another

deeper, colder, and with higher salinity, with a

slower flow (Svendsen et al., 2006).

Snake surveys and capture: We searched

for H. platurus from a low-speed boat during

the morning (7:00-11:00) and night (19:30-

23:00) from February 2009 to July 2018. Sur-

veys started at Playa Nicuesa (8°66’084’’ N &

83°28’098’’ W) following random routes across

the gulf, moving between the sectors of Puerto

Jiménez, Rincón, Playa Blanca, Punta Mogos,

Punta Esquinas and San Josecito (Fig. 1). We

also searched in the gulf entrance, between

Playa Matapalo and Punta Banco (Fig. 1). A

Fig. 1. Location of Golfo Dulce in the Southern Pacific coast of Costa Rica.

4Revista de Biología Tropical, ISSN: 2215-2075 Vol. 72: e57498, enero-diciembre 2024 (Publicado Set. 18, 2024)

total of 34 visits were made for three to four

days each month to the Golfo Dulce (102 days

in total) to sampling the abundance, dial activ-

ity patterns, and external morphology (mor-

phometry and lepidosis).

Snakes were collected manually using a cir-

cular net (Fig. 3A). Sea surface conditions and

GPS coordinates were recorded at sight. The sea

surface condition was recorded on an ordinal

scale with four categories: a) mirror-effect (i.e.,

water without surface movement, Fig. 4A); b)

clean and calm surface (minor undulations or

weak surface waves, Fig. 4B); slightly disturbed

surface (evident water movement and moder-

ate waves, Fig. 4C); d) turbulent surface (strong

waves and evident movement of the surface

water mass, Fig. 4D). We also registered the

behavior and surface activity of the snakes prior

to capture.

Body measurements: The captured snakes

were sexed manually, using the inversion of

the hemipenis in males. For all specimens,

body lengths (total length, snout-vent length,

head length, and width) were taken to the

nearest millimeter using a tape measure. The

animal’s weight is also estimated using a 500 g

electronic scale. For comparative purposes, we

took body measurements from oceanic bicol-

ored specimens captured in both Golfo Dulce

and Playa del Coco, Guanacaste Province, in

the Northern Pacific of Costa Rica, kept at the

Museo de Zoología, Centro de Investigaciones

en Biodiversidad y Ecología Tropical, Univer-

sidad de Costa Rica. To avoid duplication of

information, captured individuals were marked

by cutting a tissue clip from the end of the tail

(approximately 3 mm long). This clip was saved

for future DNA analysis.

Female reproductive status (gravid vs non-

gravid) was examined by palpation of the lower

abdomen. Sometimes, stomach contents and

freshly ingested prey were collected after manu-

ally forcing regurgitation.

Statistical analyses: Morphometric data

were analyzed using means comparisons using

ANOVA or non-parametric analogs. The

Fig. 2. Bathymetry in Golfo Dulce and the position of current meters. Depth measures in meters. (Courtesy of Omar Lizano,

Centro de Investigaciones en Ciencias del Mar y Limnología, CIMAR).

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 72: e57498, enero-diciembre 2024 (Publicado Set. 18, 2024)

comparisons were implemented in SPSS 20.0

(IBM Institute).

Research and collection permit issued by

the Instituto de Pesca y Acuacultura (INCOP-

ESCA), Resolution A.J.D.I.P 138/2010.

RESULTS

Surface conditions during captures:

Within Golfo Dulce, we found 482 yellow-pat-

tern specimens and four bicolored or tricolored

individuals during our surveys. Most of these

snakes were encountered in the gulf’s internal

sector, which coincides with its deepest area

(Fig. 2). The snakes were found floating day and

night on the surface, mostly (> 90 %) in clean,

calm, and mirror-effect waters (Fig. 3B, Fig. 3D,

Fig. 4A, Fig. 4B). However, a few individuals

were observed in slightly disturbed waters (Fig.

4C). In contrast, none were observed in the

shallower and usually turbulent surface area at

the entrance to this gulf around Cabo Matapalo

(Fig. 4D). We rarely find individuals within the

flotation strips or smooth points (drift lines or

foam cords) with accumulations of debris and

organic remains such as twigs, branches, and

leaves. In most cases, the snakes were found at

rest, with their bodies wavy or slightly coiled

Fig. 3. A. Collecting specimens of Hydrophis platurus using a circular net. B.-C. Specimen floating during the day time

on the surface in the interior of Golfo Dulce, Puntareas province. D.-E. Specimen floating during the night time on the

surface in the interior of Golfo Dulce, Puntarenas province (E. Photo by Jaime Culebras, Photo Wildlife Tours). F. A group

of 62 individuals of Hydrophis platurus colected at night on a monitoring trip of May 13, 2013, in Golfo Dulce, Puntarenas

province.

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(Fig. 3E). Rain and strong wind breezes that

cause high turbulence significantly affect the

presence of snakes on the surface, as no indi-

viduals were observed under these conditions.

Diel activity: The number of snakes

observed during the day and night differs. The

highest sightings occurred during nocturnal

surveys, with an average of 7.8 individuals

(2-62) per trip and a maximum of 122 observed

for three nights (62, 16, and 44 specimens) on

13, 14, and 15 May 2013 (Fig. 3F). In contrast,

the average number of individuals observed

during the day was 3.7 individuals (1-16).

We could not find individuals during the

censuses on a full moon, so this condition

influences their activity on the surface. How-

ever, we observed and collected specimens that

approached the boat several times at night.

These facts prove the version of local fishermen

who claim that snakes are attracted to boats’

light when anchored at various points fishing

at night.

Body size: In Golfo Dulce, yellow morph

females have snout-vent body lengths between

285 and 555 mm, while those of the captured

males varied between 275 and 473 mm. Con-

sequently, the mean body length of females is

significantly larger than that of males (Table 1,

SVL: U = 176.0, df = 1, p = 0.026). In this popu-

lation, females have more robust and elongated

heads, recording head lengths between 20 and

34 mm, while for males, this measurement var-

ies between 19.7- and 29.1 mm. Means for this

variable also differ between sexes (U = 164, df

= 1, p = 0.029).

On the other hand, snout-vent body

lengths of oceanic bicolored females varied

between 241 and 730 mm, while in males from

Fig. 4. Usual condition of the surface layer in the interior of Golfo Dulce. A. Note the effect of flat or ¨mirror¨ surface. B.

Clean and calm surface. C. Slightly disturbed waters. D. Usual turbulent surface on the entrance of Golfo Dulce in Cabo

Matapalo.

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that same population, the measurement was

between 456 and 635 mm. No differences were

retrieved between sexes in mean body size in

our sample from the oceanic population (SVL:

U = 217, df = 1, P = 0.95; TL: U = 216, df = 1,

p = 0.93). Furthermore, for this population, the

head length ranges from 18.0 to 44.0 mm for

females, while that of males varies between 25

and 34 mm. Again, this variable does not show

dimorphism in this population (Head length: U

= 177, df = 1, p = 0.38).

Thus, the values of body and head mea-

surements reported here indicate that yellow-

morph adults from the Golfo Dulce population

are significantly smaller than the bicolor adults

of the oceanic population (Fig. 5, SVL: Mann

Whitney U = 1031.5, df = 1, p = 0.001; TL: U =

1648, p < 0.001).

Color pattern variation: Individuals in

the Golfo Dulce population exhibit an entirely

yellow or yellow pattern combined with spots

(black, dark brown, or greenish), and dark or

light circles or lines predominate. In addition,

there is variation in the intensity of the yellow

tones (Fig. 6A). In the entire research period,

only four bicolor or tricolor specimens (the typ-

ical color patterns of the oceanic population)

were found within the gulf (Fig. 6B), which

represents less than 0.9 % frequency.

Diet and feeding behavior: Stomach con-

tents and prey found in the snakes’ mouths

before ingestion reveal the exclusive presence of

small fish (< 30 % snake length, Fig. 7A) from

three families: Carangidae (Caranx sexfascia-

tus), Clupeidae (Ophistonema sp.) and Priacan-

thidae (possibly Pristigenys serrula). Nocturnal

observations show snakes passively floating on

the surface with the body undulating and coiled

and with the head slightly downward and the

mouth sometimes ajar, stalking their approach-

ing prey.

Reproduction: We only found three juve-

niles between March and April 2009 with sizes

between 305 and 336 mm in total length. On

February 16, 2011, a mating pair was observed

on the surface between 10 and 11 in the

morning, made up of a yellow and a bicolor

specimen. The male (yellow) was coiled tightly

around the female’s tail (Fig. 8A). Between

Tabl e 1

Body size ( ± S.E.) for adult males and females from Golfo Dulce and the bicolored oceanic population.

Population/ Sex SVL TL Head length

Golfo Dulce

Female 452.05 ± 12.02* 506.45 ± 9.68* 27.90 ± 0.63*

Male 415.43 ± 13.62 477.54 ± 8.83 25.75 ± 0.72

Oceanic

Female 507.54 ± 17.96 570.40 ± 20.16 31.47 ± 0.90

Male 526.75 ± 19.92 596.37 ± 21.30 29.75 ± 1.19

*Sex differences, p < 0.05.

Fig. 5. Comparative body size of females (above) and

males (below) Hydrophis platurus from bicolor oceanic

population (left) and yellow population of Golfo Dulce

(right).

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Fig. 6. A. Variation in the intensity of the yellow tone in the population of Hydrophis platurus in Golfo Dulce, Puntarenas

province. B. Oceanic color specimen of Hydrophis platurus found in the inner part of the Golfo Dulce during a monitoring

trip of May 13-17, 2013.

Fig. 7. A. Stomach contents and prey found in specimens of Hydrophis platurus from Golfo Dulce, Puntarenas province

(A-photo by Alejandra Rojas-Barrantes). B. Yellow specimen of Hydrophis platurus with a small fish caught laterally and

using their venom to subjugate it.

Fig. 8. A. Pelagic seasnakes (Hydrophis platurus), a yellow male and a bicolor oceanic female, mating in the interior of Golfo

Dulce in February 2011 (Photo courtesy of Gerardo Sequeira). B. Dissection of gravid yellow female of Hydrophis platurus,

with two embryos with advanced development, February 2013, Golfo Dulce, Puntarenas province.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 72: e57498, enero-diciembre 2024 (Publicado Set. 18, 2024)

March 20 and 21, 2018, two gravid females were

collected at night, one of them with two almost

fully developed embryos of 204 and 200 mm,

respectively (Fig. 8B).

Behavior and surface activity: Observed

specimens within the Golfo Dulce generally

remain floating passively on the surface, and

most of the specimens were found in the cen-

tral part within the perimeter of greater depth

of the gulf. In the surveys during the day, we

noticed that most individuals tend to be more

evasive when approaching the boat and tend to

sink quickly. However, a shift in their behavior

occurs at night. They become much more pas-

sive, allowing observers to get closer before

showing signs of evasive behavior. This unique

nocturnal behavior was a significant finding of

our research. During our surveys, we observed

a few individuals close (< 2 km) to the coast.

Local fishermen reported that sea snakes are

rarely seen stranded on inland Gulf beaches,

and we did not observe any during this inves-

tigation. Even so, we were able to register some

reports of specimens found lifeless in the sand,

but for the local inhabitants, these encounters

are infrequent.

Predation: We did not observe any preda-

tion case on this population of yellow snakes,

and none has been documented to date. How-

ever, in May 2013, we found a specimen with

the tail cut and scarred by apparent bites, like

what was observed in the oceanic bicolor indi-

viduals in Golfo de Papagayo in the Northwest

of the province of Guanacaste (Fig. 9).

Fig. 9. A.-C. Adults specimens of oceanic Pelagic seasnakes (Hydrophis platurus) from Golfo de Papagayo in Northwestern

Guanacaste province, with tails cut off and body scarred. D. Adult yellow specimen from Golfo Dulce, Puntarenas province,

with tail cut and scarred tail, all probably from an attempted predation.

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DISCUSSION

The discovery of a population of H. pla-

turus established in the interior of the Golfo

Dulce basin of Costa Rica, where yellow mor-

phos predominate, was an unexpected event

(Bessessen, 2012; Solórzano, 2011). On the

one hand, the species is remarkably pelagic,

and in the Eastern Pacific, it is distributed

uninterruptedly close to the American con-

tinental shelf, where bicolored morphotypes

dominate. These bicolored morphs resemble

those observed in the Western Pacific and

Indian Ocean populations (Heatwole, 1987).

The recent report of a population dominated

by yellow-pattern snakes from Golfo Dulce

(Solórzano, 2011) was also surprising due to

the extensive history of herpetological exports

in Costa Rican territory (Savage, 2002). Never-

theless, the available literature shows that only a

few sea snake-collecting expeditions took place

on the Pacific coast of Costa Rica during 20th

century. These expeditions were carried out

mainly in the Northwestern sector of the prov-

ince of Guanacaste (Tu, 1976) and the North of

the province of Puntarenas (Voris, 1983). This

last author points out that in September 1970,

at the “mouth” of Golfo Dulce, 268 specimens

were collected, with 3 % of specimens being

completely yellow. There is no evidence of other

records or earlier collections in the inner part

of this gulf, which eventually explains why this

population remained anonymous for so long.

Yellow-colored individuals from the Golfo

Dulce population are significantly smaller than

the bicolored or tricolored individuals from

the oceanic population. Morphological differ-

ences in general, and body size in particular,

in isolated populations or with restricted gene

flow have been extensively reported in different

animal groups (Boback, 2003; Case, 1978), and

in snakes, this is no exception (Aubret, 2015;

Keogh et al., 2005).

Evaluating the factors that mediated the

origin and maintenance of morphological and

color characters in the Golfo Dulce population

is beyond the scope of this article. However, the

morphological variation revealed here, and the

prevalence of an otherwise rare phenotype in

Golfo Dulce could result from adaptive pres-

sure. Coloration modification in snake popula-

tions and other ectotherms is often interpreted

as adaptive, as natural selection promotes color

polymorphism on spatial and temporal scales

(Cox & Davis-Rabosky, 2013). Tanaka (2005)

found that the heating rate is negatively related

to body size in the snake Elaphe quadrivir-

gata, while the increase in body temperature

is significantly faster in the melanistic morph

than in the striped morph in that species. Simi-

larly, melanistic morphs of other snake species

maintain higher body temperatures than light-

colored morphs (Forsman, 1995; Gibson &

Falls, 1979). It is tempting to speculate that the

Golfo Dulce population’s predominant yellow

pattern and smaller body size could also rep-

resent a temperature adaptation. Golfo Dulce

is a narrow and deep gulf with surface water

temperatures higher than those recorded on

the external ocean surface (Quesada-Alpízar

& Cortés, 2006; Quesada-Alpízar & Morales-

Ramírez, 2004; Svendsen et al., 2006). Thus,

pelagic ectotherms within the Golfo Dulce

could experience higher environmental tem-

peratures than their conspecifics outside, which

could relax the pressure to maintain a higher

level of melanism and larger size necessary for

thermal regulation in colder waters.

Body size in snakes has also been consid-

ered a phenotypic trait that varies in response

to diet type and size (Boback, 2003; Tanaka &

Ota, 2002), especially in gape-limited species

or stages where selective pressure can act more

strongly (Aubret, 2012; Aubret, 2015; Meik et

al., 2010). Thus, the smaller body size and the

coloration pattern recorded in individuals from

the Golfo Dulce could result independently.

The diet of individuals in this Golfo Dulce

population consists exclusively of small fish of

several species. However, it is still being deter-

mined if there are differences in the relative size

of the prey caught by the oceanic population

since they also feed on small fish (Kropach,

1975). Likewise, the predatory behavior of

the passive stalker that floats on the surface,

waiting for fish to approach, is similar to the

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 72: e57498, enero-diciembre 2024 (Publicado Set. 18, 2024)

hunting behavior of oceanic bicolor individu-

als (Kropach, 1975; Pickwell, 1972). On the

other hand, personal observations indicate that

specimens fed in captivity generally swallow

the offered fish alive and, apparently, only use

venom when the prey is captured from the

side (Fig. 7B), taking a few seconds longer

to ingest it.

A remarkable aspect is the absence of pre-

dation on H. platurus; even previous research-

ers agree that this species lacks significant

population-limiting predators (Heatwole, 1975;

Kropach, 1975). The few recorded predation

events involve bicolored individuals. These

instances have generally occurred when snakes

are thrown onto the sand of beaches due to

seasonal winds that push the strips or water-

lines, where snakes are usually found in the

surf zone (Kropach, 1975; Solórzano, 2022).

Being exposed and with little mobility, they are

vulnerable to opportunistic predators such as

raptors (Solórzano & Kastiel, 2015; Solórzano &

Sasa, 2017). Furthermore, in the Pacific North-

west of Costa Rica, Sheehy III et al. (2011)

found individuals with scars on their bodies

that suggest the occurrence of sporadic preda-

tion attempts, possibly by seabirds. Although

no predation records are available for yellow

morphs in Golfo Dulce, during our surveys,

we recorded anecdotic versions from residents

that photographed spotted dolphins (Stenella

attenuata) playing and chasing the snakes and

sometimes throwing them out of the water but

without eating them (Fig. 10), similar to what

was reported by Durso et al. (2015) in Puerto

Vallarta, Mexico, where a group of dolphins of

this species interacted with a sea snake in the

same playful way. We were also told of a bicolor

specimen of H. platurus found in the stomach

of a common fish in the area, a spotted snapper

(Lutjanus guttatus), caught in the outer sector

of the Golfo Dulce in Punta Matapalo.

The physical characteristics of this Gulf

and the behavior of the surface currents at its

entrance (Quesada-Alpízar & Cortés, 2006;

Svendsen et al., 2006) seem to be the events

that generate the isolation and development of

the yellow-colored population in the interior of

Fig. 10. A.-B. Spotted dolphins (Stenella attenuata) interacting and chasing a yellow Pelagic seasnake (Hydrophis platurus) in

Golfo Dulce, Puntarenas province (A. Photo courtesy of Nicuesa Rainforest Lodge. B. Photo from video of Gerson Cedeño).

12 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 72: e57498, enero-diciembre 2024 (Publicado Set. 18, 2024)

Golfo Dulce (Solórzano, 2011). Given that the

movements of this pelagic species are deter-

mined mainly by surface currents (Kropach,

1975), the current circulation pattern could

constitute a natural barrier that restricts the

transit of bicolor oceanic sea snakes towards the

Golfo Dulce and of the yellow morph towards

the outside of it (Fig. 11). Likewise, the physi-

cal conditions of the Golfo Dulce explain why

snake strandings within the beaches of this

basin are extremely rare and occasional. It is

likely that the mountains surrounding the gulf

function as barriers that cushion and reduce the

effect of surface winds (Svendsen et al., 2006),

making it more stable.

Evidence of genetic structure that could

explain the reported phenotypic variation is

still pending (see below). The presence of

some bicolor or tricolor specimens inside Golfo

Dulce (even mating with yellow specimens)

suggests that the flow in both directions is not

completely interrupted. In any case, the few yel-

low specimens reported in the Gulf of Panama

(Kropach, 1975), Bahía Culebra (Tu, 1976), and

other beaches on the Pacific coast (anecdotal

reports with photographs) probably dispersed

Fig. 11. A. Surface marine currents near the Pacific coast of Costa Rica. Coastal Current of Costa Rica which flows mostly

from Southeast to Northwest is noticed. B. Behavior of the surface currents in Costa Rica from January 2014 to 2022

(Courtesy of Omar Lizano, Centro de Investigaciones en Ciencias del Mar y Limnología, CIMAR).

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 72: e57498, enero-diciembre 2024 (Publicado Set. 18, 2024)

from the Golfo Dulce through the Costa Rican

Coastal Current, which flows mostly from

Southeast to Northwest (Quesada-Alpízar &

Cortés, 2006; Solórzano, 2011).

The taxonomic status of the Golfo Dulce

seasnake population has recently attracted the

attention of different researchers. Solórzano

(2011) emphasized the need to perform addi-

tional genetic-molecular and morphometric

studies to evaluate the case. Using nuclear and

mitochondrial DNA sequences from speci-

mens from the Golfo Dulce and the North-

ern Pacific of Costa Rica (Gulf of Papagayo,

Guanacaste), Sheehy III et al. (2012) found

a low level of molecular variation and low

population structure, concluding that the local

variation at the morphological and color level

cannot be explained by isolation. These results,

in turn, support the notion that gene flow

between populations might not be as restric-

tive as previously thought. Bessessen & Gal-

breath (2017) recognized the differences in

coloration and morphology, hastening to name

the Golfo Dulce population as a subspecies

(Hydrophis platurus xanthos). Although some

authors argue that the concept of subspecies is a

valid taxonomic division that could enrich our

understanding of evolution and biogeography

by reflecting geographic variation within spe-

cies (Patten, 2015; Patten & Unitt, 2002), we

believe that the proposed trinomial nomencla-

ture could be confusing.

On the one hand, the criteria for defining

a subspecies rarely pass the multiple-character

test; they are generally defined based on one or

a few characters (Wilson & Brown, 1953). In

this case, the difference in the color pattern is

clear, but further analyses in lepidosis and mor-

phometry are still pending. On the other hand,

the low genetic differentiation unraveled by the

markers used (Sheehy III et al., 2012) prevents

a distinction sustained by molecular characters.

Similarly, Lomonte et al. (2014) found no dif-

ferences in the protein composition of the ven-

oms of yellow individuals of the Golfo Dulce

and that of bicolor individuals from the Golfo

de Papagayo in the Northwest of the province

of Guanacaste. The combined molecular data

show a vigorous gene flow between both popu-

lations, reinforced by the interbreeding between

yellow and bicolor individuals observed in our

surveys. Future studies should use nuclear mic-

rosatellites or finer-scale markers to investigate

the population structure of this species and

assess whether this Golfo Dulce population is

an incipient state of speciation.

Although the movements of this species

and sea snakes, in general, are still poorly

understood, the monitoring results suggest,

unlike what was suggested by Bessessen et

al. (2022), that this population is currently

healthy and relatively abundant within the lim-

ited perimeter of this basin despite the acceler-

ated growth of the tourism industry and other

related activities. Economic activities in the

Golfo Dulce (Morales-Ramírez, 2011; Morales-

Ramírez et al., 2015) may constitute potential

risk factors for the health and stability of this

small population of sea snakes. We believe pre-

serving and protecting this small population of

yellow sea snakes unique within this species’

wide distribution range is necessary.

Ethical statement: the authors declare that

they all agree with this publication and made

significant contributions; that there is no con-

flict of interest of any kind; and that we fol-

lowed all pertinent ethical and legal procedures

and requirements. All financial sources are fully

and clearly stated in the acknowledgments sec-

tion. A signed document has been filed in the

journal archives.

ACKNOWLEDGMENTS

Our thanks to Louis W. Porras and Wil-

liam W. Lamar for their comments and sugges-

tions on this manuscript, as well as to Gerardo

Chaves and Federico Bolaños for the facilities

provided at the Museum of Zoology of the

University of Costa Rica. We especially high-

light the key logistical support from Donna

and Michael Butler and the staff at Nicuesa

Rainforest Lodge in Golfo Dulce and Catalina

Torres in Golfito. We also thank Marcelo López,

Joe Bernini, Gerardo Sequeira, Jaime Culebras,

14 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 72: e57498, enero-diciembre 2024 (Publicado Set. 18, 2024)

Omar Lizano, Quetzal Dwyer, Alejandra Rojas-

Barrantes, and Arturo Angulo for their col-

laboration, photographs, and contributions to

different aspects of this research.

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