Hydrophis platurus

Colors of Hydrophis platurus View in CoL

There are two principal color morphs of Hydrophis

Fig. 1. Views of the bicolored Hydrophis platurus platurus featuring the black dorsum of a beached specimen (A) and the yellow underside with posterior mottling as seen from underwater (B). Photos by the author (A) and Joe Pfaller (B).

platurus , each representing a distinct subspecies. The more common is the well-known bicolored black and yellow subspecies making up the larger number of snakes that range across the Indo-Pacific oceans ( H. p. platurus ). These snakes have a black dorsum, including the head, with yellow to orange-yellow coloration on the ventral and lateral aspects of the body (Fig. 1). The lateral yellow coloration can be variably interrupted with streaks or spots of black, but the ventral body is typically completely yellow. Beginning at or near the tail, or in many cases somewhere about two-thirds or threefourths of the distance from the head, the black color is interrupted with yellow or white blotches, which almost always represent a mottling of the two colors on the full height of the tail (Fig. 1). In various views, this bicolored subspecies appears largely black when seen from above (Fig. 2A), and variably yellow when viewed from below (Fig. 1B).

The second color morph is a xanthic subspecies that is restricted to the inner basin of Golfo Dulce, southwestern Costa Rica ( H. p. xanthos; Bessesen and Galbreath 2017; Solórzano and Sasa 2024). These snakes are entirely yellow, but many individuals have various amounts of black skin limited to diffuse narrow blotches or a middorsal stripe (Fig. 2B). Compared with H. p. platurus , specimens of H. p. xanthos are notably smaller and generally exhibit a uniformly yellow color (Fig. 2B). The xanthic subspecies is significantly isolated from the

Fig. 2. Dorsal view of H. p. platurus swimming in coastal water offshore of northwestern Costa Rica (A) and a group of xanthic H. p. xanthos from Golfo Dulce being held temporarily for research purposes (B). Most specimens of the latter subspecies are all-yellow, but some individuals are marked with small spots or stripes of black pigment. Photos by Shauna Lillywhite (A) and the author (B).

broader-ranging bicolored subspecies due to the nature of the currents and bathymetry of its habitat ( Bessesen and Galbreath 2017; Bessesen et al. 2023). The xanthic snakes appear allopatric ( Sheehy et al. 2012) and likely seldom interbreed with the bicolored conspecifics, even though there is occasional exchange between Golfo Dulce and the western Pacific (see also Solórzano and Sasa 2024).

The bright yellow of the xanthic subspecies is obviously not cryptic when these snakes are at the ocean’s surface. Therefore, the discussion that follows focuses on the bicolored subspecies, while the xanthic snakes are considered only in contexts where it is appropriate.

Behavior of Hydrophis platurus at oceanic slicks

Yellow-bellied sea snakes spend most of their routine time (up to 99%) floating in the water column at depths of 20–50 m ( Rubinoff et al. 1986). Bouts of diving

Fig. 3. Scenes of ocean water in Golfo Papagayo, northwestern Costa Rica, featuring contrast between calm surface water of a slick and rougher water adjacent to the slick. The slick shown in (A) (left half of photo) shows the length of a broad slick extending to the horizon. The breadth of a large slick can be seen in (B) (much of foreground). Photos by the author.

alternate with surfacing events, which are either very brief to breathe in air or longer to forage ( Rubinoff et al. 1986). This species ambushes small fishes that are concentrated under debris while ‘float-and-wait’ foraging at the oceanic surface on slicks ( Dunson and Ehlert 1971; Rubinoff et al. 1986; Brischoux and Lillywhite 2011). Evidently, the Yellow-bellied Sea Snake is the only marine air-breathing predator that forages at the oceanic surface on labile features such as slicks or drift lines.

Slicks (or drift lines) are typically short-lived, mobile oceanic structures formed as the result of Langmuir circulations or convergent currents created by a variety of physical processes involving internal waves and eddies ( Ewing 1950; Barstow 1983). Slicks can be variable in width, relatively small, and short-lived, or they can form drift lines that extend for hundreds of meters or kilometers (Fig. 3). They typically form stretches of calm glassy water surrounded by rougher areas (Figs. 3 and 4), and they frequently have high quantities of positively buoyant flotsam containing both seaborne and terrigenous items (Figs. 5 and 6). Due to their dynamic features, planktonic organisms including larval and juvenile fish concentrate on the slicks ( Barstow 1983; Dempster and

Fig. 4. Two conditions of the surface waters of a slick featuring surface patterns of wavelets in different sea and lighting conditions. Note the wavelets shown in (A) create dark streaks that resemble outlines of the black surfaces of sea snakes that float on the ocean surface. The water conditions shown in (B) produce variable surface features that obscure the outline of floating snakes. Photos by the author.

Kingsford 2004; Kingsford and Choat 1986; Pineda 1994; Shanks 1995). How sea snakes aggregate on slicks is not clear, but they could arrive at slicks by passive drift on currents, similar to the inert flotsam ( Dunson and Ehlert 1971; Hecht et al. 1974), or they might actively seek out slicks possibly using currents, vision, or vomerolfaction underwater ( Rubinoff et al. 1988). Indeed, Yellow-bellied Sea Snakes might visually detect flotsam or the refractive properties of glassy water, or even fish, from an underwater viewpoint, as has been shown for the detection of potential prey by ‘float-and-wait’ foraging fish ( Cronin 2005). The presence of flotsam appears to be important in attracting snakes to slicks, whether actively or passively, and conceivably enhances their visual detection of such structures from beneath the water ( Brischoux and Lillywhite 2011). Hence, a reasonable hypothesis is that the presence of calm, glassy water and the presence of flotsam might influence a snake’s ‘decision’ to surface at a slick ( Brischoux and Lillywhite 2011) (Figs. 3, 5, and 6). Floating snakes are generally more abundant on slicks when atmospheric conditions include clear sky and bright light ( Brischoux and Lillywhite 2011).

Slicks with any amount of flotsam attract smaller pelagic species of fishes which habitually hide beneath floating objects, including the snakes themselves ( Hunter and Mitchell 1967). Floating snakes characteristically adopt a floating posture with the dorsal surfaces variably exposed to air and the head angled downward beneath the water’s surface (Figs. 1B and 6). Such ‘float-and-wait’ behavior employed by floating aggregations of H. platurus on slicks enhances the foraging success of this species ( Dunson and Ehlert 1971; Kropach 1971a; Brischoux and Lillywhite 2011).

Fig. 5. Views of flotsam on a slick. (A) Froth extends toward the horizon on a small slick in Golfo Papagayo, Costa Rica. (B) Debris that accumulates on slicks as result of washout from coastal river mouths during the wet season. As a result there are numerous sticks and other objects of variable colors that resemble floating snakes, although there is no snake in this scene. Photos by the author.

Fig. 6. Crypsis in snakes floating on a slick with small amounts of flotsam. The black arrows point to two snakes, and the white arrows point to floating sticks that resemble the yellow edge of a snake’s outline. The posterior mottling on the snake at the left is shown to resemble a small patchwork of leaves that are part of the flotsam. Photo by the author.

Are the black and yellow sea snakes cryptic at the ocean’s surface?

Animals that are effectively camouflaged exhibit colors, patterns, and movements that render them indistinguishable from their background or as irrelevant objects in it. Two related but distinct aspects of camouflage are matching the background pattern (crypsis; Endler 1978, 1984) and disruptive coloration ( Cott 1940; Cuthill et al. 2005; Merilaita and Lind 2005). Crypsis embodies all traits that reduce an animal’s risk of being detected when it is potentially perceivable to an observer ( Stevens and Merilaita 2009). Typically, the term crypsis includes features of physical appearance such as coloration and behavioral traits that prevent detection.

With respect to the latter, the time spent by sea snakes floating on slicks involves little swimming or movement. The swimming that I have observed involved snakes that were disturbed by humans during attempts to capture them. In the absence of provocation, the snakes floated motionless at the surface of calm waters that were part of a slick. In other situations when snakes were surfacing for short periods to breathe air, swimming entailed vertical movement up or down in the water column, and their time at the surface was brief. These statements are based on observations of two to four persons during 14 trips to Costa Rica, when boat time on slicks was additive for several hours per day over the course of about seven days per trip. The total observation time was conservatively about 900 person-hours. Dismissing active movements by floating snakes, I will now consider aspects of crypsis related to background color and pattern-matching.