Sea kraits: radical intraspecific diversity, reproductive isolation, and site fidelity

Sea snakes are awesome, and in the previous post I introduced various aspects of sea snake diversity, evolution, biology and behaviour. We looked at cutaneous respiration, the invasion of freshwater, bizarre body shapes, and breeding aggregations kilometers long. Also discussed – or at least mentioned – was the evidence indicating that sea snakes aren’t monophyletic, but that the two distinct groups (the laticaudids, or sea kraits [photo above], and the hydrophiids, or true sea snakes) instead represent two separate invasions of the marine environment. Furthermore, there are now indications that even hydrophiids aren’t monophyletic. More on this in a later post.

Laticaudids are less specialised for marine life than hydrophiids, being better able to move on land, and they are mostly (but not entirely) oviparous (whereas hydrophiids are all viviparous). This might show that they’ve taken to marine life more recently than hydrophiids have, but then it’s also possible that they’re simply conservative. And don’t go thinking that they’re any less interesting than hydrophiids. Thanks almost entirely to the field research of Richard Shine and Sohan Shetty of the University of Sydney, it’s recently been discovered that laticaudids are actually rather complex, quite variable creatures. In fact they illustrate nicely Shine & Bonnet’s (2000) point that snakes can be considered model organisms in terms of how much recent research has told us about evolution, ecology and behavioural diversity.

Six species of laticaudid are presently recognised. The Black-banded sea krait L. laticaudata (Linnaeus, 1758) is a large (1.1 m long), wide-ranging species. Populations differ in the patterning they have on the head, and while two subspecies have been distinguished on the basis of such differences, the variation actually appears to be clinal (McCarthy 1986). L. crockeri Slevin, 1934 is a denizen of Lake Te-Nggano on Rennell Island: it’s one of the freshwater sea snakes I mentioned in the previous post. It’s often melanistic. The Yellow-lipped sea krait L. colubrina (Schneider, 1799) is similar to L. laticaudata in distribution, and is highly variable across its range: in fact some workers have suggested that it should be split into as many as six subspecies. L. semifasciata (Reinwardt, in Schlegel, 1837) from the coasts of Japan, the Philippines, the Moluccas and the Lesser Sunda Islands is highly similar to L. schistorhynchus (Günther, 1874) from Niue, Tonga and Samoa, and some workers have considered them synonymous. Finally, there’s the small (less than 1 m long) L. frontalis De Vis, 1905.

Among these species, L. colubrina and L. frontalis are virtually identical, differing only in size. In fact the two are so similar that they have until recently been regarded as conspecific, despite detailed character analysis (McCarthy 1986), and not until Cogger et al.’s (1987) study was L. frontalis shown to be distinct. It had actually been first named by De Vis in 1905 and can be diagnosed on the basis of its small size (less than 1 m), ventral scale counts and banding pattern.

Both L. colubrina and L. frontalis occur sympatrically around Vanuatu. Yet, despite their similarity, hybrids have never been reported, so how do these two remain reproductively isolated? It seems that males court females when triggered by the pheromones contained within the lipids that occur on female’s bodies, and that they only court conspecifics (Shine et al. 2002). What’s particularly interesting about this study from the point of view of science history is that, while sexual selection and isolating mechanisms among sympatric species have become heavily studied, virtually no research of this sort has been published on snakes.

What’s also interesting is that the lipids concerned function in waterproofing. I assume then that they’ve been exapted to assist in pheromone distribution (that is, the lipids evolved for waterproofing first, and then became co-opted during evolution for pheromone distribution), but I can’t determine this from Shine et al. (2002).

Laticaudids occur throughout the Indo-Australasian area, ranging from southern Japan and the Bay of Bengal in the north to Tonga and Somoa in the east, and to Tasmania and New Zealand in the south. Intriguingly, there are a few unconfirmed reports that the Yellow-lipped sea krait L. colubrina occurs on the Pacific coasts of Nicaragua, El Salvador and Mexico. The claim that this species has managed to cross the Pacific is pretty radical and really requires confirmation: as McCarthy (1986) stated ‘unfortunately these records are based on material than is no longer available for examination; the presence of L. colubrina in tropical America therefore requires substantiation’ (p. 134).

It’s usually implied in the literature that laticaudids are more flexible, in ecomorphological terms, than are hydrophiids in that they are not all marine: as mentioned above, the sometimes melanistic Laticauda crockeri is restriced to the brackish Lake Te-Nggano (also called Lake Tegano) on Rennell Island (one of the Solomons: and if you’re especially interested in Solomon Islands wildlife go here). This isn’t really accurate however, given that some hydrophiids also occur in lakes. The Yellow-lipped sea krait L. colubrina, the most studied laticaudid, is reportedly quite agile on land (which fits nicely with amino acid data indicating that it is the most basal member of the group) while L. crockeri is rarely, if ever, seen on land. What does L. colubrina do when it’s on land? It seems to rest here after feeding at sea (presumably using solar heat to aid digestion) and also sloughs its skin, lays it eggs, and mates on land.

L. colubrina individuals have been shown to exhibit strong site fidelity: even after feeding in deep water many km from land, they return to the same ‘home’ island (Shetty & Shine 2002a). ‘Homing’ behaviour has been reported among other snakes (usually in species that return year on year to the same overwintering site), but these are terrestrial species that have been shown to follow pheromone trails. How then do laticaudids find their way home? I don’t think anyone knows and this is a fascinating topic for future research. Maybe they have a very good memory!

However, the terrestrial abilities of laticaudids don’t just differ among species: they also differ among individuals of a species. Intraspecific variation in locomotor ability was studied in L. colubrina by Shine & Shetty (2001a) who found that males were substantially more agile on land than were females. This sexual difference is probably due to the smaller size and more gracile proportions of males, but there might be intense selection for good terrestrial abilities in males because males do a lot of searching on land for female mates (interestingly, the good terrestrial abilities of males in this species is opposite to what’s seen in sea turtles, where the males of some species may never ever leave the water, and are hence less able to perform terrestrial locomotion than are females).

Because they have to move on land a lot more than females do, male L. colubrina have relatively longer tails than females but there’s a complex interplay between growth rate, survivability, and both aquatic and terrestrial agility in males (Shine & Shetty 2001b). In view of all this intraspecific variability, Shine & Shetty (2001a) noted how Yellow-lipped sea kraits might ‘offer exceptional opportunities to study phylogenetic shift in locomotor ability’ because they ‘display considerable intraspecific and interspecific diversity in terms of the degree to which they use terrestrial vs. aquatic habits’ (p. 338).

This reminds me of the intraspecific variation I discussed recently in Flying steamer-ducks Tachyeres patachonica: the species includes both flight-able and flightless individuals (go here to read more). Intraspecific variation in locomotor abilities among snakes is pretty special however, elsewhere being documented only where pregnancy makes big females less agile than males, and in garter snakes where individuals that have only just emerged from hibernation are less agile than warmed-up, fully active indiduals (Shine et al. 2000).

Laticaudids are also intraspecifically variable in feeding behaviour. So far as we know all laticaudids are specialist predators of moray and conger eels, but it now seems that the sexes specialize on different types of eels. Again the species concerned is the Yellow-lipped sea krait, where the larger, more broad-headed females feed primarily on conger eels, while the smaller, more slim-headed males take the smaller moray eels (Shetty & Shine 2002b). Similar sexual variation in prey preference has been reported elsewhere in snakes in file snakes and American water snakes, so niche partitioning like this isn’t unique to laticaudids.

So then… despite the fact that laticaudids aren’t as specialised for marine life as hydrophiids are, they’re clearly interesting. Is the amount of intraspecific varation we see in them unique to the group, or is it actually more widespread among sea snakes, but just undiscovered? That’ll do on sea snakes for now. I really need to get back to my oviraptorosaurs. For the latest news on Tetrapod Zoology do go here.

The photo of the Laticauda used above is from Divegallery.

Refs - -

Cogger, H. G., Heatwole, H., Ishikawa, Y., McCoy, M., Tamiya, N. & Teruuchi, T. 1987. The status and natural history of the Rennell Island sea krait, Laticauda crockeri (Serpentes: Laticaudidae). Journal of Herpetology 21, 255-266.

McCarthy, C. J. 1986. Relationships of the laticaudine sea snakes (Serpentes: Elapidae: Laticaudinae). Bulletin of the British Museum of Natural History (Zoology) 50, 127-161.

Shetty, S. & Shine, R. 2002a. Philopatry and homing behaviour of sea snakes (Laticauda colubrina) from two adjacent islands in Fiji. Conservation Biology 16, 1422-1426.

- . & Shine, R. 2002b. Sexual divergence in diets and morphology in Fijian sea snakes Laticauda colubrina (Laticaudinae). Austral Ecology 27, 77-84.

Shine, R. & Bonnet, X. 2000. Snakes, a new ‘model organism’ in ecological research? Trends in Ecology & Evolution 15, 221-222.

- ., Harlow, P. S., LeMaster, M. P., Moore, I. & Mason, R. T. 2000. The transvestite serpent: why do male garter snakes court (some) other males? Animal Behaviour 59, 349-359.

- . & Shetty, S. 2001a. Moving in two worlds: aquatic and terrestrial locomotion in sea snakes (Laticauda colubrina, Laticaudidae). Journal of Evolutionary Biology 14, 338-346.

- . & Shetty, S. 2001b. The influence of natural selection and sexual on the tails of sea-snakes (Laticauda colubrina). Biology Journal of the Linnean Society 74, 121-129.

- ., Reed, R. N., Shetty, S., Lemaster, M. & Mason, R. T. 2002. Reproductive isolating mechanisms between two sympatric sibling species of sea snakes. Evolution 56, 1655-1662.