How the urban sea snake lost its stripes

Stripy sea snakes are more susceptible to the harmful effects of water pollution than their black coloured counterparts, according to research published this week in Current Biology.

Turtle-headed sea snakes live on coral reefs in the Indo-Pacific and, across most of their geographical range, these animals are decorated with a black and white banded pattern.

However, Rick Shine, Professor of Evolutionary Biology at The University of Sydney, has worked with sea snakes for over twenty years and noticed that in more polluted parts of the reef, such as in areas close to cities, the sea snakes are jet black. 

So, why have sea snakes in urban environments lost their stripes?

The link between melanism, a dark pigmentation, and pollution is not a new concept. Henry Kettlewell first presented evidence of this phenomenon in the peppered moth during the UK industrial revolution.  Dark-coloured peppered moths were better camouflaged against soot-covered tree bark and so their numbers in urban environments increased while numbers of the once common light-coloured moth decreased.

Shine admits that this observation in sea snakes, however, “had always been a puzzle” as black colouration provides no camouflage to these reptiles and therefore does not explain the loss of banding.

Lead author of the study, Claire Goiran, from the University of New Caledonia, was struck with an idea about what may be going on when reading a recent study on the black feathered pigeons of Paris. The research showed that melanin in black feathers binds to trace metals such as arsenic and lead, which are common pollutants, allowing pigeons to eliminate these potentially harmful elements when they shed their feathers.

Since sea snakes have the ability to shed their skin, a process known as sloughing, Goiran thought that they too may be ridding themselves of pollutants via a similar process. 

Inspired by the Parisian pigeons, the team compared the concentration of pollutants in the skins of sea snakes living in industrial areas to those in nonindustrial areas.  Unsurprisingly, the comparison revealed that higher levels of pollutants are found in skins collected in the industrial sites.

The truly remarkable result came when Goiran and colleagues then compared pollutant concentrations in the white and black skin segments of another species of banded sea snake, the sea krait. Strikingly, concentrations of these potentially harmful elements were found to be higher in the dark, melanic skin than in the lighter skin - even within the same individual.

Furthermore, due to algal build up on melanic skin, the black sea snakes slough more frequently than their patterned counterparts.

The ability of melanic skin to both bind trace metals and generate an increase in sloughing suggests that black sea snakes can more efficiently eliminate pollutants from their bodies than those that are banded.

Goiran and her collaborators have provided compelling evidence of melanism as a response to industrial pollution in an aquatic species and Shine warns, “It’s a sad commentary that we are releasing pollutants into our oceans and generating such pressures that influence the evolution of these animals.”