You Scratch My Back, I'll Scratch Yours.

You would not expect whales to be friends with lice, but researchers now suggest these two animals have a very special and important relationship that benefits both the giant and the small.

There aren't many things in the modern world that you can get for free, and it seems everything has a price. Nevertheless, there are animals around the globe that gain seemingly everything, from travel and food to a mobile home, for free. Usually, these animals would be labelled as parasites, living off another animal for their own benefits. Yet for whale lice, where is the line between parasitism and commensalism?

Commensal refers to the relationship between organisms where one benefits, and the other derives neither benefit nor harm. So, in biological terms, being a commensal animal is difficult to accomplish as the cost to a host must be so minimal, that it is overlooked. Researchers from around the world are now suggesting that this case study has more implications than previously thought.

Marine mammals inhabit almost every corner of the ocean and therefore establishing this type of relationship with members of such a group, would have extraordinary benefits for any animal. This is what a family of amphipods, also known as the 'whale louse' has successfully achieved.

This remarkable crustacean can be found in numbers exceeding 7,500 on a single whale in the natural skin folds and body openings. They also gain food from the algae that settle on the skins surface, in addition to the dead tissue from open wounds.

The whale louse has evolved hooked legs that allows gripping the skin to be relatively simple. The angled ends of the limbs are slightly different to the other sections of leg, as they use thicker cuticle and a sharp point to penetrate the animals skin. Using this anchoring technique allows the louse to use three sets of fore-limbs to scavenge the algae that colonise the cetacean using their enlarged fore-arm like appendages.

If the host mammal has an open wound, then they will gain food from pinching off small amounts of tissue as well as parasites present. However, the whale lice only removes tiny amounts of flesh and in any case the removal of the dead tissue would not harm the mammal. Research has shown that there is reduced risk of infection or extensive parasitism, as the louse removes it before serious issues can develop. Whilst their weight has no significant impact on animal's overall movement.

Recently, Jon Seger and Victoria Rowntree of Utah University , along with Tessa Danelesko of the Wild Whales website, have found something that is truly remarkable. They suggested that whales, when attacked, are more likely to roll onto the side that contains the larger number of whale lice. These animals therefore, along with barnacles, could possibly produce a biological armour for their hosts and provide protection against predators.

However, this is not where the story ends for this hitchhiker. Many species of whale louse are specific to the species of whale that it lives on and with developing technology in genetic analysis, we can do some amazing things. New Scientist magazine published an article in 2005 showing the results of whale louse genome examinations which was compared to the whale they were found on. This allowed us to map three new species of right whale (Eubalaena).

Therefore, since the species of whale louse is specific, we can tell the species of animal from the louse. This was shown when a whale was killed by native people in Alaska who had removed the tissue for food and used many of the whale components before scientists could examine the carcass. When they arrived at the site only a small section of skin remained, however using the whale lice found on the skin, they were able to identify the whale species like the California grey whale (Eschrichtius robustus).

The whale lice gain everything from shelter to nutrition at seemingly no direct cost to the carrying mammal. The weight gain is minimal for animals of that size, the lice have shown usefulness by eating the organisms that would otherwise infect the very same animals that have learnt how to use this relationship for living armour.

An incredible relationship and series of behaviours, from two groups of organisms on the opposite ends of the scale.

Author: Thomas Llewellyn

References:
Kaliszewska, Z.A., Seger, J., Rowntree, V.J., Barco, S.G., Benegas, R., Best, P.B., Brown, M.W., Brownell, R.L., Carribero, A., Harcourt, R. and Knowlton, A.R., 2005. Population histories of right whales (Cetacea: Eubalaena) inferred from mitochondrial sequence diversities and divergences of their whale lice (Amphipoda: Cyamus). Molecular Ecology, 14(11), pp.3439-3456.Margolis, L., 1955. Notes on the morphology, taxonomy and synonymy of several species of whale-lice (Cyamidae: Amphipoda). Journal of the Fisheries Board of Canada, 12(1), pp.121-133.