In an earlier post, I joked (well half joked) about the need to save the whale lice, even if you don't care about the right whales. I thought today I would expand on the brief comment about the lice and their special relationship with whale and how they can actually tell us about the populations of right whales and their evolution.
In the image above you can see the characteristic white and black rough patches known as callosities on the face of a right wale. The raised dark grey bits are indeed part of the whale. Those are rough ridges that become sharper and harder. They are not really lice, but a caprellid amphipod, in other words a crustacean. Whale lice specifically are in the family Cyamidae consists of 35+ species in seven genera and are collectively known as cyamids.
On each right whale around 5000 Cyamus ovalis coat the callosities and gives them their white color is Cyamus ovalis. In the spaces between the raised callosities live around 500 C. gracilis. On adult whales approximately 2000 C. erraticus live in the genital and mammary slits. C. erraticus is highly mobile though often occupying wounds, and living in large concentrations on the heads of young calves. Of these C. gracilis is the smallest with ~6mm long adults and with the other two species measuring ~12-15mm long as adults.
The cyamids were named "whale lice" by early whalers in reference to their own head and body lice. Mmmmm fun! While not actual lice, they behave similarly in a few key ways. Cyamids have no free swimming stage and spend their entire life on one species of whales, transferring from whale to whale through intimate contact, primarily between a mother and it's calf. They were recently used, similarly to their lice namesakes, to track the population structure and evolution of their hosts.
In 2005 a team of scientists published the results of their study of the population structure and evolution of right whales based on DNA studies of the whales' cyamids. The cyamid DNA is in some ways more informative than the whales' own DNA as the cyamids complete many generation per whale generation and the population of the cyamids, especially C. ovalis, is far greater than that of the whales, offering the researchers more mutations to track.
The team collected cyamids from globally distributed right whale strandings and used variation in the mitochondrial COI gene to analyze the population structures of both the cyamids and by inference the right whales they inhabit. The first finding was that there was no obvious population structure within ocean basins. They also found high levels of haplotype diversity but low gene differentiation suggesting a large population with high transfer rate between individual whales.
The North Atlantic and Southern Ocean populations however have apparently been fully isolated for several million years. This supports the view that the North Atlantic, Southern Ocean and North Pacific right whales have been isolated for millions of years and shouldbe considered separate species. In a gene tree for the right whale cyamids the three different nominal species clustered out as seven distinct species. C. avalis, C. gracilis and C. erraticus fall out with separate North Atlantic and Southern Ocean species which diverged approximately 6.3mya. Interestingly the Northern Pacific C. ovalis form a tight clade nested within the tree of the Southern Ocean C. ovalis suggesting that there was more recent contact those two populations than between the Northern Atlantic and Southern Ocean populations.
By studying the cyamid crustaceans Jon Seger and his team were able to provide another line of evidence that each oceanic basin population of right whales is in fact a distinct species. They also found four new species of right whale cyamids and that the number of right whales in each basin was higher than originally estimated for pre-commercial whaling populations.
Classification of the nominal species Cyamus ovalis:
- Cyamus ovalis
Kaliszewska et al. (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), 3439-3456 DOI: 10.1111/j.1365-294X.2005.02664.x