Sea lampreys (Petromyzon marinus) are not just hitching a ride when they attach themselves to whales, but help themselves to a convenient meal of blood when they do so. This observation was made in a paper by Owen Nichols and Ursula Tscherter to be published in a forthcoming issue of the Journal of Fish Biology.
Like salmon, sea lampreys are born in freshwater, live their adult lives in the sea and return to freshwater to breed.
While the parasitic sea lampreys are known to attach themselves to fishes to feed (rasping away tissues and blood with their oral disc) when in freshwater, we know very little about the hosts they feed on in the marine phase of their life cycle.
Comparison with scars on freshly killed cetaceans and lamprey dentition suggests that sea lampreys attach themselves to Fin whales (Balaenoptera physalus), Sei whales (B. borealis) and Harbour porpoises (Phocoena phocoena).
More recently, sea lampreys have been photographed attaching themselves to North Atlantic right whales (Eubalaena glacialis) and Pacific humpback whales (Megaptera novaeangliae). In many of these cases, it is speculated that the lampreys are not merely along for the ride and are actually feeding on the cetaceans, but direct evidence that they are actually doing so has been lacking.
The authors provide photographic evidence that the sea lampreys attach themselves to whales to feed on them by observing Minke whales (Balaenoptera acutorostrata) in the mouth of the St Lawrence River in Canada.
Over a period of five years, the authors observed sea lampreys attaching themselves to Minke whales on 109 occasions (the observations were made on 47 individual whales), of which 77% of these occurrences were photographed.
In two instances, the authors were able to observe the whales immediately after the lampreys had detached themselves, and were able to observe bloody lesions at the attachment points, indicating that the lampreys had indeed been feeding on the whales.
Although the entire whale could not be seen during the observations, making it impossible to make many firm conclusions regarding attachment site preference for the sea lampreys, the authors note that all of the observed attachments were on the dorsal portion of the whale, mostly on the flanks below or behind the dorsal fin.
Based on our understanding of whale biology, the authors hypothesise that the prevalence of dorsal attachments is likely to be a result of the relatively thin skin and reduced water flow at this part of the body in comparison to flippers and flukes.
The continual expansion of the ventral pouch during feeding is thought to be the reason that prevents the attachment of sea lampreys to this area of the body, even though the skin is the thinnest here.
For more information, see the paper: Nichols, OC and UT Tscherter (2011) Feeding of sea lampreys Petromyzon marinus on minke whales Balaenoptera acutorostrata in the St Lawrence Estuary, Canada. Journal of Fish Biology doi: 10.1111/j.1095-8649.2010.02842.x