Blood-sucking fish feed on whales

BBC Earth News: Blood-sucking fish feed on whales
Scientists studying whales in Canada have challenged previous theories that lampreys attach to whales purely to "hitch a ride".

Photographs show bloody lesions after lampreys detached from whale hosts, indicating feeding.

Sea lampreys are parasitic fish that feed on others' blood, attaching to the skin with a suction-cup like mouth. They are sometimes called 'lamprey eels' because adults are similarly long and lack scales. Sea lampreys secrete an anticoagulant from their mouths to stop prey's blood from clotting

Sea lampreys (Petromyzon marinus) are known to feed on a wide variety of fish using a funnel-like mouth filled with teeth and a razor-like tongue.

In the past lamprey teeth-marks have been identified on whale and porpoise bodies. Sea lampreys have also been photographed attached to Pacific humpback and North Atlantic right whales.

Based on these rare glimpses, certain scientists theorised that sea lampreys feed on cetaceans, but it was not possible from this evidence to say conclusively that they were drawing blood.

Others in the scientific community argued that P. marinus could merely be using cetaceans for transport, biting into their flesh in order to travel long distances.

However, during studies in the St Lawrence estuary, where the Great Lakes enter the Atlantic Ocean in eastern Canada, researchers resolved the debate.

The long-term study of minke whales in the area provided the first ongoing observations of sea lamprey and whale interaction.

Their findings were published in the Journal of Fish Biology.

"We and others had long had reason to believe that the lampreys were feeding," said Owen Nichols, Director of the Marine Fisheries Initiative at the Provincetown Center for Coastal Studies in Massachusetts, US.

"The high frequency of minke whale observations coupled with their behaviour in the estuary, frequent surfacings with much of their bodies visible, made these observations possible."

Ursula Tscherter was responsible for the fieldwork during which the research team photographed whales before, during and after lamprey attachment.

"Little is known about lamprey fish on cetaceans especially on minke whales," said Ms Tscherter.

"To document the presence of lamprey fish on individually identified minke whales, which use the St Lawrence estuary as a summer feeding ground, helps to put the topic into a broader context."

The images of bloody lesions on the whales provided the first evidence that the parasites fed on cetacean blood.

"To our knowledge these are the first in-situ observations of such behaviour during which one can convincingly state that lampreys attached to whales are rasping through the skin and feeding rather than just 'hanging on for the ride'," explained Mr Nichols.

Sometimes dubbed the "vampire fish", P. marinus was accidentally introduced to North America's Great Lakes in the 1800s.

Like salmon, sea lampreys are "anadromous", meaning they are born in fresh water, migrate to the sea for their adult life and eventually return to freshwater to spawn and die.

Efforts are now being made to better understand the species in order to control the invasive population that threatens native fish species.