Both these terms are a little ambiguous because different people may apply them to quite a variety of crustacean plankton including some larval forms which are, or in some cases are thought to be, eaten by whales. All the larger whales, with the exception of the sperm whale, are quite without teeth and rely for their food on plankton, which are extracted from the water by the “whalebone” filtering mechanism attached to the upper jaw. The true krill, food of the blue and fin whales, is a large shrimplike form known as a Euphausid, and it was found by the early Discovery investigators that the presence of krill was a very good indicator that whales might also be present. Smaller whales, such as the sei whale, are content with the rather smaller copepod, Calanus. An interesting Euphausid is often found in New Zealand waters – Nyctiphanes, Greek for “night light”. Usually this species, in common with many other plankton, prefers deeper water in the daytime, but at night rises to the surface where it produces bright pinpoints of light in disturbed water such as the wake of a vessel. Sometimes a trawl net when raised from the water at night is covered with Nyctiphanes, and the effect is like a Christmas tree covered with hundreds of tiny candles.

by Richard Morrison Cassie, M.SC.(N.Z.), D.SC.(AUCK.), Senior Lecturer in Zoology, University of Auckland.