The Euphausiidæ in the Neighbourhood of Plymouth. III.Thysanoessa Inermis.

Abstract

In a former paper (Lebour, 1924) attention was called to the eggs, Calyptopis and first Furcilia stages of Thysanoessa inermis together with the form neglecta, which was regarded as a different species, mainly on account of the size of the eggs shed by the females in glass jars in the aquarium, those of the neglecta form being much smaller than those of the inermis form. Certain differences were also found between the males of the two forms.Having through the kindness of Mr. C. F. Hickling obtained a number of Thysanoessa from the Atlantic Slope, chiefly in the region of the Smalls trawling ground, I found both forms present, which were usually much larger than those from the Channel, and presented certain other differences. Specimens of the two forms from both localities were sent to Dr. H. J. Hansen, of Copenhagen, who has kindly examined them and pronounced them all to belong to the same species,e.g. Thysanoessa inermis. It is an interesting fact that the specimens from the Smalls district are usually much larger than those from the Channel, and the neglecta form has the long legs much longer and stouter. Specimens from Iceland, for which I am indebted to Dr. Hansen, are also of a large size.As Hansen has already pointed out (1911, 1915), Thysanoessa inermis which has a wide range of distribution has two forms, a short-legged form, formerly known as Rhoda inermisand a long-legged form, formerly known as Thysanoessa neglecta; these may be referred torespectively as the inermis and neglecta forms.