Introduction

Great numbers of parasitic and commensal protozoans have been described from marine and freshwater fishes in many parts of the world. Those of freshwater fishes include species which have been responsible for epizootics in streams and hatcheries. The prospect of gaining knowledge leading to the control of these pathogens has not only provided a stimulus to research, but has frequently induced organizations concerned with conservation for game or commercial purposes to make funds and facilities available to investigators. The nature of the marine habitat raises as yet unsurmounted difficulties in the control of pathogenic protozoans of fishes occupying it. Marine fisheries authorities have tended to concentrate their conservation research on studies designed to result in the improvement of fishing regulations. In consequence, the ecology of the Protozoa of freshwater fishes has become more completely understood than has that of the marine fish protozoans.

A wealth of information regarding the Protozoa of marine fishes has nevertheless been accumulated. The majority of this, having been gathered by systematists working on their own initiative, is scattered throughout the literature in unrelated fashion. Many of the protozoans concerned have been most inadequately described, new specific names having been accorded them merely on the basis of their occurrence in new hosts or new localities. Numerous anomalies have arisen from this practice. Unfortunately, the incompleteness of the recorded information is such that it is as yet seldom possible to do more than suspect and regret the existence of synonymy.

It became manifest during recent studies of the haematozoa of New Zealand fishes (Laird, 1951a, 1952) that in only few instances are published descriptions sufficiently detailed to allow of a species being confidently described either as new or as conspecific with an established species. Furthermore, much confusion is caused by our lack of knowledge concerning host specificity and host-induced morphological variation. These problems are complicated both by the plastic nature of the material and by the fact of our all but total ignorance of the transmission of marine fish haematozoa. Obviously, any attempt to monograph either the trypanosomes or the haemogregarines of marine fishes, based on the existing literature, could result in little more than the compilation of long lists of specific names embodying much suspected but undemonstrable synonymy.

Equally obviously, a study of the zoogeographical distribution and host specificity of these parasites could be expected to yield highly significant results. It appeared worthwhile to conduct an introductory investigation, the scope of the project being limited in some way, and the greatest possible use being made of the literature in such a manner as to enable comparisons of zoogeographical significance to be drawn.

The host preferences of haematozoans of fishes often bear little relation to the systematic positions of the hosts themselves. In the family Blenniidae, for example, the European catfish Anarhichas lupus is parasitized by Haemogregarina anarhichadis Henry, 1912, a species having close affinities with haemogregarines of certain non-blenniid pelagic fishes (Laird, 1952) but systematically remote from Haemogregarina bigemina Laveran and Mesnil, 1901, a widespread parasite of intertidal zone blennies. Again, there are species of Trypanosoma (e.g., T. blenniclini Fantham, 1930) and Haemogregarina (e.g., H. bigemina) having hosts in two or more piscine families.