Matthews, J.B.L. (1967). Investigations on the systematics and ecology of Calanus finmarchicus s.l. (Crustacea, Copepoda). Ph.D. Thesis, Oxford University, Oxford, U.K. 122 pp.

126654

Matthews, J.B.L.

1967

122 pp.

Publication

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Calanus finmarchicus sensu lato belongs in the order Copepoda of the class Crustacea. It is probably the most widespread marine copepod, being found throughout the world in all but the tropical seas. Over much of its distribution it is the dominant member of the zooplankton and is an essential part of the food web on which many economically important fish depend. An understanding of its ecology will play a fundamental part in the scientific management of fisheries. The aim of the thesis is to attempt an understanding of certain aspects of this vast subject. The scope of the thesis has been broadened, paradoxically perhaps, by previous work which has produced various opinions on the systematic status of populations in different water masses. It has been necessary, therefore, to define the main populations in systematic terms and then to use these definitions in a number of ecological studies. The scientific history of Galanus finmarchicus s.l, began two hundred years ago with its discovery and description, as Monoculus finmarchicus, by Gunnerus. Today it has been suggested that it should be regarded as a complex of seven species, each characteristic of different water masses. Three of these, C, finmarchicus, sensu stricto, C. glacialis and G. helgolandicus are known from the North Atlantic and they form the subject of the present investigations. Morphological separation of these species is based mainly on the structure of the fifth (last) pair of limbs; in adult females and stage V copepodids the shape of the inner border of the coxopodite and the size of the teeth on this border are the main charac ters. In C. finmar chicus the teeth are small and the border approx imatel y straig ht; C. glacia lis, which is larger than the other two, also has small teeth on a concav e border ; and C. helgolandic us has larger teeth on a strong ly concav e border . Adult males are distin guishe d mainly by differ ent propor tional length s of the rami of this pair of limbs. The materi al for the invest igatio ns has come for the most part from sample s obtain ed by the Contin uous Plankt on Record er in the course of the Survey of the North Atlant ic and North Sea conduc ted from the Oceano graphi c Labora tory, Edinbu rgh; these sample s have all been taken at a depth of 10 m. Some experi mental work was carrie d out at the Marine Statio n, Millpo rt, in the Firth of Clyde, and net sample s obtain ed during the Intern ationa l NORWES TLANT Survey s of 1963 have formed the basis for two ecolog ical studie s. Mixed sample s formed the basis for the compar isons of G. finmar chicus with C. glacia lis and C. helgol andicu s. The routin e analys is conduc ted by the staff of the Oceano graphi c Labora tory provid ed the inform ation on occurr ence necess ary for the select ion of sample s. Measur ements were taken of metaso me length and of all featur es of the fifth limbs mentio ned in previo us descri ptions ; the most import ant of these was the curvat ure of the toothe d border , but the number and size of teeth and the presen ce or absenc e of an extra seta on the endopo dite were includ ed as well. These charac ters were all consid ered in relati on to metaso me length and, in the case of curvat ure of the toothe d border , an angula r measur e was devised which also took account of size. The results sho?\red that there was little difference in length between G. finmarchicus and C. helgolandicus, the mean for the latter being slightly greater; values for tooth size also overlapped but there was complete separation between the two on the basis of curvature. This separation held good when measurements were made of over a thousand specimens from a wide range of different positions in the North Atlantic and North Sea. The mixed sample of C. finmarchicus and C. glacialis showed a bimodal length frequency but continuity of all morphological characters, including curvature, in relation to length. The experimental investigations demonstrated inverse relationships between length and temperature and between concavity and temperature. The individuals in the experiment thus tended at low temperatures towards features characteristic of C. glacialis. These results are considered, on the one hand, to support the specific separation of C. finmarchicus from C. helgolandicus, as is now widely accepted, but, on the other, to indicate that the apparent differences between G. finmarchicus and G. glacialis are the result of scarcity of specimens of intermediate size and that the characters are affected by temperature. The scarcity of individuals of intermediate size is considered to be explicable in terms of the comparatively small quantities of stable sub-Arctic water compared with the North Atlantic and the Arctic. Since recent authors have shown G. glacialis to be zoogeographically distinct from C. finmarchi cus, it is proposed to retain the former as a subspecies of G. finmarchicus. In introducing the ecological studies, results are presented which were obtained from a series of plankton samples taken during the NORWESTLANT Surveys. These demonstrate above all the dominance of GaJLanus f« finmarchicus among the herbivorous copepods in the region south-west of Greenland. In addition to C. f. finmarchicus and G. f. glacialis, ten species of larger copepods (greater than J> im ±n body length) were obtained and the distribution and abundance of each is recorded. The occurrence of each species is analysed further in relation to sea temperature and depth in a way which enabled them to be classified as warm, intermediate or cold water and as slope, intermediate or oceanic species. It is suggested that the temperature characteristics of C. f. finmarchicus indicate that the sampled population is relatively stenothermic. The primary aim of the Continuous Plankton Recorder Survey is zoogeographical and the abundance of Galanus finmarchicus s.l. makes it an ideal subject for distributional studies even in the areas to have come most recently within the Survey. Charts of distribution show that C. f. glacialis occurs only in those areas influenced by an outflow from the Arctic,i.e. the Labrador and Greenland coastal currents; that G. f. finmarchicus is found throughout the Survey area except for the extreme south-east and is most abundant in the colder oceanic regions of the North Atlantic; and that C. helgolandicus is a more southerly form which is most common in the south-east of the Survey area and in the V. waters of the Gulf Stream System. Low numbers of each form are generally found during the winter months. In spring, C. f, glacialis is the first to increase in numbers near the surface, starting in the regions most strongly affected by the Labrador Current and later in the more mixed waters south of the Newfoundland Grand Banks, G, f. finmarchicus is next to increase, first in the southern coastal areas and later further north and out into the open ocean, G, helgolandicus is last to increase in numbers, starting in the south and becoming progressively later towards the north. Where they overlap, C. f, finmarchicus is the springSfllSZ' summer form, G. helgolandicus the summer-autumn one. Annual fluctuations in numbers are, with a few exceptions, closely similar throughout the Survey area. The most notable exception concerns C. f. finmarchicus on the Grand Banks where the cycle is a complete inversion of the typical pattern.

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