Investigations on the systematics and ecology of Calanus finmarchicus s.l. (Crustacea, Copepoda)

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 characters. In C. finmarchicus the teeth are small and the border approximately straight; C. glacialis, which is larger than the other two, also has small teeth on a concave border; and C. helgolandicus has larger teeth on a strongly concave border. Adult males are distinguished mainly by different proportional lengths of the rami of this pair of limbs. The material for the investigations has come for the most part from samples obtained by the Continuous Plankton Recorder in the course of the Survey of the North Atlantic and North Sea conducted from the Oceanographic Laboratory, Edinburgh; these samples have all been taken at a depth of 10 m. Some experimental work was carried out at the Marine Station, Millport, in the Firth of Clyde, and net samples obtained during the International NORWESTLANT Surveys of 1963 have formed the basis for two ecological studies. Mixed samples formed the basis for the comparisons of G. finmarchicus with C. glacialis and C. helgolandicus. The routine analysis conducted by the staff of the Oceanographic Laboratory provided the information on occurrence necessary for the selection of samples. Measurements were taken of metasome length and of all features of the fifth limbs mentioned in previous descriptions; the most important of these was the curvature of the toothed border, but the number and size of teeth and the presence or absence of an extra seta on the endopodite were included as well. These characters were all considered in relation to metasome length and, in the case of curvature of the toothed border, an angular measure was devised which also took account of size. The results showed 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 3mm in 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 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 spring-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. The Continuous Plankton Recorder provides samples which can also be used in a detailed investigation of the ecology of abundant epiplanktonic species in a part of the Survey area. In the present study the annual cycle of C. f. finmarchicus on the continental shelf between Newfoundland and the Gulf of Maine has been investigated, as well as some aspects of the relationships with temperature and phytoplankton. There are two main generations in the year, in spring and summer, and a suggestion of a third one in the autumn. Timing was a little earlier in the vicinity of the Gulf of Maine than further to the north-east.Copepodids showed clearer relationships with environmental conditions than did adults, presumably because they had had less time to encounter and be affected by different conditions. An inverse relationship was apparent between metasome length of stage V and sea surface temperature two months previously. Fluctuations in the estimated biomass of younger copepodids (stages I - IV) closely followed those of estimates of the total phytoplankton at all times of year except the spring; in the four months from March to June the increase in phytoplankton was at first greatly in excess of that of young Calanus, to be followed two months later by signs of overgrazing before the balance was achieved again in July. It is suggested that the total biomass of Galanus did not fluctuate greatly through the year; stocks increased steadily during spring and summer and became depleted just as steadily during late autumn and winter. Hensen net samples taken in April, June and July, 1963» during the NORWESTLAMT Surveys, formed the basis for an investigation of the speed and geographical pattern of development of C. f&gt; finmarchicus in the waters around Greenland. The spring generation, the only important one in the year, began almost simultaneously throughout the area. It is suggested that light, rather than temperature, was associated with maturation of the population. Thereafter the speed of development varied from region to region, so that in July the generation was most advanced in the open Atlantic and southern Labrador Sea, less advanced further north and in shallower water. A temperature effect was apparent in this pattern. It was possible to define three regions within the Survey area, namely the Labrador Sea and Baffin Bay, the Irminger Sea south to the Reykjanes Ridge, and the open Atlantic south-east of the Ridge, The relationships between sea temperature and the speed of development of Galanus in each region differed in such a way that, for a given temperature, development was fastest in the Labrador Sea and slowest in the open Atlantic, This is taken to indicate acclimatisation of the populations in each of the three regions. It is suggested that in this way populations of Galanus f. finmarchicus can complete a generation during the summer months, both in the cold water of the Labrador Sea and in water, some 8 - 11 C warmer, in the open Atlantic. In conclusion, the importance of the population as the basic unit in ecological investigations is emphasised. A high degree of ecotypic adaptation of populations is suggested as the means whereby Calanus finmarchicus s.l. has become widely distributed and reached high levels of abundance. Prominence must be given, therefore, to the identification of such populations in future studies. The vitality and adaptability of this copepod remain to be explained in terms of a wide range of biological disciplines.</p

Midnight sinking behaviour in Calanus finmarchicus: a response to satiation or krill predation?

The vertical migration of Calanus finmarchicus and krill (Meganyctiphanes norvegica and Thysanoessa raschii) was monitored during the summer of 1999 in the Clyde Sea using a combination of acoustic and net sampling methods. A moored 300 kHz acoustic Doppler current profiler (ADCP) identified a sound scattering layer (SSL) that started to ascend to the surface during the last moments of daylight. Net samples showed that the SSL was mostly composed of krill. C. finmarchicus rose to the surface in the late afternoon, causing a small but detectable increase in backscatter that did not vary in time through the summer. The ascent of krill, by contrast, became earlier as day length decreased towards autumn. Net samples showed that the strong downward Doppler velocities following the rise of the SSL was caused by the descent of C. finmarchicus. The fact that this coordinated sinking of C. finmarchicus occurred earlier towards autumn, even though the time of ascent to surface remained constant, implies that the feeding ‘window’ diminished over the course of the season. Feeding conditions did not become significantly better during this same period, discounting satiation as a likely cause of descent. The close temporal coupling between the arrival of krill and the subsequent descent of C. finmarchicus from the surface suggests that midnight sinking in Calanus is a response to predation