On the biology of Calanus helgolandicus (Claus) from the Veliko jezero i. Mljet
Abstract
This paper deals with the results of the investigations on the biology of the copepod Calanus helgolandicus C l a u s obtained from the material collected during the complex oceanographic studies and fertilisation experiment in the region of i. Mljet (Veliko jezero) from 1951 to 1955. This material has been completed with the data collected in the open Adriatic during the expedition »H v a r« (1948-1949) and with other investigations in the north and middle Adriatic (Dugi otok, Kaštelanski zaljev and Stončica) from 1957 to 1963.
Systematic and distribution. Revision of the morphological charecteristics of the Calanus has shown that it is C. helgolandicus which is found in the Bay of Mljet and in the Adriatic in general. This species comes out from the Mediterranean and is spread as far as the English Channel and is the integral part of the ’Lusitanian’ fauna. In the Black Sea it appears as C. helgolandicus var. ponticus (Y a s h n o v, 1955, 1961, 1963).
According to the earlier data as well as to those obtained during these, investigations, C. hegolandicus is widely spread in the Adriatic. Great sesonal annual variations of the population density in different regions have also found. In the surface layers (from surface to the depth of 53 m) of the open Adriatic maximal quantities appear at the end of winter and the beginning of spring, when it makes up to 50% (Fig. 4,7) of the total zooplankton (number) (V u č e t i ć, in preparation).
Population dynamic - density. On the basis of the total material it has been established that there is no immigration of the copepod C. helgolandicus population into the Veliko jezero from other localites (Fig. 5,6.) This has enabled to make with greater accuracy many conclusions in connection with the population dynamics of this copepod. In the period from March 1951 to November 1954 the population density was between 42.3 and 97.1 individuals (♀, ♂, Vst) per m3. These values outstand considerably from all the others obtained in the up to the present investigated Adriatic areas. In the littoral area, in the Bay of Kaštela, the values were from 2.2 to 12.0 (♀, ♂, Vst) per m3 in the period from 1957 to 1962. In the area of the open sea at the station Stončica, i. Vis, (1961-1962), the number of C. helgolandicus was between 1.5 and 2.0 per m3. In the north Adriatic, in the area of Dugi otok, in 1960 there was an average of 11 specimens per m3 (Fig. 25).
In the Veliko jezero the increase of the total zooplankton (dry weight) shows some parallelism with the density increase of the copepod C. helgolandicus population (Fig. 8).
The increase of the Calanus population was accompanied by the increase of the bacterial population in 1952, while in 1951 there was an alternation (Fig. 9).
No relation could be determined between the changes in the density of the population C. helgolandicus and the phytoplankton (number) density variation (Fig. 10).
The increase of Calanus population density in relation to the temperature increase shows weak correlation in the negative direction. A certain relation in the positive direction was found in 1952. It was tried to explain this high density of the population during the high temperatures by the horizontal shifting of the organisms in the direction of the greatest depth of the Veliko jezero, where, at that time, relatively low sea temperatures were registered.
In 1954 again a more pronounced population density increase appeared, but not so the temperature increase. The density increase of the C. helgolandicus in 1954 might possibly be explained as the reaction of the population to the favourable conditions of growth because of better feeding conditions as a result of fertilization. The difference in C. helgoladnicus population density between 1951 and 1954 was cca 88% (page 15).
Reproduction: In order to determine to a certain degree the amount of the produced organic matter by the copepod C. helgoladicus in the period of one year, it was necessary to establish the annual quantities of eggs production and the number of generations. Following the appearance of individual copepodite stages and nauplii in the sea it has been concluded that the C. helgolandicus in the Veliko jezero on the i Mljet is reproducing all the year round. The olny break, i. e. lack of younger stages, in the sea partly appears in the period from June to August; this »diapause« has been named »estivation« (Fig. 12).
On the other hand, by staining the females and determining their rate of ripeness it has been found that ripe females are present all the year round. Although the rate of ripeness or high fertility does not reach the height of that the females of C. finmarchicus from the North Sea it is nevertheless intensive because of its longer period of reproduction. Namely, the females of the copepod C. finmarchicus in the North Sea, i. e. their V stage of the autumm generation, has not time to ripen until winter. Only in spring, when the phytoplankton starts blooming, they mature and from the V stage ripe females develop. In our case in May the V stage is represented in large quantities, and later, in June, July and August, there is an accumulation of ripe females (Fig. 13, 14).
Looking for the causes which might have brought about an interruption of reproduction from June to August, a striking correlation was noticed between this phenomenon and the variation of the sea water density. At the time when, the young copepodit stages of C.helgolandicus can no more be detected, the sea-water density in the surface layers (from 0 to 20 m) drops to the annual minimal values (Fig. 16). The earlier data (V u č e t i ć, 1961) show that the females at this time of the year do not approach the surface layers. Knowing both this and the fact that for the development of the nauplii from eggs (with Calanus sp), besides a certain sea water density, also a certain thickness of the vertical layer through which the eggs sink is necessary (M a r s h a l l & O r r, 1955), it has been concluded that the interruption of reproduction in the Veliko jezero was brought about because the eggs had been destroyed. Namely, in normal conditions the females rise at night to the surface levels where they feed more intensely while laying eggs. The eggs are developed while sinking to the bottom through the layers about 40 m thick. At the time of the thermocline, which appears in June, July and August, the females rise at night to the higher levels only to the depth of 20 m from the surface. If there they feed intensely and lay eggs, the nauplii has no chance to develop (nauplii hatch), because the eggs sink too quickly to the sea bottom. This results in an abrupt interruption of their development and they perish.
Studying the ratio of sexes it has been recorded that the females are always more numerous. Besides, the males mature much quicker so that their maximal quantities always appear somewhat earlier than those of the females, which probably explains why the females of the C. helgolandicus also live longer than the males, as M a r s h a l l and O r r (1955) had already established for C. finmarchicus (Fig. 13).
The obtained data allow to suppose that, from the ecologic point of view, C. helgolandicus in the Veliko jezero can develop 5 generations annually in the following order: I from December to February; II from February to April; III from April to June; IV from August to October and V from October to December. These periods are supposed to change as a result of the influence of the predominant ecological factors in each year (Fig. 26).
Size distribution — From the size (cephalothorax length) frequency distribution of the adults it has been stated that in length the females differ considerably from the males. The males are considerably shorter, their length being from 1.92 mm to 2.32 mm. The length of the females varies from 2.02 to 2.54 mm (Fig. 18).
Seasonal differences of the female and male have also been found. In cold periods the organisms are always of smaller dimensions, while in the warm season they are much larger. There are certain differences between individual years, so that a considerable increase in length was noticed in 1954 (Table VI).
Studying the growth mechanism by means of moulting it became necessary to examine their growth in relation to some biotic and abiotic factors. (Fig. 19-22). It was established that the changes in the density of the total zooplankton had influenced the changes in the length of individual members of C. helgolandicus population. Also the changes in the Calanus population density resulted in the changes in the length of individual members. At the time of a higher density of the total zooplankton, as well as of the Calanus population itself, there was also registered an increase in the length of the individuals of this copepod. The conclusion has been come to that the above results might have been the reflection of a slower maturation of adults (which causes the prolonged growth) as a consequence of shortage of food which was distributed among a greater number of individuals.
While analyzing graphically the total size data (for all the investigated years) in relation to the food changes (phytoplankton density), an inconsiderable increase in the growth of organisms in relation to food increase was obtained. But from regression curves for individual years statistically significant relation in the positive direction only for females was established in 1952. For males also, nearly in all the years, it was in the positive direction, only in 1953 it was in the negative direction and statistically significant. (Table XIII, XIV).
Examining the relation between growth and temperature, already graphically the positive correlation between the length increase od C. helgolandicus and the sea-water temperature increase was obtained. A statistically significant relation was established in 1952. (Table XII, XV).
Salinity changes were not important but they also showed the correlation in the changes in length of the adults, i. e. with the salinity increase the organisms show some length increase. The relation between the sea-water density and the changes in length of males and females is very striking in the negative direction. Namely, larger organisms were obtained at the time of the lowest sea-water density (Fig. 22). But, considering all together, growth is the result of interaction of all these factors.
In these examinations temperature played a decisive role at determining the sea-water density, beside the salinity which was of lesser importance. Most of the correlations between temperature and the length increase of the copepods, stated up to now, might be the result of the influence of temperature on the sea water density, which influence the osmotic regulation of the organism. This brings about either the increase or decrease of the body volume, because of the different capacity of water absorption, while moulting.
The growth increase in the adults of the copepod C. helgolandicus in 1954 (Fig. 17, 18; Table XVI) could not be ascribed to the temperature increase as it was considerably lower that year than in 1952. The influence of the sea-water density was also excluded because that year it was considerably higher than in other years. Therefore, it has been suggested that the increase in the length of organisms was in some way the result of fertilization, i. e. fertilization through the phytoplankton increase had effected the length of the individual members of this population in the following way: first - natality could have been increased through the greater number of eggs, and its result was a density increase of Calanus population. This brought about the decrease of phytoplankton density which resulted later on in the slower maturation of the females from the later spawns, causing their prolonged growth. Second - the phytoplankton growth after fertilization could have had a postive effect on the growth of the copepodite stages, which C u s h i n g & V u č e t i ć (1963) had already established with C. finmarchicus. This could have effected later on the increase in the length of adults.
Vertical migrations and feeding - High population density of Calanus in the Bay of Mljet enabled us to obtain more data on the vertical migration than this could have been done according to the data from the open Adriatic (H u r e, 1955, 1961).
It is important that in the time of homothermia and less intense insolation in winter the adults of C. helgolandicus appear near the surface by day (V u č e t i ć, 1961). During intense insolation and warming up of the surface layers they disappear from the surface even at night. In summer, at the time when a strong thermocline appears they rise towards the surface at night, but they do not cross the thermocline but stop at the levels with the temperature of 15 - 18°C (Fig. 23, 24).
It was found that light conditions and temperature limits the vertical migration. But feeding is more intensive in the surface layers where phytoplankton production takes place. More intensive feeding at night is emphasized especially at the time when only at night Calanus is found in the surface layers. It has been supposed that feeding depends on the degree of the active migration besides its dependency on the rate of phytoplankton density. Superfluous feeding was found only when the phytoplankton density was very high (November 1952).
In the food of C. helgolandicus from the Veliko jezero the remains of diatoms Centricae and Penatae were found, especially some tihopelagic forms. Several times it was noticed that the faecal pellets consisted of some green amorphous mass of undeterminable origin. Numerous faecal pellets were noticed in April, May and June 1951, and especially so in November 1952, when surely superfluous feeding took place.
In summer a great number of phytoplankton was found in the suface lavers just at the time when the Calonus did not appear in these layers. In winter it was vice versa, a greater number of phytoplankton was found in the bottom layer, while the Calanus (generally with a smaller density) was detained in all the layers.
It is supposed that the decrease in laying eggs by C. helgolandicus in the Veliko jezero could have come about in summer because of less intensive feeding. In summer some phytoplankton tihopelagic species of smaller dimensions were present. They are known to be of lesser use as food to the Calanus, or that it cannot consume them at all.
