| Author||Dick G. Troost|
|Title||Onderzoek aan enkele groepen uit het plankton verzameld in het kader van “Cooperative Investigations in the Caribbean and Adjacent Regions” (Cicar-project)|
|Journal||Verslagen en Technische Gegevens|
|Abstract||In February 1970 plankton sampling has been started as a section of the “Cooperative Investigations in the Caribbean and Adjacent Regions” (CICAR-project). Sampling was executed by ornithologists on board the M.S. “Luymes”. Open plankton nets with meshes of 0.056 mm diam. were used for sampling between 0 and 18 metres. Part of the samples made in 1970 has been examined. 1. The samples examined came from the area around the islands Aruba, Bonaire and Curacao (area 1, fig. 1), visited in summer and winter, and from the coast off the Guyana’s (area 2, fig. 2), visited in autumn. The second area is strongly influenced by the outflow of the Amazon, which in summer is nearly three times as much as in winter (fig. 3). 2. For area 1 only the temperature of surface layers was measured, in area 2 also the salinity, current velocity, and primary production. A temperature salinity diagram was made based on data from 42 stations (fig. 4). From this diagrim, and the position of these stations (fig. 5), it was concluded that three different watermasses are sampled in area 2. They are called “river-water” (mainly from the Amazon), “upwelling-water”, and “mixed-water” (a mixture of “river-water” with “upwelling-water”, and/or oceanic surface-water). With “upwelling-water” is meant, water coming in the coastal region from deeper layers, though this water may also originate from a divergence or turnover of a watermass. 3. A project was started concerning the distribution and relative abundance of the Crustacea Lucifer typus, an “oceanic” species, and Lucifer faxoni, a “neritic” species. The differences in their distribution were striking. In summer L. typus was in area 1 two and sometimes more than three times as abundant as L. faxoni, depending depth. However, in winter L. faxoni was about twenty times more abundant than L. typus (fig. 7). The great influence of the Amazon and the wet season in area 1 are expected to be responsible for the seasonal differences. In area 2 L. faxoni was nearly five hundred times as abundant as L. typus. The abundånce in the different watertypes was totally different (figs. 8, 9). In “mixed-water” we found about 12 specimens of L. faxoni in each sample, in “upwelling-water” 103 specimens, and in “river-water” 732 specimens. So, the “neritic” character of this area in autumn is not equally distributed. This confirms the generally accepted theory that water from the Amazon is penetrating the area in “bubbles”. 4. The following species and formae of the Euthecosomata were found: Limacina inflata, L. lesueuri, L. trochiformis, L. bulimoides, Creseis acicula forma acicula, cf. Cr. acicula forma clava, Cr. virgula forma conica, cf. Cr. virgula forma virgula, Styliola subula, Hyalocylis striata, Cuvierina columnella forma atlantica, Diacria trispinosa forma trispinosa, D. quadridentata quadridentata forma danae, Cavolinia longirostris forma longirostris, cf. Cav. longirostris forma strangulata, Cav. uncinata uncinata forma uncinata, and Cav. inflexa forma inflexa.|
All of these were present in area 1 during summer; L. inflata was by far the most abundant at that time (73,1%). In winter this species has also the greatest abundance (58,3%); L. trochiformis, Cr. acicula forma acicula, Cr. virgula s.l., St. subula, Cav. longirostris forma longirostris/strangulata, and Cav. uncinata forma uncinata were the only species also present in winter.
In area 2 in autumn Cr. acicula forma acicula showed the greatest abundance (91,4%). The following species were also found: L. inflata, L. trochiformis, Cr. virgula s.l., H. striata, Cav. longirostris forma longirostris/strangulata, Cav. uncinata uncinata forma uncinata, and Cav. inflexa forma inflexa. 5. Cr. acicula forma acicula without shell and without intact columellar muscle turned out to be distinguishable from other Creseis without shell. Only this forma has the wing protusion separated from the wing gland (fig. 12).
The distribution of juveniles of Cav. inflexa was obviously different from that of the adults of this species and rather in accordance with the distribution of adults of Cav. longirostris. However, the very small juveniles of this species were not described, and could resemble those of Cav. inflexa (Van der Spoel, pers. comm.). Investigation of the soft parts of the juveniles was started and series of animals with increasing length were selected. Most juveniles turned out to be young specimens of Cav. longirostris instead of Cav. inflexa (Troost & Van der Spoel, 1972). 6. The already known distribution of the Euthecosomata (Van der Spoel, 1967) could be corrected for L. inflata (1), L. lesueuri (2), L. trochiformis (3), L. bulimoides (4), Cr. acicula forma acicula (5), (probably also the forma clava (6)), Cr. virgula forma conica (7), and Cav. longirostris (8). Numbers 2, 4, 6 and 7 occur in area 1 (between 67° – 70° W and 12° – 13° N); numbers 1, 3, 5 and 8 occur in area 1, and also in area 2 (between – 47° – 63° W and 05° – 12° N). 7. The presence in different watermasses made it possible to enlarge the already known ranges for temperature and salinity of L. inflata, L. trochiformis, Cr. acicula forma acicula, Cr. virgula s.l., H. striata, and Cav. longirostris. These ranges are now 26.9°C to maximal 28.6°C and 33.2 ‰ S (average value) to more than 36 ‰ S, respectively. 8. After having comparised samples collected during day-time and night-time, a diurnal vertical migration became evident for several species, except for Cr. virgula s.l., and Cav. longirostris. The already known double diurnal migration in Cr. virgula could not be proved with the present samples. 9. Biometric differences in shell dimensions have been found for all formae of Creseis (fig. 18), some of which were difficult to identify, even with intact shells.
For nearly all other species and formae biometric data were collected (figs. 15, 16, 20, 22, 25). 10. Some specimens of Cav. longirostris resemble the forma flexipes from the Red Sea as to the lateral spines and sometimes the dorsal shell lip; they differ, however, in size. A comparable selective pressure in both areas may result in similar phenotypic features. For a more complete description of this variation more investigations are required. 11. Ten species of Pseudothecosomata, Gymnosomata, and Nudibranchiata were found.
Of the Prosobranchiata-larvae specimens of 31 different types were found; they have been illustrated (figs. 27-54). 12. To simplify handling and comparison of fish larvae a new method has been developed. It was obvious, that identification was difficult. Only eight species belonging to seven families have been distinguished, and numerous specimens belonging to another eight families. The larvae of the Anquilliformes have been divided into ten different groups (Pl. I) on ground of the arrangement of the chromatophores and the total number of myomeres. It was striking, that most of the larvae of the Anguilliformes from area 2 came from stations, situated near the continental slope.
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