Agulhas Bank Plankton
211
A similar westward shift from January to April was seen in Simorhynchotus antennarius, Tetrathyrus forcipatus, Parascelus typhoides, P. edwardsi and Amphithyrus glaber,
whereas A. similis underwent the westward displacement between October and January.
A remarkable chronological succession of species was seen in the genus Amphithyrus.
With A. similis at its maximum in October and January, and A. glaber in January only,
A. sculpturatus had its widest dispersal in April and A. bispinosa in July. A further
movement pattern was seen in Brachyscelus crusculum and Vibilia armata, which were
very rare on the Bank but much more common in the western and southwestern parts of
the survey area. They tended towards the coast in July and October and kept farther
offshore in January and April. The amplitude of this movement was greater in Vibilia than
in Brachyscelus. Their distribution pattern suggests that they may belong to the plankton
of the southern eddy system.
6. Cladocera
The Cladocera of the Agulhas Bank have not yet been the object of a formal taxonomic
study. The presence of Evadne spinifera, E. tergestina and E. nordmanni, and of at least
one Podon sp. has been established. We know that over the western extremity of the Bank
Podon sp. and Evadne nordmanni are common. The latter is present throughout the year
in False Bay and had a clear maximum in early spring (September 1959), with lesser
maxima at intervals of 2-3 months (February, May, July, December 1959). Evadne
spinifera made a sudden appearance in False Bay in December 1959 (444 specimens in one
hau!), but had vanished completely by the following March. E. tergestina appears to be
very rare and confined to the summer months. The latter two species seem to be associated
with colder water around centres of upwelling.
The dominant cladoceran on the Bank, however, is Penilia avirostris, whose center of
dispersion appears to be situated in the central area of the Bank. At times in this area
Penilia by far outnumbers all other zooplankton species caught in the N70-nets, its peak
season being summer. Occa.sionaliy Penilia extends its distribution to the west and rounds
the Cape, penetrating northward, both in- and offshore, as far as the latitude of St. Helena
Bay (32
0
S).
The Penilia population of the Bank is now under study by DELLA CROCE and his
collaborators (DELLA CROCE and ANGELINO, 1968). They found that the Bank was one
of very few localities where Penilia is present throughout the year and recognized here for
the first time that the annual cycle is supported by both parthenogenetic and sexual reproduction, as in the case of a seasonal cycle. The authors consider that the resting eggs may
here accomplish their function in a different way from that observed in the areas where
this species shows a seasonal cycle.
7. Copepoda
This group is the only one among the zooplankton for which a reasonable coverage
of the whole Bank has been obtained. CLEVE (1904) identified 110 species in a series of 21
vertical hauls taken in inshore waters between Cape Town and Durban. His 4 samples
taken west of the Cape Peninsula contained 43 species and 6 samples taken off the east
coast yielded 92 species, whereas 17 samples taken over the Bank contained only 27
species. This is another example of the low level of diversity in the plankton groups over
the Bank.
211
A similar westward shift from January to April was seen in Simorhynchotus antennarius, Tetrathyrus forcipatus, Parascelus typhoides, P. edwardsi and Amphithyrus glaber,
whereas A. similis underwent the westward displacement between October and January.
A remarkable chronological succession of species was seen in the genus Amphithyrus.
With A. similis at its maximum in October and January, and A. glaber in January only,
A. sculpturatus had its widest dispersal in April and A. bispinosa in July. A further
movement pattern was seen in Brachyscelus crusculum and Vibilia armata, which were
very rare on the Bank but much more common in the western and southwestern parts of
the survey area. They tended towards the coast in July and October and kept farther
offshore in January and April. The amplitude of this movement was greater in Vibilia than
in Brachyscelus. Their distribution pattern suggests that they may belong to the plankton
of the southern eddy system.
6. Cladocera
The Cladocera of the Agulhas Bank have not yet been the object of a formal taxonomic
study. The presence of Evadne spinifera, E. tergestina and E. nordmanni, and of at least
one Podon sp. has been established. We know that over the western extremity of the Bank
Podon sp. and Evadne nordmanni are common. The latter is present throughout the year
in False Bay and had a clear maximum in early spring (September 1959), with lesser
maxima at intervals of 2-3 months (February, May, July, December 1959). Evadne
spinifera made a sudden appearance in False Bay in December 1959 (444 specimens in one
hau!), but had vanished completely by the following March. E. tergestina appears to be
very rare and confined to the summer months. The latter two species seem to be associated
with colder water around centres of upwelling.
The dominant cladoceran on the Bank, however, is Penilia avirostris, whose center of
dispersion appears to be situated in the central area of the Bank. At times in this area
Penilia by far outnumbers all other zooplankton species caught in the N70-nets, its peak
season being summer. Occa.sionaliy Penilia extends its distribution to the west and rounds
the Cape, penetrating northward, both in- and offshore, as far as the latitude of St. Helena
Bay (32
0
S).
The Penilia population of the Bank is now under study by DELLA CROCE and his
collaborators (DELLA CROCE and ANGELINO, 1968). They found that the Bank was one
of very few localities where Penilia is present throughout the year and recognized here for
the first time that the annual cycle is supported by both parthenogenetic and sexual reproduction, as in the case of a seasonal cycle. The authors consider that the resting eggs may
here accomplish their function in a different way from that observed in the areas where
this species shows a seasonal cycle.
7. Copepoda
This group is the only one among the zooplankton for which a reasonable coverage
of the whole Bank has been obtained. CLEVE (1904) identified 110 species in a series of 21
vertical hauls taken in inshore waters between Cape Town and Durban. His 4 samples
taken west of the Cape Peninsula contained 43 species and 6 samples taken off the east
coast yielded 92 species, whereas 17 samples taken over the Bank contained only 27
species. This is another example of the low level of diversity in the plankton groups over
the Bank.