SEVEN SPECIES OF RED SEA ONCAEA 



53 



Ecological notes 



O. venusta f. venella is much more abundant than f. typica in the Red 

 Sea, exceeding the population densities of the latter by a factor of 

 between 3 to 100, when both forms co-occur (Bottger-Schnack, 

 1990b, 1995). 



Geographical distribution 



In the Red Sea, O. venusta f. venella is more widespread than O. 

 venusta f. typica, it occurs throughout the main basin (Bottger- 

 Schnack, 1990a, b, 1995) and was also found in samples from the 

 northernmost part of the Gulf of Aqaba (unpubl. data). O. venusta 

 f. typica is mainly restricted to the southernmost Red Sea and the 

 Gulf of Aden, reaching the central Red Sea only during the winter 

 months, when a strong seasonal inflow of southern Red Sea waters 

 influences the plankton fauna in the central area (Bottger-Schnack, 

 1990b; see also Weikert, 1987; Beckmann, 1996). 



Vertical distribution and vertical migration 

 O. venusta is mainly distributed in the epipelagic zone in the Red 

 Sea, with maximum abundances in the 0-100 (150) m depth layer 

 (Bottger-Schnack, 1990 a, b). Occasional finds of the species in the 

 bathypelagic zone (unpubl. data, see also Beckmann, 1996) were 

 usually regarded as moribund specimens or may be due to contami- 

 nation of the nets. 



When co-occurring, the two forms of O. venusta tend to be 

 vertically separated: Both sexes of O. venusta f. typica were concen- 

 trated in the upper epipelagic zone at 0-20 m (Strait of Bab al 

 Mandab) or at 0^-0 m (Gulf of Aden) during summer, whereas 

 female f. venella occurred deeper in the water column, with maxi- 

 mum concentrations at 20-60 m ( Bab al Mandab ) and ( 20)80- 1 00 m 

 (Gulf of Aden), respectively. Male f. venella occurred at the same 

 depth horizon as females in the Gulf, but were concentrated some- 

 what shallower than females (0^40 m) in the Strait, thereby extending 

 into the depth horizon where f. typica dominates. No corresponding 

 vertical separation of the two forms became obvious in the central 

 Red Sea during winter, when both forms stayed in the upper 50 m 

 during day and night (Bottger-Schnack, 1990b). However, the sam- 

 pled depth strata in the epipelagic zone were broader during winter 

 (50 m-intervals) than during summer (20 m-intervals) and thus a 

 vertical segregation may not have been detected due to the limita- 

 tions in the sampling strategy. 



In the central and northern Red Sea, O. venusta f. venella was 

 mainly concentrated in the lower epipelagic zone, within and 

 below the strong seasonal thermocline, during autumn (Bottger- 

 Schnack, 1990a). Diurnal vertical migration of moderate intensity 

 was observed during this season, with specimens showing a 

 stronger tendency to concentrate within the depth range of maxi- 

 mum temperature gradients during the night than during the day. 

 From autumn to winter, a conspicuous shift of the population 

 centre from the lower epipelagic zone (40- 100 m) to shallower 

 depths (0-50 m) was observed, which coincided with the weaken- 

 ing of the thermocline during the winter season (Bottger-Schnack, 

 1990b). Males of O. venusta f. venella generally exhibited the 

 same depth distribution as females, but were evaluated semi-quan- 

 titatively during the autumn survey only. 



Seasonal variation in abundance (central red sea) 

 Seasonal variation in abundance in the central Red Sea was most 

 conspicuous for O. venusta f. typica, which occurred in the central 

 area only during winter, but was absent during summer and autumn 

 (Bottger-Schnack, 1990b, 1995). By this, a strong inflow of south- 

 ern Red Sea populations into the central area was indicated, and the 

 species has been regarded as an indicator species of southern Red 

 Sea waters, similar to other copepod species, such as species of 



Eucalanus (Beckmann, 1984, 1996). Abundances of O. venusta f. 

 venella in the central Red Sea were highest during winter, but 

 moderately high densities were also found during autumn, thereby 

 indicating a lesser influence of the southern inflow than was observed 

 for f. typica. 



Oncaea mediterranea (Claus, 1863) 



Antaria mediterranea Claus (1863): 159-160, Tafel XXX, Fig. 1-6 



(9). 7(d). 



Onccea mediterranea (Claus, 1863); Onccia mediterranea (Claus. 

 1863). 



OTHER DESCRIPTIONS. Giesbrecht (1892) [as Oncaa mediter- 

 ranea]; Heron (1977); Heron & Bradford-Grieve (1995); for further 

 details see Bottger-Schnack & Huys (1997b). 



Type LOCALITY. Tyrrhenian Sea; Messina. 



PRELIMINARY NOTE. A detailed redescription of O. mediterranea, 

 based on material from the Red Sea and the Eastern Mediterranean, 

 has recently been published by Bottger-Schnack & Huys (1997b), 

 including a review of the taxonomic history of the species. The 

 following corrective note describes additional morphological details, 

 which were not mentioned in the previous account, but might be of 

 importance for constructing phylogenetic relationships within the 

 genus Oncaea s. str. 



Corrective note 



(1) The labrum of O. mediterranea exhibits an additional patch of 

 long spinules on posterior face at posterior part of each lobe, 

 similar to O. venusta (cf. Fig. 3C), which was not figured by 

 Bottger-Schnack & Huys. 



(2) The endopodal spines of PI exhibit a subapical tubular exten- 

 sion, similar to O. venusta (cf. Fig. 4A). These extensions seem 

 to be widespread among oncaeids as they have been found in 

 distantly related species, such asArchioncaea arabica (Bottger- 

 Schnack & Huys, 1997a), species of Triconia{ Bottger-Schnack. 

 1999). O. subtilis (Bottger-Schnack & Huys, in press) and 

 species of the atlantica-gvoup (unpubl. data). 



(3) The coxa of P4 is ornamented with a patch of long setules on 

 posterior face, similar to O. venusta (cf. Fig. 4D). 



(4) The female P6 is ornamented with 1 spine and 2 spinous 

 processes, as in all other species of Oncaea s.str., not only 1 

 spinous process as erroneously figured by Bottger-Schnack & 

 Huys. 



(5) In addition to the sexually dimorphic characters described by 

 Bottger-Schnack & Huys, sexual dimorphism is expressed in 

 the endopods of P1-P3, in the ornamentation of P5, and in the 

 length of caudal setae: (1) in PI, the spinous outgrowth at the 

 distal margin of the endopod is relatively longer in the male, 

 reaching half the length of the distal spine, (2) in P2-P3, the 

 conical projections on enp-3 are relatively longer in the male 

 as compared to the female, similar to O. venusta (cf. Fig. 7B, 

 C, E, F), (3) the outer long seta on P5 exopod is ornamented 

 with triplicate row of minute spinules along entire length, not 

 naked as in female, (4) caudal setae VI and IV are relatively 

 shorter in the male as compared to the female, which was 

 correctly figured by Bottger-Schnack & Huys (their Fig. 4A), 

 but erroneously described as being equal in length to the 

 female in the text. 



