SEVEN SPECIES OF RED SEA ONCAEA 



69 



Fig. 18j). Males of oncaeid species usually exhibit a naked 

 maxillipedal claw, with few exceptions only [Epicalymma schmitti 

 and Oncaea englishi (Heron, 1977); O. mollicula (unpubl. data)]. In 

 the case of O. englishi the ornamentation seems to be variable, as it 

 was lacking in some individuals (Heron, 1977). A further difference 

 between the original description and the present account is the 

 sexual dimorphism in spine lengths on enp-3 in P2-P4, which was 

 not reported for O. scottodicarloi before. The sexual dimorphism of 

 this character seems to be typical for species of the genus Oncaea 

 s.str., however, as it was found in almost all species described in the 

 present account. Nonwithstanding these differences between the 

 two descriptions, both sexes of the species found in the Red Sea are 

 regarded as conspecific with O. scottodicarloi. 



O. scottodicarloi is closely related to O. media, O. waldemari, 

 and O. carta. Major distinctions between the four species have been 

 summarized in Table 5. Female O. media can most easily be 

 separated from the three other species by the length ratio of urosome 

 somites, particularly of the genital double-somite, which is much 

 longer. This character enables rapid identification of the species 

 during quantitative counts. The three remaining species are morpho- 

 logically very similar, except for differences in the length ratio of 

 urosome somites, the length of exopodal seta on P5 and the relative 

 lengths of caudal setae. The morphological relationships of O. carta 

 within this species complex remain uncertain, however. Sars' origi- 

 nal account of O. curta is the only reliable description of the species. 

 He described several important characters, such as the unequal 

 exopodal setae on P5, which need to be affirmed, as they are difficult 

 to discern. The species could not be re-examined during the present 

 study due to the lack of material. Heron & Bradford-Grieve ( 1995) 

 recorded O. carta from the Gulf of Naples, but their summary of 

 morphological distinctions between the species and O. scottodicarloi 

 was based on Sars' original account, rather than on own observ- 

 ations (G. Heron, pers. commn to R. Huys). The authors noted 

 several as yet undescribed species close to O. scottodicarloi in their 

 samples from Naples, one of which probably had been O. waldemari. 

 which is also present in the Mediterranean. However, Heron & 

 Bradford-Grieve were not aware of this species, because the original 

 description of O. waldemari (dated 1994) did not appear until 1996. 



Males of O. scottodicarloi can easily be separated from those 

 of O. media by the lack of sexual dimorphism in the coxobasal 

 seta on the antenna, which is long and plumose as in the female. 

 Males of O. waldemari, however, share this character and can be 

 distinguished from O. scottodicarloi only by minor details in the 

 form and ornamentation of seta IV on the antenna as well as in 

 proportional lengths of caudal setae (Table 5). For males of O. 

 curta corresponding sexual differences still need to be investi- 

 gated. 



Size variation 



Female O. scottodicarloi from the central and northern Red Sea 

 were considerably smaller (0.48-0.52 mm, Bottger-Schnack et al., 

 1989) than those from the southern Red Sea and Gulf of Aden, which 

 measured between 0.54-0.61 mm (numerous specimens measured 

 by the traditional method used by Bottger-Schnack etai. 1989). No 

 morphological differences other than size became apparent for 

 specimens from the two regions. Re-examination of specimens from 

 the central populations confirmed that the small morph had not been 

 confused with O. waldemari, which is similar in size. The observed 

 regional size difference of O. scottodicarloi (which occurred also in 

 the males) does not seem to be a temporary phenomenon, as the 

 lengths of specimens in the central Red Sea did not vary during 

 different seasons. Generally, O. scottodicarloi seems to exhibit a 

 wide range in body length: Heron & Bradford-Grieve reported 



females between 0.59-0.72 mm, with distinct differences between 

 different oceanic areas. Female O. scottodicarloi from the northern 

 Arabian Sea and Eastern Mediterranean Sea as well as those col- 

 lected of Australia's NW Cape (eastern Indian Ocean) measured 

 between 0.58-0.62 mm (unpubl. data) and were thus in the upper 

 size range of specimens from the Gulf of Aden. Itoh [in: Chihara & 

 Murano ( 1 997)] recorded females between 0.50-0.64 mm and males 

 between 0.38-0.54 mm in size, however, the author was not aware 

 of O. waldemari, which might have been included in the lower size 

 range of his specimens. [A recent examination of two forms of 

 female O. scottodicarloi from the Western Subtropical Pacific kindly 

 made available by H. Itoh and S. Nishida showed that females of the 

 smaller form (0.49-0.62 mm) were conspecific with O. waldemari, 

 while the larger form (0.58-0.7 1 mm) belonged to O. scottodicarloi. ] 

 Compared to these data, specimens of O. scottodicarloi from the 

 central Red Sea appear to be even smaller than the lowest values 

 reported for this species so far. The occurrence of the small morph 

 seems to be a local phenomenon for this area. A possible explanation 

 for the reduced size of central populations may be the more extreme 

 environmental conditions in the central Red Sea as discussed by 

 Bottger-Schnack et al. (1989). Similar to O. venusta f. venella. the 

 small morph of O. scottodicarloi in the central Red Sea seems to be 

 a self-sustaining population, with no substantial recruitment from 

 the South. 



Other records of o. scottodicarloi 



Heron & Bradford-Grieve (1995) recorded O. scottodicarloi from 

 various regions in the Atlantic and Pacific and recently Itoh [in: 

 Chihara & Murano ( 1997)] described it from Japanese waters. The 

 species was also found in great abundance in the Eastern Mediter- 

 ranean as well as in the northern Arabian Sea (Bottger-Schnack, 

 1994, 1996, as O. media f. minor). Sewell ( 1947) had reported O. 

 media f. minor from the Indian Ocean, however, the identity of his 

 size morph with O. scottodicarloi cannot be confirmed as the 

 specimens of Sewell are not available. The specimens might have 

 been confused with O. waldemari, which was found to co-occur 

 with O. scottodicarloi in both areas (see below). The identity of O. 

 media f. minor (sensu Malt, 1982b) will be discussed below under 

 O. waldemari. 



Geographical distribution 



Oncaea scottodicarloi is distributed throughout the Red Sea proper 

 (Bottger-Schnack, 1988, 1990a, b, 1995, as O. media f. minor) and 

 can also be found in the northernmost part of the Gulf of Aqaba (own 

 unpubl. data). The species is one of the most abundant oncaeids in 

 the central Red Sea, usually accounting for 2-10% of the total 

 number of Oncaeidae in the water column down to 450 or 1050 m 

 depth, depending on region and season. In narrower depth layers, 

 even higher percentages, up to 15%, of all oncaeids are found 

 (epipelagic zone, winter, central Red Sea). Similar to O. media, O. 

 scottodicarloi exhibited a 2-tailed horizontal distribution during 

 summer, with high abundances in the central-northern Red Sea as 

 well as in the Strait of Bab al Mandab and Gulf of Aden, but low 

 abundances in the intermediate southern parts of the Red Sea 

 (Bottger-Schnack, 1995: Table 6). Consequently, its relative abun- 

 dance in the southern Red Sea was rather low (0.4/1.6%), but 

 strongly increased again further south. The proportion of O. 

 scottodicarloi in the Gulf was possibly underestimated, since sam- 

 ples in that area covered only the upper 250 m of the water column, 

 thereby excluding a potential mesopelagic population. In Table 8 of 

 Bottger-Schnack (1995), the abundance values of O. scottodicarloi 

 in the uppermost depth zone (0-100 m) were inadvertently changed 

 between stations. The correct data for O. scottodicarloi are given in 

 Table 6 of the present account. 



