SEVEN SPECIES OF RED SEA ONCAEA 



57 



in specimens from the Red Sea than had been reported by Giesbrecht 

 (1892, PI. 47, Figs. 32, 33) and Heron & Bradford-Grieve (their Fig. 

 17g-i). In particular the outer distal spine on P2enp-3 was longer in 

 Red Sea specimens, measuring about 3/4 the length of the outer 

 subdistal spine (cf. Table 4), whereas it was figured being only 1/3- 

 1/2 that length by the authors mentioned above. Proportional spine 

 lengths on P2 enp-3 of O. media males from Japanese waters as 

 recorded by Itoh (in: Chihara & Murano, 1997: his Fig. 365f) were 

 similar to those found in Red Sea specimens. 



O. media is closely related to O. scottodicarloi, with which it has 

 often been confused. According to Heron & Bradford-Grieve (1995), 

 important distinctions between females of the two species are found 

 in (1) proportional lengths of urosome segments, (2) form and 

 location of sclerotization between genital apertures, and (3) relative 

 lengths of endopod spines of P4. Comments on the first two characters 

 will be given below. A further character separating females of the 

 two species as observed during the present study is the proportional 

 length of exopodal setae on P5, the outer one being relatively shorter 

 in O. media than in O. scottodicarloi. 



( 1 ) The typical elongate form of the genital double-somite of O. 

 media as figured by Heron & Bradford-Grieve appears to be of 

 great significance, because it enables unequivocal separation of 

 O. media from other species of the media complex. In the original 

 account of Giesbrecht ( 1 892), however, the urosome is concealed 

 underneath the paired egg-sacs of the ovigerous specimen in the 

 illustration of female habitus ( Plate 47, Fig. 1 ) and his illustration 

 of the female urosome in dorsal aspect ( Fig. 1 1 ) was assigned to O. 

 scottodicarloi by Heron & Bradford-Grieve. Therefore it cannot 

 be decided definitely, whether this character had also been present 

 in Giesbrecht's specimen and the conspecificity of Giesbrecht's 

 O. media with O. media sensu Heron & Bradford-Grieve can only 

 be confirmed after re-examination of his type material. The lateral 

 view of O. media as figured by Giesbrecht (Plate 2, Fig. 1 2) shows 

 the genital segment being twice as long as the remaining part of the 

 urosome, which is similar to O. media sensu Heron & Bradford- 

 Grieve; also, the proportional lengths of spines on P4 enp-3 P4 

 figured by Giesbrecht are more similar to O. media than to O. 

 scottodicarloi. Since both characters support Heron & Bradford- 

 Grieve's conclusion, their opinion is followed in the present 

 account. 



(2) The integumental sclerotization located between the genital 

 apertures is a reinforced attachment site for the insertion of the 

 dorsal longitudinal trunk muscles (R.Huys, pers. commn.). These 

 usually insert on the anterior rim of each somite, or in the case of 

 the female genital double-somite near the genital apertures, 

 marking the original plane of fusion. The sclerotization is 

 always located at about 1/3 to 40% the distance from the anterior 

 margin and does not differ between species, but as the position 

 of genital apertures on the genital double-somite is different, the 

 sclerotization changes its position relative to the genital aper- 

 tures. In O. media, the genital apertures are located more laterally 

 than in O. scottodicarloi, and thus the sclerotization changes its 

 position accordingly. 



Males of O. media can be separated from those of O. scottodicarloi 

 most easily by the sexual dimorphism in the coxobasal seta of the 

 antenna, which is short and naked (Fig. 16D) in O. media, but long 

 and plumose as in the female in O. scottodicarloi. Giesbrecht and 

 Heron & Bradford-Grieve did not notice the sexual dimorphism of 

 this seta in O. media. In the present study, however, this character 

 was found in all species of Oncaea s.str., except for O. scottocarloi 

 and O. waldemari. The sexual dimorphism in spine lengths of P2-P4 

 enp-3, on the other hand, which had already been reported for males 

 of O. media in both previous account mentioned above, cannot be 

 used to distinguish between males of both species, since it was also 

 found in O. scottodicarloi during the present study (Fig. 23J, M) and 

 seems to be a typical character for males of Oncaea s.str. (cf. O. 

 venusta, O. waldemari). 



Other records of o. media 



A summary of the world-wide records of O. media has been com- 

 piled by Heron & Bradford-Grieve (1995). They point to the great 

 confusion caused by Sewell (1947), who described two size vari- 

 ants, f. major sad f. minor, from the Arabian Sea. The major form ( 9 

 9: 0.73-0.82 mm, 66: 0.53-0-62 mm) exhibited a smaller length to 

 width ratio of caudal ramus (2:1) than the minor form (3: 1 ), which 

 would correspond to the observed differences between O. media and 

 O. scottodicarloi. However, Sewell described the distal spine on P4 

 enp-3 off. major measuring about 2/3 the length of distal endopod 

 segment (p. 26 1 ), which is longer than reported by Giesbrecht ( 1 892) 

 and Heron & Bradford-Grieve for O. media, the spine being only 



Table 5 List of important characters separating O. media and related species. Data of O. curta after Sars (1918). remaining data from present study. 



Species 



media 



scottodicarloi 



curta 



waldemari 



Female 













GDS 













-l:w ratio 



1.9:1 





1.5:1 



1.5:1 



1.7:1 



- Form 



very el. 





oval-el. 



squarish 



elongate 



- ratio GDS : rest of urosomites' (excl.CR) 



3.9:1 





2.5:1 



1.5:1 



1.9:1 



- position gen.ap.. distance from lateral margin 



1/4 





1/3 



ca 1/5 



1/4 



L. ratio caudal setae 













-setaVFIV 



2/3 





1/2 



ca 1/2 



3/4 



-setaVECR 



3.8:1 





2.2:1 



ca 1 : 1 



1.6:1 



-setaVIFIII 



slightly 



longer 



slightly longer 



? 



longer 



Labrum, anterior face, paired patch of setules 



no 





yes 



? 



yes 



P5 exopod 













- l:w ratio 



1.9:1 





1.7:1 



cal.5:l 



1.3:1 



- outeninner seta 



equal 





slightly longer 



l/21ength 



equal 



Male 













Antenna 













- coxobasal seta 



short naked 



long, plumose 



? 



long, plumose 



- endopod 2. element IV 



strong-c 



urved 



curved 



? 



slightly curved 



1. ratio caudal seta VI:VII 



longer 





longer 



? 



equal 



GDS = genital double-somite: CR = caudal ramus; gen.ap.= genital apertures; el. = elongate; 1 = length: w = width 

 'Calculated by traditional method (i.e. telescoping of segments not considered) 



