SEVEN SPECIES OF RED SEA ONCAEA 



51 



Table 3 Comparison of morphological characters of Oncaea venusta Giesbrecht from the Gulf of Naples with two forms, f. typica and f. venella, from the 

 Red Sea. 









Gulf of 



Naples 







Red Sea 



Species/form 













f. typica 





f. venella 



Female 



















Ornamentation on surface of prosome ( 



ridges 



etc) 



very strong 





strong 





present, less strong 



P2-bearing somite with dorso-posterior 



swelling 



no 







no 





yes 



Genital double-somite 



















1 : w ratio 







1.4: 1 







1.5: 1 





1.8: 1 



anterior part produced dorsal ly 







yes 







yes 





no 



sclerotization between gen.ap. 



















- form 







double-scalloped 



paired s- 



shaped 



double-scalloped 



- location 







between 



gen 



ap. 



anterior to gen.ap. 



between gen.ap. 



Caudal ramus. 1 : w ratio 







3.0: 1 







3.5: 1 





2.8: 1 



P4enp-3. ratio of DS:ODS 







1.5:1 







1.3: 1 





1.5: 1 



Male 



















Genital segment 



















1 : w ratio 







1.7: 1 







1.5: 1 





1.7: 1 



no. of pores on dorsal surface 







5 







11 





5 



Caudal ramus. 1 : w ratio 







2.2: 1 







2.5 : 1 





1.9: 1 



DS = distal spine: ODS = outer distal spine; gen.ap. = genital apertures; P2. P4 = swimming legs 2. 4; enp-3 = third endopod segment; I = length: w = width; no. = number 



a smaller length to width ratio of caudal ramus in the male ( see above 

 under 'Generic diagnosis'). The length to width ratio of the male 

 genital segment was greater in f. venella f 1 .6 : 1 ) than in f. typica ( 1 .3 

 : 1 ), which is in accordance with results from the Red Sea. The body 

 lengths of males in Tanaka's study ranged between 0.74-1.07 mm, 

 with no separation given for the two groups. 



Corral Estrada ( 1970) recorded two forms of female O. venusta 

 from the NE Atlantic, near Tenerife, which differed mainly in size 

 (1.05-1.25 mm and 0.87-0.95 mm). Small differences were also 

 found in overall body proportions, the venella form being more 

 slender. 



Ferrari (1975) recorded two size groups for both sexes of O. 

 venusta from the Gulf of Mexico, measuring 1 . 1-1 .2 mm or 0.92- 

 0.99 mm (females) and 0.76-0.86 mm or 0.57-0.63 mm (males). 

 Both groups occurred over the entire period (4 yrs) of his study, 

 without exhibiting any overlap in size. No morphological differ- 

 ences other than size were noted by the author. His illustration of the 

 male antenna of the typical form (his Fig. 6D) shows a long, plumose 

 seta on the coxobasis, not the small naked one usually found in O. 

 venusta (cf. Fig. 6H, 14G). 



Boxshall (1977b) reported both size morphs of female O. venusta 

 from the NE Atlantic, near the Cap Verde Islands, and in another 

 report gave detailed information on their vertical distribution and 

 diurnal vertical migration (Boxshall 1977a). The two forms differed 

 only in size, with a mode length of 1 . 1 3 mm for f. typica and 0.98 

 mm for f. venella. No other detectable differences were recorded. 

 The length frequency distribution of both forms (his Fig. 13) shows 

 very little overlap in size between the two groups. Males were not 

 distinguished into size groups during his study, their length ranged 

 between 0.96-1.08 mm with a mean of 1.01 mm. Both sexual 

 dimorphic characters of the male antenna (naked seta on coxobasis 

 and modified seta on lateral armature) were illustrated by Boxshall 

 (his Fig. 12b). The terminal accessory seta on the male caudal 

 ramus, however, was figured as being less than twice the length of 

 CR, whereas it is more than twice the length in O. venusta from 

 Naples and the Red Sea. In addition to the two forms known at that 

 time, Boxshall also recorded a few female specimens of a third 

 'robust form', which was more squat in general appearence and 

 differed in the length to width ratio of the caudal ramus. The length 

 of the specimens ranged between 0.88-1.4 mm, thus covering the 

 length range of both other varieties of O. venusta. The 'robust form' , 



however, seemed to have a limited distribution range, because it was 

 not discovered again in subsequent collections from other North 

 Atlantic localities (Malt 1983c). 



Recently, Itoh [in: Chihara & Murano (1997)] recorded both size 

 morphs of O. venusta from Japanese waters. His descriptions of 

 female and male habitus exhibit the same differences in overall body 

 morphology between the two morphs as recorded here for Red Sea 

 specimens. Also, Itoh's illustration of the male antenna [Plate 223, 

 Fig. 37 1 (right) c] shows the two sexual dimorphic characters typical 

 for the species. No overlap in size between the two groups was noted 

 by Itoh. females measured 1.09-1.23 mm (/ typica) or 0.86-0.94 

 mm (f. venella) and males 0.89-0.98 mm (f. typica) or 0.62-0.65 

 mm (f. venella). Specimens of female O. venusta from the Sea of 

 Japan examined during the present study, however, covered a wide 

 size range from 0.80-1.3 mm, and many specimens intermediate in 

 size (about 0.96 mm) occurred. Specimens at the upper and lower 

 end of the size range exhibited nearly all morphological characters 

 off. typica and f. venella. respectively, while specimens intermedi- 

 ate in size could be assigned to neither morph. [The same phenomenon 

 was observed in O. venusta specimens from Australian waters 

 (McKinnon material).] 



In summary, the two distinct size morphs of O. venusta. f. typica 

 and f. venella, are widespread in tropical and temperate regions of 

 the Atlantic (Corral Estrada, 1970; Ferrari. 1975; Boxshall. 1977b) 

 and the western Indian Ocean and adjacent seas, including the Red 

 Sea (Sewell, 1947; Tanaka, 1960; this report). Records of two 

 corresponding size groups from the Eastern Indian Ocean and the 

 Pacific are rare, because intermediate size forms occur, which make 

 a clear separation of the groups more difficult. This had already been 

 noted by Farran (1936), who found specimens of O. venusta from 

 the Great Barrier Reef covering a wide size range, without being 

 able to separate them into the two groups. Further studies on O. 

 venusta from Pacific localities are required in order to define their 

 morphological relationship to Atlantic and western Indian Ocean 

 populations. The present results, based on both light and SEM 

 microscopy, pointed to several morphological differences between 

 the two venusta forms, which had not been noted before. However, 

 these were not regarded as sufficient to warrant recognition of the 

 two forms as separate species. Alternative taxonomic techniques, 

 such as molecular analysis, may permit examination of any genetic 

 differentiation between the forms of this ubiquitous species. A study 



