50 



R. BOTTGER-SCHNACK 



Specimens of O. venusta from the Red Sea exhibited a corre- 

 sponding difference in size (Table 2) with no overlap between the 

 two groups. No distinct regional or seasonal differences in the 

 lengths of the two forms were observed, although specimens of f. 

 venella in the central Red Sea tended to be smaller in summer as 

 compared to autumn and winter (Table 2). Generally, Red Sea 

 specimens off. venella are smaller than elsewhere, which might be 

 related to the extreme environmental conditions in this basin (Bottger- 

 Schnack et al. , 1 989). Sizes off. typica, on the other hand, correspond 

 well to those reported from other regions (Bottger-Schnack et al., 

 1989). A possible explanation for this may be that the large morph 

 penetrates the Red Sea during a limited period only (see below under 

 Ecological notes) and does not survive as an indigenous population 

 in this area. 



Alternative explanation could be that smaller individuals of f. 

 venella may not have been collected during earlier investigations, 

 because nets with fairly large mesh sizes of about 0.3 mm were used 

 (e.g Boxshall, 1977b). However, this would not explain the absence 

 of intermediate sizes, measuring between 0.9-1 .0 mm ( 9) and 0.75- 

 0.85 mm (6*) in length, in O. venusta from the Red Sea. 



Morphological differences other than size between the two forms 

 of O. venusta females from the Red Sea were similar to those 

 recorded by Farran (1929) and Sewell (1947). Additional differen- 

 ces in body morphology found in the present study include (1) the 

 length to width ratio of female genital double-somite, which is 

 smaller in f. typica (1.5:1) than in f. venella (1.8:1), (2) the small 

 dorso-posterior protrusion of the P2-bearing somite in female f. 

 venella, which is not found in f. typica, (3) small differences in the 

 relative lengths of distal spines on P4 endopod, which are likewise 

 found in both sexes, and (4) the length to width ratio of male genital 

 somite, which is smaller in f. typica (1.5:1) than in f. venella (1.7: 

 1 ). The latter two characters are the only differences other than size, 

 by which males of the two forms can be separated. The dorsoposterior 

 swelling on the P2-bearing somite in f. venella was consistent for all 

 specimens from the Red Sea and was also found in f. venella from 

 the northern Arabian Sea. Altogether, the results pointed to several 

 additional morphological differences between the two forms, which 

 had not been noted in the literature before, but these were not 



Table 2 Body length (mm) of O. venusta in the Red Sea. 



Form variant 



Sex 



n 



X 



R 



AUTUMN 



Northern Red Sea 









f. venella 



F 



15 



0.80 



0.75-0.85 





M 



6 



0.58 



0.57-0.59 



Central Red Sea 









f. venella 



F 



24 



0.80 



0.75-0.85 





M 



4 



0.61 



0.57-0.73 



WINTER 



Central Red Sea 









f. venella 



F 



12 



0.80 



0.75-0.88 





M 



16 



0.59 



0.55-0.65 



f. typica 



F 



14 



1.11 



1.00-1.23 





M 



7 



0.92 



0.88-0.95 



SUMMER 



Central Red Sea 









f. venella 



F 



M 



10 



1 



0.76 

 0.56 



0.70-0.80 



Gulf of Aden + Strait of Bab al Mandab 







f. venella 



F 



14 



0.83 



0.74-0.92 





M 



3 



0.60 



0.58-0.63 



f. typica 



F 



5 



1.15 



1.10-1.22 



n = no. of individuals measured; X = mean; R = range 



regarded as sufficient to warrant distinction of the two forms as 

 separate species. 



Identification of O. venusta f. typica males during routine counts 

 in plankton samples is facilitated by their great size and overall 

 robust appearance. Males of f. venella, on the other hand, are very 

 similar to males of O. clevei, which are described in the present 

 account for the first time (see below). 



Comparison of o. venusta form variants with specimens 

 from the mediterranean 



Total body length of specimens from Naples recorded by Giesbrecht 

 (1892) and Heron & Bradford-Grieve (1995) ranged between 1.09- 

 1.27 mm for females, and 0.8-0.95 mm for males. This size range 

 corresponds to that of O. venusta f . typica and most previous authors 

 have regarded the mediterranean specimens as being conspecific 

 with the typical form (e.g. Sewell, 1947; Tanaka, 1960). A compa- 

 rison of morphological characters other than size between the two 

 form variants from the Red Sea and specimens from the Gulf of 

 Naples are summarized in Table 2. The Mediterranean specimens in 

 fact seem to be more similar to the typical form than to the venella 

 form. However, they also share some characters with f. venella, such 

 as the form of the sclerotized structure between female genital 

 apertures and the pore pattern of the male urosome. Two morpho- 

 logical characters of the Naples specimens were intermediate between 

 the two Red Sea forms, the length to width ratio of the caudal ramus 

 in both sexes and the length to width ratio of the genital somite in the 

 male (Table 2). Based on these observations, it cannot be confirmed, 

 which of the two form variants from the Red Sea is conspecific with 

 O. venusta Giesbrecht sensu Heron & Bradford-Grieve. The length 

 to width ratio of caudal ramus has been found to be very variable 

 among specimens of O. venusta (Boxshall, 1977b) and this might 

 also apply to the pore pattern. Malt (1983c) investigated the 

 integumental pore patterns of females of the two venusta forms, 

 based on material collected in the Atlantic, however, did not find any 

 significant differences between them and/or the third 'robust' form 

 variety. Males were not investigated during her study. 



Other records of o. venusta form variants 

 The geographical distribution of the size morphs of O. venusta is 

 poorly documented (Malt. 1983a). Farran (1936) reported a great 

 size variation in specimens from the Great Barrier Reef, but could 

 not distinguish the two form variants, which he previously had 

 described from the temperate and tropical Atlantic and off New 

 Zealand, because many specimens intermediate in size occurred. 



Sewell (1947) recorded two variants of O. venusta from the 

 northern Arabian Sea (discussed above), and concluded that they 

 might have slightly different breeding seasons, since both exhibited 

 different proportions of ovigerous females and of females bearing 

 spermatophores in the samples. He summarized the geographical 

 distribution of O. venusta known at that time and concluded that the 

 smaller f. venella form was absent in the Mediterranean. However, 

 both forms of O. venusta were recorded from Lebanese waters by 

 Malt et al. (1989) without further descriptive details. In the quanti- 

 tative study of Bottger-Schnack (1996) conducted in the Eastern 

 Mediterranean, O. venusta was totally absent. 



Tanaka (1960) recorded two size groups (1.13-1.39 mm and 

 0.90-1.0 mm, respectively) among female O. venusta from the 

 Indian Ocean and off Cape of Good Hope, as well as from Antarctic 

 waters; specimens from Japanese waters (South China Sea) be- 

 longed to the typical form only. He did not find any structural 

 differences between the two forms, except for a somewhat more 

 slender prosome in f. venella as compared to f. typica. The caudal 

 rami were four times longer than wide in both forms, irrespective of 

 sex; this is unusual for species ot'Oncaea s. str., which typically have 



