146 



R. BOTTGER-SCHNACK AND R. HUYS 



Many more records of O. mediterranea from different locali- 

 ties in the world ocean are known [see Malt (1983a) for a 

 review], but are not considered here, because they did not 

 include figures or a description that positively identified the 

 species. 



FORM VARIANTS 



In the eastern Mediterranean, O. mediterranea exhibits two 

 forms in both sexes: a smaller and more slender form and a 

 larger, more robust form. These forms differ only in the general 

 appearance and in the length : width ratio of the caudal ramus in 

 the females. The difference in general habitus between the two is 

 caused by the greater width of the leg 2-bearing and leg 3- 

 bearing somites in the larger form (Fig. 5A). In the females, the 

 length to width ratio of the caudal rami is larger in the broad form 

 (4:1) (Fig. 5C) as compared to the small form (3:1, cf. Fig. IF), 

 whereas in the males no corresponding difference could be 

 found. No other morphological differences were discernible 

 between the two morphs. The small form of O. mediterranea 

 corresponds to the small variant described by Malt (1983a), 

 while the large form seems to be similar to her 'typical' 

 mediterranea form. Only one size morph of O. mediterranea 

 was found in the Red Sea, which can be identified as the small 

 form found in the eastern Mediterranean and on the basis of the 

 length to width ratio of caudal rami seems to resemble the small 

 form of O. mediterranea recognized by Malt (1983b). A small 

 colourless form of O. mediterranea (in addition to the larger 

 'orange-golden' form) was found south of New Zealand by 

 Farran ( 1 929) but no further description was provided. 



Ferrari (1975) reported the existence of three distinct size 

 groups in O. mediterranea of the Gulf of Mexico. The two larger 

 forms differed only in size, but had a similar length to width ratio 

 of the caudal ramus of 4:1. On the basis of their morphometry 

 they might be assigned to the typical form of O. mediterranea 

 {sensu Malt). The smaller size group had a length to width ratio 

 of the caudal ramus of 3.0-3.5: 1 and might thus be similar to the 

 small form sensu Malt and to that found in the Red Sea. The 

 smaller morph was initially regarded as a potential new species 

 (Ferrari, 1973) since it differed both in the proportional lengths 

 of body somites as well as in total body size. Subsequently, 

 Ferrari (1974), quoting O. conifera Giesbrecht, 1891 as an 

 exemplar of gross relative size variation in oncaeids, considered 

 it as merely another size group of O. mediterranea since not 

 structural differences could be detected in the appendages. The 

 recent outstanding work on the com/era-complex by Heron and 

 Bradford-Grieve (1995) has amply demonstrated that much of 

 the 'variation' in O. conifera can be explained by the fact that 

 morphologically similar species can co-exist and that this mor- 

 phological similarity can even cause anomalies in the mate 

 guarding configurations of certain species. It is conceivable that 

 many of the form variants represent genuine species which can 

 no longer be discriminated using traditional oc-taxonomical 

 methods. For example. Malt (1983c) mapped the pore signature 

 pattern of the two female forms of O. mediterranea from the 

 North Atlantic but failed to reveal any significant difference 

 between them. It seems therefore that the question whether the 

 large morph found in the Eastern Mediterranean represents a 

 sibling species of O. mediterranea can perhaps only be resolved 

 by breeding experiments or alternative methods using molecular 

 data such as enzyme electrophoresis, immunological distance 

 methods or ribosomal RNA sequencing. 



ECOLOGICAL NOTES 



Geographical distribution. 



O. mediterranea is distributed throughout the Red Sea (see review in 

 Halim (1969); Bottger-Schnack, 1990b, 1995). It was also found in 

 small mesh net samples from the northernmost part of the Gulf of 

 Aqaba (Bottger-Schnack, unpubl.). 



Vertical distribution and vertical migration. 



During summer and autumn, when a strong seasonal thermocline is 

 developed, the core of the O. mediterranea population during the 

 day is situated in the zone below the thermocline, at 50-150 m 

 (Bottger-Schnack, 1990a, unpubl.). Parts of the population migrate 

 into the upper 50 m during the night, with males showing a greater 

 proportion of migrating individuals than females. For females, a 

 bimodal vertical distribution can be found during these seasons, with 

 the lower part of the population dwelling in the 250-400 m layer, in 

 the core of the oxygen minimum zone. In the northern Red Sea the 

 mesopelagic populations of females are found at greater depths than 

 in the central area, corresponding to the regional differences in the 

 depth of minimal oxygen concentrations (Bottger-Schnack, 1990b). 

 These deep dwelling populations are not found during winter 

 (Bottger-Schnack, 1990b). 



Seasonal variation in abundance (central Red Sea). 



No consistent seasonal variation in abundance was found for O. 

 mediterranea in the central Red Sea, indicating that the populations 

 are not substantially recruited by those from the south (Bottger- 

 Schnack, 1995). 



Acknowledgements. We wish to thank Prof. J. Lenz, Prof. D. Schnack 

 and Dr H. Weikert for support and help. Dr G. A. Boxshall (NHM) and Dr F.D. 

 Ferrari (Smithsonian Institution) reviewed earlier drafts of the manuscript. 

 This study was supported by a Deutsche Forschungsgemeinschaft grant Le 

 232/18-1. 



REFERENCES 



Alldredge, A.L. 1972. Abandoned larvacean houses: Unique food source in the pelagic 



environment. Science, 177: 885-887. 

 Bottger-Schnack, R. 1988. Observations on the taxonomic composition and vertical 



distribution of cyclopoid copepods in the central Red Sea. Hydrobiologia, 167/168: 



311-318. 



— 1990a. Community structure and vertical distribution of cyclopoid copepods in the 

 Red Sea. I. Central Red Sea, autumn 1980. Marine Biology, 106:, 473^t85. 



— 1990b. Community structure and vertical distribution of cyclopoid copepods in the 

 Red Sea. II. Aspects of seasonal and regional differences. Marine Biology, 106: 487- 

 501. 



— 1992. Community structure and vertical distribution of cyclopoid and 

 poecilostomatoid copepods in the Red Sea. III. Re-evaluation for separating a new 

 species of Oncaea. Marine Ecology Progress Series, 80: 301-304. 



— 1995. Summer distribution of micro- and small mesozooplankton in the Red Sea and 

 Gulf of Aden, with special reference to non-calanoid copepods. Marine Ecology 

 Progress Series, 118: 81-102. 



— 1997. Vertical structure of small metazoan plankton, especially non-calanoid 

 copepods. II. Deep Eastern Mediterranean (Levantine Sea). Oceanologica Acta, 20: 

 399^119. 



— Schnack, D. & Weikert, H. 1989. Biological observations on small cyclopoid 

 copepods in the Red Sea. Journal of Plankton Research, 11: 1089-1 101. 



Bourne, G.C. 1889. Report on the pelagic Copepoda collected at Plymouth in 1888— 

 89. Journal of the Marine Biological Association of the United Kingdom, new series, 

 1: 144-151. 



Boxshall, G.A. 1977a. The planktonic copepods of the northeastern Atlantic Ocean: 



