Journal of Plankton Research Vol.23 no.9 pp.1029-1035, 2001
© Oxford University Press 2001
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The microcopepod fauna in the Gulf of Aqaba, northern Red Sea: species diversity and distribution of Oncaeidae (Poecilostomatoida)
1 Marine Zoologie (fb2), Universität Bremen (nw2), Postfach 330440, D-28334 Bremen; 2 Postal Address: Moorsehdener Weg 8, D-24211 Rastorf-Rosenfeld, Germany; 3 Alfred-Wegener-Institut Für Polar- Und Meeresforschung, Postfach 120161, D-27568 Bremerhaven, Germany
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A total of 23 species and three groups of form variants of oncaeid copepods have been identified, which were collected in small mesh net samples taken along a transect from the northernmost Red Sea (>27°N) to the Gulf of Aqaba down to a maximum depth of 800 m. Two of the species were present only in the Red Sea main basin, but were not recorded in the Gulf. By comparing the present results with previously published data from the central and southern Red Sea a first assessment of the zoogeographical distribution of Oncaeidae in the entire Red Sea has been achieved. The species diversity of Oncaeidae in the Gulf of Aqaba appears to be considerably lower compared to the southern Red Sea, where 31 oncaeid species and four groups of form variants have been found. The observed latitudinal gradient in species numbers of Oncaeidae generally corresponds to the gradients observed for various other zooplankton taxa in the Red Sea. First data on the vertical distribution of oncaeid species in the Gulf are provided.
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Introduction |
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TOPAbstractIntroductionREFERENCES |
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The Gulf of Aqaba, the northern extension of the Red Sea, is a moderately oligotrophic sea with oceanic gyre centre characteristics, in contrast to the southern Red Sea and the Gulf of Aden, which in their characters resemble productive oceanic gyre margins (Reiss and Hottinger, 1984
). Salinity increases considerably from south to north in the Red Sea, from ca. 37 psu at Bab al Mandab to more than 40 psu in the Gulf of Aqaba (Morcos, 1970). These characteristics and differences are also reflected in the plankton. With increasing distance from the Strait of Bab al Mandab at the southern entrance of the Red Sea, the species diversity of plankton taxa generally decreases to the north (Halim, 1969; Weikert, 1987). Among pelagic copepods, for instance, the number of calanoid species in the Gulf of Aqaba (about 35 species) is only about half that in the southern Red Sea, where 60 species are found (Delalo, 1966; Almeida Prado-Por, 1983). For non-calanoid copepod taxa a corresponding comparison could not be achieved so far, because studies on these taxa were restricted to the southern and central-northern Red Sea up to 25°N only (Böttger-Schnack, 1990b, 1995). However, as the decrease in species numbers between the southern and central area was much lower compared to calanoids, non-calanoid copepods seemed to be less sensitive to the extreme environmental conditions in the Red Sea (Böttger-Schnack, 1995). Correspondingly, the smaller zooplankton fraction, including a large non-calanoid component, had a generally higher relative abundance and a greater mass-specific metabolic rate in the central Red Sea as compared to the Gulf of Aden (Schneider and Lenz, 1991; Schneider et al., 1994).
The component of microcopepods (i.e. those <1 mm in length) within the marine plankton community does not only include the numerous juvenile stages of larger species of calanoid taxa, but also a high number of adults of very small species, most of which belong to non-calanoid taxa, such as the cyclopoid and harpacticoid genera Oithona, Paroithona and Microsetella or the poecilostomatoid family Oncaeidae (Judkins, 1980; Böttger, 1987; Cowles et al., 1987; Paffenhöfer, 1993 and literature cited therein; Hopcroft et al., 1998). The oncaeids, which are between 0.2 and 1.2 mm in length in the adult stage, are one of the most abundant and diversified groups among the marine microcopepod fauna (Böttger-Schnack, 1994; Kr
ini
, 1998).
The community of oncaeid copepods in the Red Sea has been investigated in great detail with regard to ecological and biological questions during the past years (Böttger-Schnack 1990a, b, 1994, 1995; Böttger-Schnack et al.1989; Ohtsuka et al.1996). More recently, a comprehensive taxonomic study of Red Sea Oncaeidae was started, first results of which have beenpublished (Böttger-Schnack and Huys, 1997; Böttger-Schnack, 1999, 2001). These studies, however, did not include the northernmost Red Sea (>25°N) and/or its northern extensions, the Gulf of Aqaba and Gulf of Suez. Thus, the species diversity of Oncaeidae in the Gulf of Aqaba is not well known. Two species, Oncaea venusta and O. dentipes, were identified in an ecological study on reef zooplankton (Vaissière and Seguin, 1984), and one species, O. media, was reported during a small mesh net study on the surface community of coastal zooplankton in the Gulf of Aqaba (Khalil and Abdel-Rahman, 1997). In a recent check-list of pelagic copepods in the Gulf, based on sampling with fine mesh nets of 0.05 mm mesh size in the upper 100 m of the water column, four species of Oncaeidae were recorded (Abdel-Rahman, 1999). In view of the great number of over 30 oncaeid species known from the Red Sea main basin so far (Böttger-Schnack, 1994, 1999), this number appears to be rather low, even when taking into account some reduction in species numbers between the main basin and the Gulf.
The present paper provides new results on the species diversity of Oncaeidae from a recent zooplankton survey in the Gulf of Aqaba and the northernmost Red Sea (>27°N) obtained as part of a more detailed ecological and taxonomic study on the microcopepod fauna in the Gulf. The entire water column has been sampled, allowing the inclusion of some initial information on the depth distribution of oncaeid species. By comparing the present results with previously published data from the central and southern parts, a first assessment of the zoogeographical distribution of Oncaeidae in the entire Red Sea has been achieved.
Zooplankton samples were collected during cruise 44/2 of RV ‘Meteor’ between March 2 and March 4, 1999, at four stations along a transect from the northernmost Red Sea (>27°N) to the Gulf of Aqaba (Figure 1). A multiple opening-closing net (Weikert and John, 1981) with a mouth area of 0.25 m equipped with five nets of 0.055-mm-mesh size was used to sample consecutive depth strata at vertical hauls down to a maximum depth of 800 m. Samples were taken mostly at 50 m intervals between 0 and 450 m and at 50–150 m intervals between 450 and 800 m. More detailed sampling data and first results of the environmental conditions during the sampling period are provided in a cruise report of RV ‘Meteor’ 44/2 (Pätzold et al.2000).
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The plankton was initially fixed in 4% formaldehyde–seawater solution buffered with borax. For sorting, the samples were transferred into a preservation fluid of 5% propylene glycol, 0.5% propylene phenoxetol, and 94.5% filtered sea water (Steedman, 1976
); they were then transferred back into a 4% formaldehyde–seawater solution. Entire plankton samples were scanned for the various oncaeid species and several adult specimens of each species were sorted out for reference. Individuals were stored in Steedman's preservative fluid, to which a small amount of a concentrated (40%) formaldehyde solution was added, resulting in a final concentration of ca. 2–4% formaldehyde. Species identification was conducted using the taxonomic literature given by Böttger-Schnack (Böttger-Schnack, 1990a) and the morphological descriptions of Red Sea Oncaeidae provided by Böttger-Schnack (Böttger-Schnack, 1999, 2001) as well as unpublished data. For selected species, a more detailed taxonomical analysis was carried out to confirm the identification (e.g. Triconia recta). Adult specimens were dissected in lactic acid, mounted on slides in lactophenol, and sealed with high-quality nail varnish.
The species diversity of Oncaeidae in the northernmost Red Sea and the Gulf of Aqaba is listed in Table I. In total, 23 species and three groups of form variants were identified, which belong to eight different species groups within the Oncaeidae as defined by Böttger-Schnack and Huys (Böttger-Schnack and Huys, 1998, 2001). The total number of oncaeid species in the Gulf of Aqaba was somewhat lower as compared to the reference station in the northernmost Red Sea (Table I). Two species, Triconia elongata and Oncaea ivlevi 2, were not found in the Gulf. Their sibling species T. dentipes and O. ivlevi 1, however, were distributed throughout the entire Gulf region. No differences in oncaeid species numbers were observed in the Gulf along the south–north transect. This corresponds to the species composition of calanoid copepods, which also appears to be fairly uniform in the Gulf region (Almeida Prado-Por, 1985).
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Oncaea venusta Philippi was represented in the Gulf of Aqaba by its smaller form, f. venella Farran, whereas the larger form, f. typica, was not found. This result confirms the considerable difference in geographical distribution observed for the two size variants of O. venusta in the Red Sea (Böttger-Schnack, 1990b
), which may represent different populations in this area. An application of alternative taxonomic methods (DNA analyses; A. Bucklin and D. Elvers, unpublished results) is currently in progress, which might help to differentiate the two venusta forms genetically. The occurrence of Triconia recta Böttger-Schnack in the Gulf of Aqaba is noteworthy, because the species was previously assumed to be restricted to the southern Red Sea (Böttger-Schnack, 1994, 1999). The identity of T. recta in the Gulf was confirmed by a detailed taxonomic analysis, including the description of the hitherto unknown male (Böttger-Schnack, 2000). Upon re-examination of earlier fine-mesh net samples from the central and central-northern Red Sea collected during different seasons, the occurrence of T. recta in the transition zones was verified, but the data indicated strong seasonal variations in abundance at these areas (Böttger-Schnack, 2000).
The actual number of 21 species of Oncaeidae in the Gulf of Aqaba is much higher than that previously recorded. In a recent study of the copepod species diversity in the Gulf of Aqaba, based on fine mesh net samples, only four species of oncaeids were listed (Abdel-Rahman, 1999). The author's records of Triconia conifera, T. dentipes, Oncaea venusta and O. media in the Gulf were confirmed by the present study, but the body length of T. dentipes reported in his study (1.3 mm) seems to be erroneous, as the adults of this species measure only between 0.4 mm (male) and 0.5 mm (female) (Böttger-Schnack, 1999). Oncaea media belongs to a complex of very similar species, the morphological differences of which were summarized by Böttger-Schnack (Böttger-Schnack, 2001). Two species of the media-complex, O. media and O. scottodicarloi, were identified in the Gulf of Aqaba during the present study. The latter species was found to be extremely abundant in the northernmost part of the Gulf (unpublished results). The third species of the media-complex, O. waldemari, was not found in the Gulf of Aqaba and the northernmost Red Sea, thereby confirming its restriction to the southern Red Sea (Table I
).
Compared to the southern Red Sea, the species diversity of Oncaeidae in the Gulf of Aqaba appears to be considerably lower. Ten of the 31 oncaeid species and one of the four groups of form variants, so far known from the southern area, were not recorded from the Gulf (Table I). The number of species in the southern Red Sea might further increase as soon as the taxonomic status of the numerous form variants of the atlantica-group is clarified (>14 forms, cf. Table I), as most of them seem to be confined to this area. Thus, the latitudinal gradient in species numbers of oncaeids generally corresponds to the gradient observed for various other zooplankton taxa, such as chaetognaths (J.-P. Casanova, 1985, 1990a), thecosomatous pteropods (Rampal, 1988, 1990) and euphausiids (B. Casanova, 1990); see also (Halim, 1969; Weikert, 1987). For calanoid copepods, however, the regional decrease in species numbers between the southern Red Sea and the Gulf of Aqaba, from 60 species to about 35 species (Delalo, 1966; Almeida Prado-Por, 1983), is much more pronounced as compared to Oncaeidae.
First data on the overall depth distributions of oncaeid species in the Gulf of Aqaba and the northernmost Red Sea are summarized in Table II. In general, most oncaeids were found in the same habitat zones as in the central and southern Red Sea (Böttger-Schnack, 1988, 1990a, Böttger-Schnack, b, 1995). Within the range of their habitat preferences, however, the vertical distribution patterns of species differed between the Gulf of Aqaba and the main basin of the Red Sea. This difference was most conspicuous for several epipelagic species, which showed a considerable downward extension of their vertical depth range in the Gulf. The epipelagic Oncaea venusta f. venella, for instance, which was confined mainly to the upper 100 m in the Red Sea main basin (Böttger-Schnack, 1990a, b), occurred in fairly large numbers down to about 300 m depth in the Gulf. A different vertical distribution was also observed for female O. scottodicarloi, which had a uniform depth distribution in the upper 300 m in the Gulf of Aqaba, whereas they exhibited a conspicuous bimodal vertical distribution in the epi- and upper mesopelagic zone (within the oxygen minimum zone) in the central Red Sea [(Böttger-Schnack, 1990a, b), as Oncaea media f. minor]. The observed differences in vertical distribution of oncaeid species between the Red Sea main basin and the Gulf correspond well with differences in the environmental conditions. In the Gulf of Aqaba, the upper water column was nearly homogeneous and well oxygenated from the surface down to 350 m depth (Pätzold et al.2000). In contrast, in the main basin of the Red Sea the depth of the mixed layer varies between 50 m and 150 m according to the season, and a strong oxygen minimum zone is situated between 200 and 450–600 m (Pätzold et al.2000), which has been shown to greatly influence the vertical distribution of zooplankton taxa in this area (Weikert, 1980, 1982; Böttger-Schnack, 1990b).
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In conclusion, our results on the biodiversity of oncaeid copepods in the Gulf of Aqaba and the northernmost Red Sea show a considerable reduction in species numbers as compared to the southern Red Sea. This can be explained by the more extreme environmental conditions prevailing in the North. In the Gulf of Aqaba the diversity of oncaeids is generally similar to the adjacent northernmost Red Sea main basin, showing little further reduction in species numbers. Preliminary data on the vertical distribution of oncaeid species, however, indicate considerable differences between the Red Sea main basin and the Gulf, which can be related to deviating hydrographic conditions. A more detailed investigation of the vertical distribution of the microcopepod fauna in the Gulf of Aqaba and northernmost Red Sea is currently in progress. It focuses on small oncaeid species, which comprise one of the most important components of the microcopepod community in these areas.
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Acknowledgments |
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We wish to thank Professor G. Hempel, the initiator and chief scientist of RV ‘Meteor’ cruise 44/2, for his encouragement and support. Sincere thanks are given to Professor D. Schnack for continuous help during the study and to Dr R. Huys for fundamental taxonomic advice. The constructive comments of the referees helped to improve the manuscript. This study was supported by Deutsche Forschungsgemeinschaft grant Ha 1706/5 to W. Hagen.
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Received on July 1, 2000
; accepted on April 9, 2001
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