View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by OceanRep Marine Biology (2004) 144: 1127–1140 DOI 10.1007/s00227-003-1272-8 RESEARCH ARTICLE R. Bo¨ttger-Schnack Æ J. Lenz Æ H. Weikert Are taxonomic details of relevance to ecologists? An example from oncaeid microcopepods of the Red Sea Received: 31 March 2003 / Accepted: 18 November 2003 / Published online: 20 January 2004 Ó Springer-Verlag 2004 Abstract The marine microcopepod family Oncaeidae in abundance and vertical distribution of selected oncaeid the Red Sea has been the subject of comprehensive siblings obtained during a recent cruise in the northern ecological studies over the past 15 years, providing for Red Sea are provided to exemplify the changes in the the first time insights into their community structure, knowledge of oncaeid community structure attributable vertical distribution and feeding ecology. Owing to to the improved taxonomic resolution. The potential taxonomic problems in species identification, however, ecological importance of a more differentiated consid- many of the earlier ecological results were based on eration of oncaeid species in marine microcopepod provisionally named species or morphotypes. A recent, communities is discussed. ongoing taxonomic study of Red Sea Oncaeidae resulted in a considerable increase in the estimated numbers of species, since many of the species had not been described before. The present paper focuses on the potential sig- Introduction nificance of an improved taxonomic resolution of on- caeids with respect to various ecological aspects in this The classification of individuals into groups (species, area, such as indicator species, community analysis and form variants) with ecologically uniform reactions is a vertical distribution. The progress in our knowledge of fundamental requirement for the analysis of structures the diversity of Red Sea Oncaeidae is summarized, and functions of marine plankton communities and their including latest findings on the taxonomy and zooge- variation in space and time. Among marine planktonic ography of very small species (<0.5 mm), and the copepods, the taxonomy of larger species in the size importance of sibling species in the family is pointed out. range 1–3 mm (mostly calanoids) is much better known The south–north gradient in species diversity of On- than that of microcopepod species <1 mm in body caeidae within the Red Sea appears to be greater than length, most of which belong to non-calanoid taxa. Even previously assumed, since several of the newly described though an increasing consideration of small-sized species were restricted to the southern part. The number metazoans in marine zooplankton studies has led to the of endemic species among Red Sea oncaeids is very low, recognition of microcopepods as an important fraction however, most of the new species being also recorded of the plankton community (e.g. LeBrasseur and outside the Red Sea. New quantitative data on the Kennedy 1972; Judkins 1980; Cowles et al. 1987; Hopcroft et al. 2001), information on their community structure and vertical distribution is still limited (e.g. Communicated by O. Kinne, Oldendorf/Luhe Bo¨ ttger-Schnack 1990a, 1990b, 1995; Hopcroft et al. R. Bo¨ ttger-Schnack (&) 1998; Krsˇ inic´ 1998) and the few available biological data Moorsehdener Weg 8, 24211 Rastorf-Rosenfeld, (e.g. on grazing) are often based on higher taxonomic Germany categories, such as genera or even families (e.g. Arinardi E-mail: [email protected] et al. 1990; Sutton et al. 2001). J. Lenz The ecosystem of the Red Sea has been studied Institut fu¨ r Meereskunde an der Universita¨ t Kiel, intensively over the past 25 years (Head 1987; Beck- Du¨ sternbrooker Weg 20, 24105 Kiel, Germany mann 1996, and literature cited therein), providing fundamental insights into the vertical structure (e.g. H. Weikert Weikert 1982, 1987; Beckmann 1984, 1996; Bottger Institut fu¨ r Hydrobiologie und Fischereiwissenschaft, ¨ Universita¨ t Hamburg, Zeiseweg 9, 22765 Hamburg, 1987; Bo¨ ttger-Schnack 1994) and function (e.g. Lenz Germany et al. 1988; Schneider et al. 1991) of the pelagic 1128 communities. This unique environment is characterized abundant taxa, such as the conifera group of oncaeids, by constant and exceptionally high deep-sea tempera- which presently contains 11 species differing only in very tures of 21.7°C and salinities of 40.5 PSU between slight morphological details (Heron 1977; Heron and 200 m and the near-bottom layer at 2000 m depth Bradford-Grieve 1995; Heron and Frost 2000). The (Morcos 1970; Edwards 1987). Within the oceanic zoo- taxonomy of smaller oncaeid species (<0.5 mm body plankton community, small-sized organisms (size-frac- length) was insufficiently known until recently. Species tion 100–300 lm) are of particular importance, of this size spectrum have been found to represent one of exhibiting greater relative abundances and a higher the most abundant groups in the oceanic microcopepod respiration rate in the central Red Sea than in the communities of low latitudes (Bo¨ ttger-Schnack 1994, adjacent Gulf of Aden (Schneider and Lenz 1991; 1995; Krsˇ inic´ 1998). Schneider et al. 1994). Among the small metazoan To overcome the difficulties in species identification plankton of the open Red Sea, microcopepods of the and resulting problems in the interpretation of ecolog- poecilostomatoid family Oncaeidae, measuring between ical data on oncaeid copepods in the Red Sea, a de- 0.2 and 1.2 mm long as adults, represent one of the most tailed taxonomic study was started some years ago, abundant and diversified taxa in the lower epipelagic focusing on 26 Oncaea species and morphotypes known and the meso- and bathypelagic zones (Bo¨ ttger-Schnack at that time (Bo¨ ttger-Schnack 1994). The taxonomic 1990a, 1990b, 1994, 1995). results of larger and medium-sized Red Sea oncaeids The ecological data on the diversity and vertical (Bo¨ ttger-Schnack 1999, 2001; Bo¨ ttger-Schnack and distribution of oncaeid species in the past from the Red Huys 1997, 2001) and first data on the taxonomy of Sea included several uncertainties attributable to diffi- smaller species <0.5 mm in body length (Bo¨ ttger-Sch- culties in species identification. Some oncaeid species, nack 2002, 2003) were published and further studies are for instance, exhibited a conspicuous bimodal vertical in progress. Within the frame of a phylogenetic study, distribution, with peak abundances in the epi- and the diphyletic status of the family Oncaeidae sensu lato upper mesopelagic zone, not found in other Red Sea was recognized by Huys and Bo¨ ttger-Schnack (1996), zooplankton species (Bo¨ ttger-Schnack 1990a, 1990b). It who proposed the new family Lubbockiidae to remained uncertain, however, whether this pattern accommodate Lubbockia Claus, 1862 and related gen- could be related to vertical differences in hydrographic era, retaining only Oncaea Philippi, 1843, Conaea conditions, e.g. oxygen gradients, or reflect the different Giesbrecht, 1891 and Epicalymma Heron, 1977 in the depth distributions of two closely related species Oncaeidae. The large type genus Oncaea sensu lato was (Bo¨ ttger-Schnack 1990a). Another example was the recognized as a paraphyletic assemblage (Huys and latitudinal decrease in species numbers in the Red Sea, Bo¨ ttger-Schnack 1996) and preliminary results of an which was found to be much smaller for non-calanoids, ongoing phylogenetic study of the Oncaeidae sensu especially Oncaeidae (Bo¨ ttger-Schnack 1995), than had stricto at the species level identified about 20 mono- been reported for other zooplankton taxa (Halim 1969; phyletic lineages, each of which may deserve a generic Weikert 1987; Beckmann 1996). It was hypothesized status (Bo¨ ttger-Schnack and Huys 1998, 2001). In that oncaeid copepods may be less sensitive to the addition to the three genera retained in the revised strong horizontal gradients in environmental conditions family Oncaeidae s.str., four new oncaeid genera have (especially the increasing salinity) in the Red Sea and been established to date: Archioncaea Bo¨ ttger-Schnack do not follow the latitudinal trend of a reduced species & Huys, 1997, Triconia Bo¨ ttger-Schnack, 1999, Mono- richness from south to north in this area. However, thula Bo¨ ttger-Schnack & Huys, 2001, and Spinoncaea within the Oncaea species a number of form or size Bo¨ ttger-Schnack, 2003. variants were found in the southern parts, which The present paper focuses on the potential sig- seemed to be rare or absent in the central area (Bo¨ ttger- nificance of an improved taxonomic resolution of on- Schnack 1995). Since most of these variants have not caeids with respect to various ecological aspects of the yet been described in the literature, their taxonomic Red Sea, such as indicator species, community analysis status had to be ascertained. and vertical distribution. The progress in our knowledge Species of Oncaeidae are difficult to identify, owing to of the diversity of Red Sea Oncaeidae is summarized, the small size of the species, a very high species diversity, including latest findings on the taxonomy and zoogeo- and the lack of fundamental taxonomic knowledge. The graphy of the smaller species (<0.5 mm) many of which morphology of medium-sized and larger oncaeid species have been defined as new sibling species. New data on between 0.5 and 1.0 mm in length has been studied in the abundance and vertical distribution of selected on- more detail during the past two decades (e.g. Heron caeid siblings obtained during a recent cruise in the 1977; Heron et al. 1984; Boxshall and Bo¨ ttger 1987; northernmost Red Sea and the Gulf of Aqaba are pre- Heron and Bradford-Grieve 1995; Heron and Frost sented to exemplify the changes in the knowledge of 2000), and many new species have been described. The oncaeid community structure attributable to the im- number of known species has risen from about 70 (Malt proved taxonomic resolution.