“Oncaeidae” is the name of a family of small planktonic copepods, which are very abundant in marine pelagic habitats. Species of this family are distributed in all regions of the world ocean from tropical to polar seas and from the surface to deep-sea layers. The family includes the smallest pelagic copepod species known, many of them are smaller than half a millimeter in size as adults and the largest measure about 1.3(1.5) mm.The number of species is very high, more than one hundred species have been described to date. Due to the difficulties in sampling and examining these small individuals, many undescribed morphospecies are still awaiting description.
The role of the small oncaeid copepods in the marine ecosystem is far from being understood. They differ from other pelagic copepod taxa (such as the larger calanoids or the cyclopoid family Oithonidae) by a more pseudopelagic lifestyle, often being associated with gelatinous organisms such as salps or appendicularians upon which they also feed. Otherwise they have also been observed attacking organisms much larger than themselves, such as chaetognaths or larger copepods. The structure of their mouthparts denotes that they are not filter-feeders, but their feeding habits appear to be quite opportunistic with prey ranging from particle-loaded mucus and small algae to other crustaceans. The copepods themselves are known as food for fish larvae and mesopelagic lanternfishes (Myctophidae).
The reproduction capacity and the mortality of oncaeid species in the ocean are still largely unknown. Female Oncaeidae carry their eggs in sacs (not free spawning) and the number of eggs per sac depends on the size of the species, it ranges from up to 50 eggs per sac in large species to very few or even only one single egg per sac in small species. The reproductive capacity thus seems to be rather low for the smallest species, but the egg production rate has not yet been estimated. The surprisingly high abundance of these species may be related to low mortality and correspondingly long life span; at least some deep-living oncaeids seem to be able to endure very long starving periods, but natural mortality rates are not known. Thus, the basis for the high numerical abundance of small oncaeid species has still to be discovered.
In order to improve our knowledge about the biology, the life strategy and the distributional aspects (zoogeographical and depth distribution) of Oncaeidae in the world ocean, a correct species identification is indispensable. The existing identification keys for the family in the printed literature as well as the few available online keys are still insufficient, however, because they (1) are restricted to certain regions (e.g. Antarctic, Mediterranean Sea), (2) mostly do not include the smaller species less than 0.5 mm in size, and (3) are erroneous or outdated. A special warning should be given with regard to the book on non-calanoid marine copepods in "Fauna Ibérica" by Vives and Shmeleva 2010 (*see below). In the chapter “Familia Oncaeidae Giesbrecht, 1893” numerous figures are presented, which are inadequately redrawn from the original descriptions (often not considering improvements and corrections by subsequent re-descriptions of species) and include a great number of substantial errors. Unfortunately, this book chapter cannot be used as reliable source of information for the family Oncaeidae.
Over the past three decades, taxonomists have gained much more detailed information on the morphological and molecular characteristics (barcoding) of oncaeid species. The hitherto unresolved systematics of the family, i.e. the generic system, has been partly elucidated. This new information can now be used to ease the identification process for species of this family. Within a research project supported by the German Science Foundation (DFG), a computer-based identification key has been built up for all known species of the family Oncaeidae. It may help to consider this very important but largely underrepresented microcopepod family in basic and applied ecological studies on genus/species group or species level. At present the online-version 2.0 of this key is available on this web site (see menu "Identification Key for Oncaeidae") and can be used by any interested person after registration. The structure of the key allows regular updating according to any further improvements in taxonomic knowledge about Oncaeidae.
* Vives,F. and A. Shmeleva 2010. Crustacea, Copépodos marinos II. Non Calanoida. Familia Oncaeidae Giesbrecht, 1893. In: Fauna Ibérica vol. 33, pp 258-352. Ramos, M.A. et al. (Eds.). Museo Nacional de Ciencias Naturales. CSIC. Madrid