Order ONYCHOPODA G.O. Sars, 1865

Museums

Regional Maps

• Onychopoda Sars, G.O. 1865. Norges ferskvandskrebsdyr. Første Afsnit. Branchiopoda. 1. Cladocera Ctenopoda (Fam. Sididae & Holopedidae) [Norway's freshwater crustaceans. First part. Branchiopoda. 1. Cladocera Ctenopoda (Fam. Sididae & Holopedidae)]. Christiania : Brøgger & Christie's Bogtrykker pp. i-viii, 1-71, pls 1-4. [https://wellcomecollection.org/works/s4qp43x7] [vii].

Introduction

Onychopodans live in fresh , brackish and sea-water. They are of worldwide occurrence but the freshwater species are restricted to the temperate zone of the Holarctic. Currently several Caspian species are invading the brackish and fresh waters of Europe and North America. The Onychopoda are monophyletic and supported by at least two convincing synapomorphies – four trunk limbs and marginal vesicles in the spermatozoa (Olesen, 1998).

Diagnosis

Body short. Head relatively large. Thorax short, with segmentation indistinct by fusion. Abdomen short, with segmentation indistinct, or long, with segmentation distinct, terminating - in short-bodied forms - in a simple furca on the postabdomen (which is usually small), or in long-bodied forms in a rather long caudal process. Carapace in females reduced to a dorsal brood pouch. Eye single, internal, very large, occupying most of the head, with many lenses. Ocellus lost. Labrum large. Antennules tubular, of 1 segment, more or less long. Antennae biramous, natatory; exopod of 4 segments, endopod of 3 segments, armed with 12–15 long natatory setae. Mandibles of a modified grinding type, adapted for biting. Maxillules much reduced, maxillae absent. Four pairs of cylindrical, prehensile thoracic limbs, without branchial epipodites but with exopodites (lost in Cercopagidae) and gnathobases. First trunk limbs of male with copulatory hook. No true food groove. Alimentary canal straight, generally without caeca (rarely with 2 very small anterior caeca). Ventral nerve ganglia largely fused. Ovaries and testes paired. Paired vasa deferentia opening between legs four and anus on a mound, or else paired, penes present. Resting eggs, usually two or more, carried temporarily in a brood pouch before being shed freely. Parthenogenetic eggs nourished in brood pouch by a Nährboden. No larval stages; all eggs develop into replicas of the adults. Some Caspian Apargis and some marine Evadne and Pleopis are probably obligate parthenogens. Otherwise parthenogenesis and gamogenesis alternating. Body length 0.26–6.6 mm in ovigerous females, 0.32–5.0 mm in males (Pleopis the smallest, Bythotrephes the largest. In Cercopagidae, the caudal process may be 2–4 times as long as the body in Bythotrephes against 1.5–2.5 times in Apagis, and 5–10 times in Cercopagis.