Alona rectangula, Sars. We, 1861

Damme, Kay Van & Dumont, Henri J., 2008, Further division of Alona Baird, 1843: separation and position of Coronatella Dybowski & Grochowski and Ovalona gen. n. (Crustacea: Cladocera), Zootaxa 1960 (1), pp. 1-44 : 28

Allocation of A. rectangula and related species to Coronatella Dybowski and Grochowski

The two African taxa studied in this paper are related to European Alona rectangula , but species in their own right (see Table 1 for differences). We can now list synapomorphies for limbs of C. rectangula and related species (former Alona rectangula- group; Table 2): 1. First limb (P1) with dorsal seta on first endite, a reduction of the anterior elements (setae present in A. quadrangularis- or A. affinis -groups), an IDL with two setae (third one strongly reduced), 2. exII seta well developed (not C. holdeni ), with specialisation of scrapers 5–8, no additional setae on second limb (P2), 3. exopodite of P3 with six setae (vs. seven in Alona sensu stricto) of which the third longest, 4. exopodite of P5 with two long dorsal setae, a peculiar elongated element behind the second endite seta (also in Armatalona ; Sinev, 2004b) gnV reduced (0–1 seta) and 5. P6 absent. External synapomorphic features include a ventral notch on the posteroventral corner of the valves, a short, round naked labrum; three main head pores; unmerged marginal denticles and medium to long basal spine on a postabdomen with S-shaped dorsal margin.

This is clearly distinct from “ true ” Alona , focused around the Alona quadrangularis -group (Van Damme & Dumont in press: Table 2) and the Alona rectangula -group underwent a separate evolution. These are not specialised benthic species like A. quadrangularis , the type species of Alona , but more generalists. On limbs of Coronatella , most striking are general reductions and the absence of specialisations. The small West-African endemic C. holdeni is the most distant in morphology for the genus, indicating a strong isolation. Based on these synapomorphies, species related to Alona rectangula form a separate genus, for which a name is available (Van Damme & Dumont in press). Alona coronata Kurz (= rectangula ?) is the type species of Coronatella Dybowski and Grochowski 1894 . Kurz’ (1875) drawings leave no doubt that A. coronata is directly related to (maybe form or subspecies of) A. rectangula . Therefore, Coronatella is the oldest name for a group of medium-sized alonines related to rectangula . So, Alona rectangula no longer belongs to Alona but is now named Coronatella rectangula . Classifying these animals presents a major difficulty because differences on limbs may be subtle and several are still in Alona (e.g., limbs of C. rectangula vs. A. monacantha ). That they are distant from Alona is illustrated by earlier separation of species with similar limb morphology to Coronatella (e.g., Leberis in Sinev et al. 2005). In assigning the A. rectangula group to Coronatella , we would like to avoid creating another “lump genus”, well aware that morphological boundaries are sometimes very small.