Alona major

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 : 30-31

A major division in Alona sensu lato: Coronatella - vs. Hexalona- branch

Coronatella is distinct from true Alona ( quadrangularis -group), which has undergone a separate evolution and benthic specialisation (Van Damme & Dumont in press). Coronatella is morphologically closer to A. monacantha , A. dentifera , Leberis (including former A. diaphana and A. davidi ), and even Celsinotum (related to Leberis , see Frey 1991). Two taxa with two main head pores, the A. verrucosa -group and Karualona , share similar limb reductions as Coronatella and may belong to the same lineage. Pending phylogenetic analysis, we group them here, but we cannot exclude that similarities may be the result of convergence (see below). In Table 2, we list main differences between these taxa (not Celsinotum ) and several Alona species groups that appear much closer to the type, A. quadrangularis . The table shows that reductions are more common in Coronatella and “related” taxa. Limb characters of the “ Coronatella- branch” are (Table 2): 1. two IDL setae on P1 having stronger denticles in most taxa (not Karualona ), no anterior setae on P1, 2. specialisation of scrapers (heterogenous in size and denticulation) on P2 (most obvious in A. verrucosa -group) and absence of additional (soft) setae on P2 (spine in Karualona ), 3. six setae in exopodite of P3, 4. size reduction of flaming torch setae in endite IV (loss of one in Karualona ), 5. straight to convex margin of exV (not concave), reduction of gnV (0–1 setae) and 6. absence of P6.

These characters, mainly limb reductions, may have happened several times independently in Aloninae evolution. However, there are a few shared ‘external’ characters in the Coronatella- branch: overall shape of the postabdomen (rather compressed and “S-shaped” dorsal margin; not Leberis ), unmerged marginal denticles (merged in main Hexalona -branch), a long basal spine on postabdomen in several species (e.g., A. monacantha , A. dentifera ), relatively short labral plate lacking ventral setules. So, the Coronatella- branch seems supported by a number of characters. This may indicate a separate evolution. Karualona is clearly a marginal member that has less reductions and the A. verrucosa -group still needs revision. We think that all species (groups) in this branch still belonging to Alona , need separation. Although there is a large variability in external features (e.g., dorsal keel in Leberis and Celsinotum or body shape and head pores of A. dentifera ), the morphological characters may result from common ancestry and these may represent a group of Aloninae under radiation. We cannot exclude convergences, e.g. between two- and three pored taxa ( Karualona and Alona verrucosa- group vs. other taxa of the Coronatella -branch).

The “ Coronatella -branch” differs strongly from a group of six-limbed taxa in Alona sensu lato, which we call here the “ Hexalona- branch” (from “hexa-“, “six” and Alona ). It contains species morphologically closer to A. quadrangularis : A. affinis-, A. guttata-, A. costata-, A. rustica- (under A. costata -group here) and A. intermedia -groups (Table 2). This larger assembly is characterized by 1. merged marginal teeth on the postabdomen, 2. a labral keel with ventral setules, 3. anterior setae on first and second endites of P1 (reduced in size in intermedia and guttata ), IDL with three setae (third hooklike in affinis ), 4. third exopodite with seven setae, 5. a filter comb on P5 of three setae and 5. a P6 (Table 2). The number of head pores may differ in this lineage (two to three). The oldest available genus name in this group is Biapertura Smirnov, 1971 ( A. affinis ), but we use “ Hexalona” to indicate the “six-limbed” Alona ’s. A number of characters suggest these species groups may be intertwined with other genera (e.g., Alonopsis , Acroperus, Graptoleberis ), but further phylogenetic analysis is necessary. Within the “ Hexalona -branch”, A. guttata -group shows most limb reductions (Table 2). We think these reductions may be correlated with a miniaturization in A. guttata and occurred independently from the Coronatella -branch.

Separation of a Coronatella - and Hexalona- branch is a major division in Alona sensu lato, based on more than one character. Besides five vs six limb pairs or setulated vs unsetulated labral keel, an important character is the presence of merged or unmerged marginal denticles on the postabdomen. The latter character was suggested first by Kotov & Sanuamoang (2004) as phylogenetically relevant. In combination with the other characters listed here, a pattern emerges. We think that the presence of six ( Coronatella ) vs seven setae ( Alona ) on exopodite of the third limb is also phylogenetically important. All members of Coronatella- branch have six setae here. The reduction may have occured more than once in the Aloninae . The six-limbed Armatalona for example, has six setae as well in the third exopodite ( Sinev 2004b).

So, we think that Alona sensu lato contains at least two major lines of radiation, resulting in morphological parallelisms: the Hexalona- and Coronatella -branch. The main purpose in delineating them is to investigate larger patterns in Aloninae evolution. We cannot decide on a taxonomic rank for the Hexalona- and Coronatella- branch until these are put in a wider context (phylogeny of Aloninae ), as they contain several other Aloninae genera and the situation is more complex. Two important species groups cannot be assigned directly to either: the A. quadrangularis -group belongs to the “ Hexalona -branch” in general characters but lacks a filter comb on P5 and a sixth limb. A second, the A. pulchella -group (five limbs but with less limb reductions; merged or mixed denticles on postabdomen) may be closer to Coronatella (Table 2). Positions of both are open to interpretation. The A. quadrangularis- group is likely a specialised benthic lineage of the Hexalona- branch, whereas the A. pulchella -group seems intermediate between the A. quadrangularis -group and the Coronatella -branch (see also further on A. azorica ). Also the A. elegans -group, on which more later, can be considered a relatively marginal member of the Coronatella -branch (hence the dotted line in Table 2). Latter seems morphologically closer to Coronatella than for example Karualona . Also Leberis or Alona monacantha are close in morphology to Coronatella . Their positions should be tested by a phylogenetic analysis, preferably including a molecular analysis (some work has been done, see below on A. dentifera ). For a morphological analysis, studies of alonine limbs by Kotov (2000), recent detailed descriptions (e.g., by Sinev) and the present study provide comparative data.