The samples examined contained 13 species and two infraspecific variants of Collothecidae (Table 1). This corresponds with ca. 28% of the world fauna of Collotheca species and all Stephanoceros species known to date (Segers 2007). Two of the species identified could not be ascribed to any known species and we conclude that the specimens pertain to new species, one of which is described below. Of the second possibly new species we opine that insufficient material is at present available to warrant a full description, hence we only provide a brief illustration to enable future recognition. One more species, Collotheca ferox (Penard) is new to the Oriental region and Collotheca ornata f. cornuta (Dobie) is new to Thailand. These results indicate a relatively diverse Collothecidae fauna in the studied region of Thailand, and the record of one, and possibly two new species leads us to surmise that an even higher and incompletely documented diversity can be expected to occur in Southeast Asia.

We found specimens matching the descriptions of two taxa in Stephanoceros, Stephanoceros fimbriatus (Fig. 3H) and Stephanoceros millsii (Fig. 3G, 5F) (see Kellicott 1887, Koste 1978). Stephanoceros fimbriatus has five very long, stout corona lobes carrying transverse rows of robust setae along their length, while Stephanoceros millsii has five relatively slender corona lobes carrying longitudinal rows of long, fine setae. The corona lobes of Stephanoceros fimbriatus are relatively shorter than those of Stephanoceros millsii, when compared to their trunk length. Regarding trophi, the unci tips of Stephanoceros millsii are acutely pointed whereas those of Stephanoceros fimbriatus have arrow-shaped tips, and the unci are more strongly curved in Stephanoceros millsii (compare Fig. 5F with Fig. 1B in Sørensen and Giribet 2006).

Because the morphological characters of these two taxa enable a reliable diagnosis and because the two have wide and overlapping distribution ranges, we argue that these two taxa are distinct species, in contrast to Koste (1978) who considered Stephanoceros millsii an infrasubspecific variant (“Anscheinend Ökotyp”) of Stephanoceros fimbriatus. Ours are the first photographs of living animals and trophi of Stephanoceros millsii.

Stephanoceros millsii is common in Thailand whereas Stephanoceros fimbriatus is quite rare in our survey. Both species are cosmopolitan (Koste 1978).

The uncinate trophi type is one of nine trophi types recognized in phylum Rotifera (Wallace et al. 2006). This trophi type is characterised by unci possessing few teeth and by weakly developed manubria and fulcrum (Koste 1978) and has hardly been considered in the taxonomic analysis of Collothecacea (Families Collothecidae and Atrochidae). We examined the uncinate trophi of 6 species of Collothecidae to evaluate whether morphological differences, which might be taxonomically relevant, exist.

We found that, in all species examined, the uncinate trophi are composed of two pairs of large and sturdy unci teeth, whereas manubria, rami and fulcrum are less developed components (Figs 5A–F). Of the unci, the distal tips can be gradually sharpened (5C–D), stout (5B), or with set-off tips (5A), and the tips may carry a terminal, median groove (e.g., 5E–F). The unci are mostly strongly curved, either more or less evenly (e.g., 5A, E) or in their proximal third (5B), or terminally (5C), and the terminal tips may be slightly incurved (5A), straight (5E), or outcurved (5B). The unci pairs can be relatively equal (5A, D–F) or strongly unequal (5B, C) in length. The unci teeth are quite sturdy, as they are not easily dissolved by low concentration of commercial bleach (lower than 5% final concentration). The manubria, rami and fulcrum, on the other hand, are very weak and dissolve easily in bleach making it particularly hard to reliably compare their morphology. Nevertheless, the rami scleropilli usually remain after treatment (5E–F).

As illustrated here, the uncinate trophi, in particular the unci, do exhibit features that might be useful for taxonomic analysis. We suggest that 1) shape of the head of the unci; 2) shape of the unci teeth; and 3) relative size of the two pairs of unci teeth might be registered in future studies of Collotheca rotifers. Of course, the inclusion of these features in taxonomic analysis requires addition of information on more species of Collotheca, and evaluation of the intraspecific variability by comparing different populations of Collotheca species.

As mentioned above, Collothecidae species are essentially ambush predators. They remain immobile until a prey organism, guided by their long cilia and infundibulum that forms a fyke, and water current created by the beating of short cilia, comes in range of a sensory organ situated dorsally on the inner side of the infundibulum. When this organ is triggered, the cilia, corona lobes and infundibulum contract which restrains the prey organism within the infundibulum, and the prey is finally ingested whole. We observed that some species of Collotheca, and these appear to be species that have an enlarged funnel-shaped infundibulum, arrange their corona near the surface of the substrate they are attached to (e.g., Collotheca sp., Figs 3D; Collotheca ferox, Fig. 2A, B - note that the specimen in Figs 2A, B was not in normal position; Collotheca orchidacea sp. n., Figs 2C, D). Other species, mostly those that have a relatively smaller infundibulum but well-developed bands of cilia along the corona or on knobs, and a long foot, expose their expanded corona in the water column (e.g., Collotheca campanulata f. longicaudata, Fig. 3B; Collotheca ornata, Fig. 3C; Collotheca tenuilobata, Fig. 3F). We hypothesize that the two groups may have different diets. The latter group probably feeds on free-swimming, planktonic/periphytic organisms, while species of the former group may target browsing animals, in a way that is strikingly similar to Cupelopagis vorax (Leidy, 1857) (Bevington et al. 1995).

The keys presented here include all species recorded hitherto from Southeast Asia (Brunei, Cambodia, Indonesia, Malaysia, Myanmar, Philippines, Singapore, Thailand, Timor Leste, Vietnam), as included in De Ridder and Segers (1997) and more recent publications. To facilitate identification and discovery of species not included in the key, we provide both a dichotomous as well as a formula key to the Southeast Asian Collothecidae.

Dichotomous key

Characters

1. Species (a) free-living (pelagic); (b) living attached to a substratum (fixosessile)

2. Corona edge: (a) circular, smooth; (b) with well-defined knobs (Figs 3C, F); (c) with lobes (Figs 3B, E)

3. Number of corona projections: (a) one dorsal; (b) two: one dorsal, one ventral (Fig. 3D); (c) three: one dorsal, two lateral (Fig. 3E); (d) five: one dorsal, two lateral, two ventral (Figs 3B, 4F); (f) seven (Fig. 4A)

4. Length of corona projections: (a) much shorter than trunk (Figs 3C, D); (b) strongly elongated and parallel sided (Figs 3G, H)

5. Diversification of corona projections: (a) none, all projections more or less equal (Figs 3A, F–H); (b) differentiated (Figs 3B–E)

6. Lateral corona lobes: (a) absent (3E); (b) indicated (sinus between dorsal and ventral lobe is not smoothly concave or indicated by presence of a distinct group of particularly long cilae: Figs 4E, F); (c) well-developed (Figs 2A–F)

7. Ventral corona projections: (a) with one midventral lobe, (b) with two knobs (Fig. 4B); (c) two rounded triangular lobes (Fig. 4F); (d) two semicircular lobes (Figs 3B, E)

8. Ventral corona sinus: (a) shallow, narrow (Fig. 4J); (b) shallow, broad (Figs 3B, 4F); (c) deep, broad, U-shaped (Fig. 4E)

9. Special features: (a) elongate projection dorsally on dorsal corona lobe (Figs 4G, H); (b) peduncle (attachment stalk) longer three times diameter of foot than foot (Fig. 4I); (c) cilia inserted in parallel sets of transverse rows (Figs 4C, D); (e) group of elongate cilia dorsally on dorsal corona lobe (Fig. 3D)

Collotheca algicola: 1b, 2c, 3d, 4a, 5b, 6b, 7c, 8b

Collotheca ambigua: 1b, 2c, 3d, 4a, 5b, 6b, 7d, 8c

Collotheca campanulata: 1b, 2c, 3d, 4a, 5b, 6b, 7d, 8b (+9b: f. longicaudata)

Collotheca edentata: 1b, 2a

Collotheca ferox: 1b, 2c, 3d, 4a, 5b, 6c, 7c, 8a

Collotheca heptabrachiata: 1b, 2b, 2c, 3f, 4a, 5b, 6a, 7b

Collotheca libera: 1a, 2c, 3a, 4a

Collotheca orchidacea sp. n.: 1b, 2c, 3d, 4a, 5b, 6c, 7d, 8c

Collotheca ornata: (1b), 2b, 2c, 3d, 4a, 5b, 6b, 7b (+9a: f. cornuta; 1a: f. natans)

Collotheca pelagica: 1a, 2a

Collotheca stephanochaeta: 1b, 2c, 3c, 4a, 5a, (6b), 7d, 8b, 9c

Collotheca tenuilobata: 1b, 2b, 2c, 3d, 4b, 5a

Collotheca trilobata: 1b, 2c, 3c, 4a, 5b, 6a, 7d, 8c

Collotheca sp.: 1b, 2c, 3b, 4a, 5b, 6a, 7a, 9e

Stephanoceros fimbriatus: 1b, 2c, 3d, 4b, 5a, 9c

Stephanoceros millsii: 1b, 2c, 3d, 4b, 5a