A pictorial key to Southeast Asian species of Collotheca (Rotifera, Collothecidae), with a new species from Thailand.

A new species is described from Thailand, Collotheca laddae sp. nov., and a pictorial key to all Collotheca species and subspecies recorded in Southeast Asia is provided. The new species was discovered during a freshwater zooplankton identification workshop. This species is characterized by a five-lobed infundibulum with very small lateral lobes and short vibratile setae. The pictorial key was developed from a review of original descriptions and illustrations of all valid species of Collotheca and is designed to facilitate species-level identification based on live specimens. The discovery of C. laddae sp. nov., together with the absence of new regional records for more than a decade, indicates that sessile rotifer diversity in Southeast Asia remains insufficiently explored.