Rotifers are microscopic freshwater invertebrates found in ponds, lakes and rivers around the world. The group to which the new species belongs, the order Collothecaceae, is among the strangest of them all. During metamorphosis these animals lose the corona, the wheel of beating cilia that gives rotifers their name, and replace it with an entirely different feeding structure called the infundibulum. This foregut-derived organ forms a cup- or funnel-shaped expansion at the anterior end of the body, which the animals use to trap their prey. Most collothecaceans are sessile, living attached to submerged plants and other surfaces, and their hunting style is ambush predation: prey that wanders into the infundibular chamber is enclosed as the structure folds over it, then drawn inward with the help of forceps-like jaws known as the uncinate trophi.

The family Collothecidae contains two genera of this predatory kind: Stephanoceros with two species, and Collotheca with 48 currently recognized species, making it the most species-rich genus of sessile rotifers. Members of Collotheca bear setae, fine hair-like bristles, along the margin of the infundibulum, and the number, shape and size of the lobes on that margin, together with the distribution of the setae, are the characters on which species identification largely depends. Because these features are best observed in living, expanded specimens, species-level identification requires specific laboratory procedures and considerable expertise, resources that remain scarce in much of the tropics.

The researchers organized their workshop in Thailand from 11 to 15 May 2026, covering copepods, cladocerans and rotifers. One session was devoted to sessile Gnesiotrocha, a group of rotifers that is generally not treated as zooplankton except for a few species and their juvenile dispersal stages. Working through samples of the aquatic plants Ceratophyllum demersum and Azolla filiculoides subsp. cristata, participants found abundant specimens of the known species Collotheca ambigua, along with a smaller population of an unknown Collotheca. Unknown individuals also appeared on both plant types. A careful review of the taxonomic literature confirmed that the mystery specimens matched none of the 48 valid species of the genus.

The new species, found on both plants in the pond, was formally described from ten female specimens. The holotype and three paratypes, mounted in Permount on permanent slides, have been deposited at the Princess Maha Chakri Sirindhorn Natural History Museum at Prince of Songkla University in Songkhla, Thailand. Collotheca laddae is characterized above all by its five-lobed infundibulum, in which the two lateral lobes are very small, appearing as low, broad-based pointed elevations, and the setae are short and vibratile, arranged continuously around the margin. The dorsal lobe is broad-based and subtriangular, while the two ventral lobes are small and semicircular, each only about a quarter the width of the dorsal lobe, with a shallow ventral sinus between them.

Distinguishing a new species in a genus of 48 required a meticulous comparison. Among the five-lobed species, C. laddae differs from species with prominently projecting lateral lobes such as C. ferox and C. orchidacea, and it lacks the marginal spines of C. spinata. Its broad, subtriangular dorsal lobe sets it apart from the short, U-shaped dorsal lobe of C. monastica and from the strongly inward-curved dorsal lobes of C. campanulata and C. torquilobata. Its closest resemblance is to C. bilfingeri, but that species carries a protuberance on the dorsal lobe and very short setae restricted to the tips of the five lobes, whereas the new species lacks the protuberance and bears short setae evenly distributed around the entire infundibular margin. Total body length of the measured females ranged from 290 to 620 micrometers, with the foot roughly as long as the rest of the body and the infundibulum about one and a half times as wide as the trunk.

The species was delimited solely on the external morphology of adult females, because molecular data for close relatives are scarce in GenBank, particularly for the COI gene commonly used in species delimitation, and trophi morphology could not be imaged. This gap is itself telling: for a genus this widespread, the absence of genetic reference data is a serious obstacle to modern taxonomic work. The specimen and occurrence dataset generated by the study has been published through GBIF to make it available for future research.

The description is accompanied by a pictorial key to all Collotheca species and subspecies recorded in Southeast Asia, developed through a review of original descriptions and illustrations of all valid species listed in the Rotifer World Catalog. Where photographs of living specimens were available, they served as the primary basis for the drawings; several species were drawn directly from photographs of live animals, while others were redrawn from historical publications dating back as far as the nineteenth century. The key first separates free-swimming, pelagic species from sessile ones, then works through the number of infundibular lobes, the presence or absence of knobs, lobe shapes, and the length and distribution of setae. Because identification in this genus often hinges on subtle differences in lobe shape and setal arrangement, pictures accompanying each step can make the difference between a confident identification and a dead end.

The regional picture that emerges from the study is strikingly uneven. Thailand harbors the highest recorded Collotheca diversity in Southeast Asia with 12 species, followed by Cambodia with five, and Myanmar, Indonesia and Singapore with three each. Malaysia has two recorded species, while Laos and Vietnam each have one, and no species at all have been recorded from Brunei, the Philippines or Timor-Leste. Notably, in the more than a decade since the last comprehensive regional treatment was published in 2013, only five Collotheca species have been reported from subsequent studies, with no new regional records at all. The authors point to the rotiferologist effect, a phenomenon in which recorded rotifer diversity reflects the distribution of taxonomic expertise and sampling effort rather than actual species occurrence. Southeast Asia likely harbors the second-richest freshwater fauna in the world in species richness and endemism, yet its sessile rotifers remain barely explored.

The discovery also carries evolutionary interest. The short setae of C. laddae beat actively, generating a strong water current around the infundibulum opening that may help draw algal cells toward the mouth of the trap. Most Collotheca species have long, largely immobile setae and are regarded as pure ambush predators, but several species with short setae show this active beating, raising the possibility that these animals combine ambush predation with a degree of current-assisted suspension feeding. The picture is complicated by the fact that the collothecid infundibulum appears to be derived primarily from the foregut rather than from the larval corona, meaning its setae are not homologous to the cilia of other rotifers. Whether this behavior represents an ancestral retention or an independent innovation remains an open question. For now, the finding stands as a reminder of how much undiscovered life can hide in an ordinary campus pond, and how a few hours of expert observation can reveal it. Collotheca laddae is named in honor of Professor Ladda Wongrat, a pioneer of zooplankton studies in Thailand, and the species is currently known only from its type locality.

Subject of Research: Taxonomic description of a new sessile rotifer species, Collotheca laddae, and a pictorial identification key to Southeast Asian Collotheca

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

Article References: A pictorial key to Southeast Asian species of Collotheca ( Rotifera , Collothecidae ), with a new species from Thailand. (n.d.). https://doi.org/10.3897/zookeys.1294.206505

Image Credits: AI Generated

DOI: 10.3897/zookeys.1294.206505

Keywords: rotifers, Collotheca laddae, new species, Southeast Asia, Thailand, taxonomy, pictorial key, sessile rotifers, freshwater zooplankton, ambush predation, infundibulum, biodiversity