A new species of Calanoida belonging to the genus Boholina Fosshagen & Iliffe, 1989 was found in a freshwater pool within a cave of the Satun province, South Thailand. It is the first record of the genus and of a stygobiotic representative of calanoid fauna in this country. The new species is most similar to B. crassicephala Fosshagen & Iliffe, 1989, based on position of genital pores, structures of P4 and P5 in both sexes, relative length of subapical spine vestige on the male right P5, and shape of the male left P5 endopods. However, this new species is distinguished from its known congeners by: (1) relatively longer distal outer spines on the male right P5 exopods, (2) smaller endopods of the male left P5 and (3) elongated apical spines on the distal exopodal segment of the female P4 and P5. Furthermore, the distinctive characteristic of the Thai Boholina is the presence of inner minute seta on the distal segment of the male right P5 exopod. Detailed descriptions of the new species and a key to all six known species of the genus Boholina is provided.

Iyengar was considered to be a phylogenetic link between and the type species Shaw because it has small somatic cells distributed from the anterior to posterior poles in 64- or 128-celled vegetative colonies. However, cultural studies and molecular and ultrastructural data are lacking in , and this species has not been recorded since 1951. Here, we performed light and electron microscopy and molecular phylogeny of based on newly established culture strains originating from Thailand. Morphological features of the present Thai species agreed well with those of the previous studies of the Indian material of and with those of the current concept of the advanced members of the Volvocaceae. The present strains exhibited homothallic sexuality; male and facultative female colonies developed within a single clonal culture. Chloroplast multigene phylogeny demonstrated that was sister to two other species of ( and Nozaki) without posterior somatic cells, and these three species of formed a robust clade, which was positioned distally in the large monophyletic group including nine taxa of sect. and (sect.) Ehrenberg. Based on the present phylogenetic results, evolutionary losses of posterior somatic cells might have occurred in the ancestor of and . Thus, might represent an ancestral morphology of , rather than of.

We investigated the diversity of brachionid rotifers in Thailand. Rotifers were collected from a wide range of habitat types (294 sites, 508 samples) including canals, lakes, peat swamps, ponds, reservoirs, rice fields, rivers, swamps and temporary ponds from April 1998 to January 2004. Twenty-eight species including 11 infrasubspecific forms belonging to five genera were identified. The most frequently encountered species was Brachionus falcatus (50.7% of the sampling sites), followed by Keratella cochlearis (50%), B. angularis (49.7%), K. tropica (46.3%) and B. forficula (45.9%). Less frequently encountered species were B. caudatus, B. durgae and B. urceolaris. These species were found in low abundances and only in a single site. Two sites in the Northeast -Nong Changpeuak and Bueng Srithat -recorded the greatest richness with 15 taxa, whereas no brachionid rotifers were found in 31 sites. On average, the species richness was five taxa per sampling site. A key to the genera and species of Thai brachionid rotifers is provided.

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Many herbicide products are commonly used in agricultural areas to prevent and eliminate weeds. Contamination from these toxicants in water might affect aquatic organisms not only in the active stage, but also in the diapause stage. To test the effect of herbicide on the resting eggs of zooplankton, we prepared two rice fields: one field without the application of pesticides (RF−NPA) and one with the application of pesticides (RF−PA) in a sampling year. We conducted a hatching experiment for 30 days. Twenty–four taxa of zooplankton were found. Sixteen species of these were rotifers, seven species were cladocerans and one taxon was an unidentified nauplius copepod. The species richness of zooplankton between RF–NPA (17 taxa) and RF–PA (16 taxa) was close, but species compositions between RF–NPA and RF–PA were different, indicated by the similarity index of 0.545. Lecanidae was the most diverse family of rotifers in both rice fields with nine species, while Chydoridae was the most diverse family of cladocerans (four species). The total abundance of zooplankton of RF−NPA was higher than RF−PA with 1,897 and 1,286 individuals, respectively. The Shannon–Wiener diversity index (H´) and Pielou’s evenness (J) in RF−NPA were higher than in RF−PA. The high species richness of zooplankton in both rice fields occurred on days 18 to 30. On the other hand, the highest abundance was recorded on day 18 for RF−NPA and on day 24 for RF−PA. The non-metric multidimensional scaling (NMDS) demonstrated significant differences in zooplankton community composition between RF–NPA and RF–PA (p < 0.05; ANOSIM test). According to the diversity indices, the RF–NPA has more diversity than the RF–PA, which might be a result of herbicide application in the sampling year. This study suggests that the toxicity of glyphosate should be a concern in terms of the biodiversity of rice field ecosystems.