A snake eel is described on the basis of a single specimen collected near Dongsha Atoll in the South China Sea. Its large eyes and the dorsal-fin origin behind the pectoral fins place it in the Ophichthus megalops group, despite its lack of a blackened anal-fin base in advance of the tail tip. The following combination of characters separates this new species from all of its Indo-West Pacific congeners: body bicoloured, grey-brown dorsally and uniformly pale ventrally; dorsal-fin origin situated 2.2 times pectoral-fin length behind tip of pectoral fin; eye large and snout short and blunt; a knob present between nostrils; rictus situated slightly anterior of vertical through posterior margin of eye; and teeth biserial on upper jaw and uniserial on vomer and lower jaw. A specimen of Ophichthus kusanagi Hibino, McCosker & Tashiro, 2019 was also collected nearby, representing the first record of this species from the South China Sea.

Heterocypris sarakhamensis, new species, is described from a temporary pond in Maha Sarakham Province in the Northeastern part of Thailand. The new species resembles Heterocypris indica (Battish, 1981). The following are the main indicative features of the new species: the subelliptical carapace in dorsal view without anterior compression, posterior half of the right valve dorsal margin with inward arch resulting in a thick margin aspect, the evenly arch dorsal margin of the left valve without angulated corner, the serrated bristles on the third endite of the maxillula, the short d1 seta on the second thoracopod, and the long Ga claw of the caudal ramus.Heterocypris sarakhamensis, new species, is the first record of the identified Heterocypris species in Thailand and it is the third Heterocypris species in Southeast Asia.

Obliquogobius eptactis, new species (Teleostei: Gobiidae) is described on the basis of four specimens (40.7–50.1 mm in standard length) collected by the R/V Dr. Fridtjof Nansen from the Andaman Sea, off Myanmar, in depths of 181–184 m. The new species is characterised by the following combination of characters: 2nd dorsal-fin rays I, 9 or 10; head somewhat large, length 32.9–34.4% of SL; lateral surface of nape scaled; postorbital pore G present (in anterior oculoscapular canal); gill opening relatively narrow, anteroventral point extending slightly forward to vertical level of preopercle margin; caudal fin dorsoventrally asymmetrical, rays in upper half much longer than those in lower half, giving obliquely pointed appearance; seven bright yellow bars (pale whitish in preserved specimens) on body, two under 1st dorsal-fin base, remainder under 2nd dorsal fin (from origin) and on caudal peduncle; dorsal fins pale brown with bright yellow barred pattern; 1st dorsal fin without broad black margin; pectoral fin bright yellow; three bright yellow vertical bars on upper part of caudal fin; distinct black spot on centre of caudal-fin base absent. Although the new species is similar to Obliquogobius yamadai Shibukawa & Aonuma, 2007, the former can be distinguished from the latter by having seven bright yellow bars on the body [vs. five bars (rarely six, including indistinct partial bar located above anus) in O. yamadai], a bright yellow barred pattern on the dorsal fins (vs. 1st and 2nd dorsal fins lacking barred patterns: 1st dorsal fin with distinct broad black margin, 2nd dorsal fin generally faint yellow), bright yellow pectoral fins (vs. translucent white or faint yellow), no distinct small black spot centrally on the caudal-fin base [vs. a small black spot (variable in size) with following indistinct yellow partial bar present], three bright yellow irregular vertical bars restricted to upper part of caudal fin (vs. three longitudinal yellow stripes on entire fin; stripes faded out soon after capture), and larger head, length 32.9–33.3 (vs. 28.3–32.2) and 34.4 (vs. 29.1–33.5) % of SL in males and females, respectively.

We describe Periophthalmus pusing sp. nov., a mudskipper species from the Lesser Sunda Islands. This species closely resembles, and was previously identified as, its congener Periophthalmus gracilis Eggert, 1935. A black spot on the posterior portion of the first dorsal fin, a diagnostic character for P. gracilis, is also present in the new species, and thus led to the earlier confusion. Adults of Periophthalmus pusing sp. nov. (> 30 mm SL) differ from those of Periophthalmus gracilis in having XI–XV spines in the first dorsal-fin (vs. VI–XII in P. gracilis), first dorsal fin taller than depth of body at anus (first dorsal fin shorter than depth of body at anus in Periophthalmus gracilis), interdorsal distance less than half the length of the first dorsal-fin spine (interdorsal distance more than half the length of the first dorsal-fin spine in Periophthalmus gracilis).

Phenacostethus sikat, new species, is described from coastal drainages of Kalimantan Selatan, the southeastern province of Indonesian Borneo, that flow into the Java Sea and the Straits of Makassar. This discovery and description brings the number of valid species of priapiumfishes, the atheriniform subfamily Phallostethinae, to 24 and extends the known distribution of the phallostethines to drainages on the eastern margin of the modern Sunda Shelf. Phenacostethus sikat differs from congeners in features of the priapium, the complex bilaterally asymmetric, subcephalic copulatory organ of males, and is readily distinguished by a unique, brush-shaped seminal papilla in males, as opposed to a simpler, ruffled seminal papilla in Phe. smithi, its inferred sister species based on morphology, and a smooth seminal papilla in males of Phe. posthon and Phe. trewavasae. Males of Phe. sikat and Phe. smithi are either sinistral or dextral, the hooked toxactinium arises on the right or left side of the head and curves strongly under the head towards the aproctal side of the body. Males of Phe. posthon and Phe. trewavasae are exclusively dextral or sinistral, respectively. Males of Phe. sikat have an ossified inner pulvinular bone which is cartilaginous or absent in Phe. smithi. Males of Phe. sikat and Phe. smithi have a curved and pointed or blunt second ctenactinium as opposed to a highly reduced second ctenactinium in males of Phe. posthon and Phe. trewavasae. Females of the new species lack rudimentary pelvic fins which are present in adult females of Phe. smithi, Phe. posthon, and Phallostethus. The new species is also diagnosed by a colour pattern with few or no melanophores along the horizontal septum anterior to the anal-fin origin. The mtCOI sequences of Phe. sikat differ from those of congeners by 13.85–24.23%, with an average of 21.54%, an extraordinarily high level of divergence comparable to that of other diminutive fishes. Like congeners, Phe. sikat is a miniature species. The largest specimen known is an adult female just 15.3 mm SL

Quadrivisio meufong, new species, is described from Songkhla Lake in Thailand and is seen to co-occur with the more widely documented Q. bengalensis Stebbing, 1907. Unlike other species of Quadrivisio Stebbing, 1907, the dorsal carination pattern is consistent with growth stage in Q. meufong, new species. An updated key to the eight species of Quadrivisio is provided.

A new species of freshwater crab in the genus Setosamon Yeo & Ng, 2007, is described from Sakon Nakhon Province, northeastern Thailand. Setosamon sakon, new species, is most easily distinguished from congeners by differences in the male first gonopod. A key to the species of Setosamon is also provided.