Least Rasbora (Boraras Urophthalmoides)

Boraras urophthalmoides, commonly known as the Least Rasbora or Exclamation Point Rasbora, is a very small, peaceful, and visually striking freshwater fish native to Southeast Asia . It is a popular choice for nano and planted aquariums .

Here is a detailed overview of this species, covering its scientific classification, appearance, natural habitat, and care requirements.

Scientific Classification & Names

• Scientific Name: Boraras urophthalmoides (Kottelat, 1991) 

• Common Names: Least Rasbora, Exclamation Point Rasbora, 蚂蚁灯鱼 (Chinese: "Ant Light Fish"), Red Ants Lamprey 

• Family: Danionidae (subfamily Rasborinae), formerly classified under Cyprinidae 

• Etymology: The species name comes from urophthalmus (Greek for "tail" and "eye", referring to the tail spot) and -oides (Greek for "likeness") .

Physical Appearance & Size

• Size: This is a miniature fish species. While scientific records note a maximum total length of up to 4.0 cm , in aquarium settings and according to hobbyist sources, they are often reported to be smaller, around 2 cm .

• Coloration: The body is predominantly reddish, featuring a prominent black or dark brown lateral stripe that runs from the snout, through the eye, to the base of the tail . Above this dark stripe, there is a lighter, sometimes slightly iridescent, stripe . Additional dark spots are present at the base of the caudal fin (tail) and the anal fin .

• Similar Species: It is very similar to its relative, Boraras brigittae (Chili Rasbora), but generally has a less intense, bright red coloration .

Natural Habitat & Distribution

• Geographic Range: The species is widely distributed in mainland Southeast Asia. Its native range includes Thailand, Cambodia, Vietnam, Peninsular Malaysia, and the lower parts of the Mekong and Chao Phraya basins .

• Habitat: In the wild, Boraras urophthalmoides inhabits slow-moving or stagnant freshwater environments like marshes, swamps, peatlands, and ponds . These waters are typically characterized by dense growth of fine-leaved aquatic plants and are often stained brown by tannins from decaying organic matter .

Aquarium Care & Husbandry

This species is well-suited for peaceful community tanks, particularly nano aquariums.

Reproduction (Breeding)

• Difficulty: Breeding is considered very difficult in a home aquarium .

• Method: They are egg-scatterers. Spawning is triggered by soft, acidic water and warmer temperatures (up to 28°C) .

• Behavior: Adults will eat their own eggs and fry, so they must be removed from the breeding tank immediately after spawning . Eggs are typically laid among fine-leaved plants like Java moss .

• Fry Care: The fry are extremely tiny and require infusoria or other microscopic foods for the first few days .

Conservation & Human Use

• IUCN Red List Status: Near Threatened (NT) . The primary threats are habitat loss due to the development of wetlands for agriculture and logging, as well as harvesting for use as food fish in aquaculture .

• Human Use: While it is a commercial species in the aquarium trade , it is more extensively used as a live food source for larger farmed fish in parts of its range, such as southern Vietnam and Thailand .