INTRODUCTION

• The dwarf pufferfish (Carinotetraodon travancoricus), also known as the Malabar pufferfish, pygmy pufferfish, or pea pufferfish, is a small freshwater pufferfish endemic to Kerala and southern Karnataka in Southwest India. 

• They are popular in aquaria for their bright colours and small size. 

• At a maximum total length of 3.5 cm, dwarf pufferfish are one of the smallest pufferfish in the world. 

• They closely resemble the related Carinotetraodon imitator, and the two can be difficult to distinguish. C. imitator was not recognised as a different species until 1999.

• Dwarf pufferfish dwell at the bottom of heavily vegetated waterways, predating small animals. 

• Unlike other species of pufferfish, they are found in large groups in the wild. They breed throughout most of the year, with spawning pairs producing 1–5 eggs in 1–4 day intervals. 

• Habitat loss and overharvesting for the aquarium trade threaten wild populations of dwarf pufferfish.

DESCRIPTION

• Dwarf pufferfish have dark, iridescent patches on their flanks and dorsal surfaces

• The maximum documented size is 3.5 cm (1.4 in) total length with individuals typically reaching less than 2.5 cm (0.98 in) making dwarf pufferfish one of the smallest pufferfish in the world.

• Both sexes are primarily greenish-yellow, with dark green to brown-black iridescent patches on the flanks and dorsal surface. Patterns and colouration vary considerably between individuals. 

• Their pectoral fins are short, fan-shaped, and described as "slightly emarginate", that is to say, slightly indented at the tip. 

• The dorsal and anal fins are situated opposite each other toward the posterior of the fish, both short and round, while the caudal fin is larger than the other fins and truncate, in that it terminates in a more-or-less vertical edge. 

• Their bodies are round and oblong. As with other members of the genus, sexual dimorphism is apparent in mature fish, with males being more brightly coloured than females and having a yellow ventral surface.

• Males can also have a dark stripe down the centre of their pale belly and iridescent, blue "eye wrinkle" patterns that females do not have. 

• Females are more rounded, tend to be larger than males, and may have more small spots between their larger dark markings. Their abdomens are white, and they may have a yellow patch on their throat.

NATURAL DEFENSES

• All pufferfish can inflate, or "puff up", their bodies by quickly ingesting large amounts of water (or air when necessary) into their highly elastic stomachs.

• Inflating tires pufferfish and can put them at greater risk of predation.

• Like many other pufferfish, in the absence of scales, dwarf pufferfish have skin spines on most of their body. These spines become erect when the fish is inflated. 

• Dwarf pufferfish, like many other Tetraodontiformes, produce tetrodotoxin, a highly lethal neurotoxin for which there is no antidote.

• The neurotoxin accumulates in the glands of the epidermis (surface layer of skin), ovaries, and livers of pufferfish.

• These traits are anti-predator adaptations. The pufferfish's ability to inflate makes it difficult to swallow or bite, and the tetrodotoxin makes it unpalatable or even deadly for other fish. This is because of the slow swimming speeds of pufferfish.

RESEMBLANCE TO CARINOTETRAODON IMITATOR

• Carinotetraodon imitator is a species of related pufferfish closely resembling dwarf pufferfish. 

• They are of similar size, shape, patterns, and colouration. Both species are found within the same region and may be sympatric, possibly inhabiting the same waters. 

• Dwarf pufferfish can be distinguished from their congener (a member of the same genus), as C. imitator have smaller, faint blotches compared to dwarf pufferfish, as well as greatly reduced body spination. 

• Male dwarf pufferfish have a darker yellow colouration and an iridescent "eye wrinkle" not found in males of C. imitator.

DISTRIBUTION AND HABITAT

• Although closely related to marine pufferfish, they are not found in salt water.

• Dwarf pufferfish are one of only 27 known species of Tetraodontidae adapted to freshwater.

• They are a potamodromous species, meaning they migrate within freshwater river systems, and are endemic to rivers, lakes, and estuaries in Kerala and southern Karnataka in the Western Ghats of Peninsular India.

• They can be found in waters with a pH of 7.5–8.3 and temperatures of 22–28 °C (72–82 °F).

• Inhabiting heavily vegetated waters with beds of gravel and rock or clay loam with silt and sand, the species is reported from 13 rivers in Kerala, lakes such as Vembanad, the Nilambur harbours, and Kallar Stream, part of the Neyyar Wildlife Sanctuary.

• They have been reported as rare in Bharathapuzha and the Nilgiri Biosphere Reserve.

• The species was also found in inundated brickyards in Puthukkad, as well as ditches, ponds, irrigation channels, and artificial tanks or abandoned water bodies in paddy fields. 

• Their presence in small, isolated water bodies is likely due to unintentional dispersal by waterfowl.

DIET AND BEHAVIOUR

• Dwarf pufferfish are euryphagous carnivores in that they consume a wide variety of animals. 

• Dwarf pufferfish favour insect larvae but will rely on crustaceans and annelids as alternate feed when the availability of preferable prey decreases.

• Their diet in the wild mainly consists of small animals such as water fleas, rotifers, copepods, and seed shrimp, and aquatic larvae such as those of Odonata, Hemiptera, mayflies, and flies, with some amounts of plant matter, largely diatoms and green algae. Sand and detritus, presumably ingested by mistake when feeding on small, bottom-dwelling animals, have also been found in their gut.

• In captivity, dwarf pufferfish benefit from a varied diet and will eat small freshwater snails, shrimps, as well as foods like frozen mussel meat or other frozen shellfish. 

• They will also readily eat live foods such as red and white mosquito larvae and other small animals they would predate in the wild.

• Dwarf pufferfish are commonly associated with plants in the genus Cabomba, and the presence of these plants has been shown to reduce mortality among captive specimens.

• Dwarf pufferfish are a slow-swimming, demersal (bottom-dwelling) species.

• Unlike many pufferfish species, which are primarily solitary and potentially aggressive or territorial between conspecifics (members of the same species), dwarf pufferfish are found in large shoals, occasionally consisting of hundreds of individuals.

• They are found mainly during the summer months of January to May and rarely during the rainy season.

• Shoaling fish experience stress or weight loss when kept in solitude or in groups too small.

REPRODUCTION

• In the wild, males with ripe gonads have been found during all months of the year but December and January, with the peak spawning period extending from May to August, concurring with the South-West monsoon period.

• Within the Pamba River, the minimum size at which half the population becomes sexually mature is approximately 1.83 cm (0.72 in). Environmental and dietary conditions may influence the maturation rate of individuals.

JUVENILE

• The mid-dorsal and mid-ventral skin ridges of males become brown in colour during the spawning season. The belly of the female will swell, and the courting male will frequently chase the female and nibble at its belly. The female will then search for a suitable location to spawn while the male chases away other males.

• A female will scatter approximately 1–5 eggs, 1.43 mm (0.056 in) in mean diameter. The eggs are adhesive and appear transparent and round, with a mass of small oil globules. After laying, the eggs are then fertilized externally by the male.

• Spawning has been observed in the evening, with the female resting on the spawning site and the male slowly approaching. After spawning, both fish will leave the site. The male will then quickly return to guard the eggs.

• Sneaking ejaculation by other males has also been observed. The pair may spawn multiple times in 1–4 day intervals.

• Eggs hatch after five days at 27 °C (81 °F), with larvae and fry in captivity initially fed infusoria, Brachionus (a genus of rotifers), frozen bloodworms, and brine shrimp when they are a week old.

• Hatched larvae are a mean of 3.15 mm (0.124 in) total length, with eyes incompletely developed and the body a red-brown. The yolk sac is consumed in four days, and the larvae commence swimming after six days, at which point their eyes are completely developed.

ASSOCIATION WITH HUMANS

• When first described in 1941, K. Nair noted that dwarf pufferfish were a favourite of children, who would catch and use the fish as playthings.

• They were regarded as "frog tadpoles" by local fishermen and otherwise given little consideration.

• They are of no interest to fisheries, are not a food fish, and are only valued as ornamental fish in aquaria.

• Dwarf pufferfish have become popular as aquarium fish thanks to their attractive colours, small size, "puppy dog eyes", and relative ease of maintenance.

• The dwarf pufferfish is also one of the few aquarium fish to regularly eat small, live snails and thus can be helpful in controlling snail populations.