Hisonotus depressicauda

Hisonotus depressicauda is distinguished within the genus by its markedly depressed — dorsoventrally flattened — caudal peduncle, a morphological character reflected in its species name and believed to be an adaptation to fast-flowing, shallow water environments. It is described from cool subtropical streams of southern Brazil, placing it among the cool-water tolerant members of the Loricariidae.

Like H. insperatus, this species inhabits clear, well-oxygenated streams in southern Brazil characterised by temperate-subtropical temperatures and a substrate of flat stones and fine gravel. It grazes biofilm, diatoms, and periphyton from flat rock surfaces, using its ventral sucker disc to maintain position against moderate current. The flattened caudal region may reduce drag in faster-flowing water, allowing the fish to occupy exposed positions on smooth stones.

In the aquarium, H. depressicauda is an unusual and subtle loricariid for the cool planted aquarium. Its preference for flat stones, moderate current, and cool, well-oxygenated water aligns it with hillstream loach care as much as with standard Otocinclus husbandry. It is best appreciated in a southern Brazilian or Uruguayan biotope setup with smooth stones, fine gravel, and live cold-tolerant plants such as Egeria or Lilaeopsis.

Water: Soft to moderately hard at pH 6.5–7.5 and 16–24°C; very clean, well-oxygenated; moderate flow replicates natural stream current. Tank: 40 litres minimum with flat smooth stones, fine gravel, and cold-water plants. Feeding: Diatoms and biofilm on stones primary; supplement with algae wafers and blanched vegetables. Breeding: Not documented in captivity. Compatibility: Peaceful with all cool-water community species.