Jansen’s Wrasse

Interesting Info

• Jansen’s Wrasse is a striking reef fish found in northern Australia from Western Australia through the Northern Territory to Queensland, including the Great Barrier Reef.
• They are one of the larger and more robust members of the genus Thalassoma.
• Colour and pattern are highly variable. Some individuals display a rainbow mix of blue, green, and yellow with a pale belly, while others are boldly patterned in black and white bars. Males are the brightest, while females and juveniles are smaller and duller.
• Juveniles often appear pale with black vertical blotches, a camouflage that helps them blend into rubble and seaweed before they develop adult colours.
• Like many wrasses, they are protogynous hermaphrodites, beginning life as females and later changing into males. When the dominant male in a group dies, the largest female quickly transforms into a replacement male.
• They feed on a wide range of prey, including crabs, shrimps, snails, worms, sea urchins, fish eggs, and even small reef fishes, making them active mid-level predators on reefs.
• By consuming invertebrates and egg masses, Jansen’s Wrasses help maintain ecological balance, preventing outbreaks of reef pests and regulating small-animal populations.
• Males hold territories and court females with fast, flashing colour changes, dashes, and fin flaring. Spawning occurs high in the water column, with eggs released to drift with the currents until hatching.
• They are extremely fast swimmers and can change their colours in seconds, shifting from pale camouflage to intense rainbow tones depending on mood, dominance, or breeding behaviour.
• During the day they patrol lagoons, reef slopes, and rubble zones, but at night they wedge themselves deep into reef crevices or rubble piles to avoid predators.
• The species typically lives for 5–8 years in the wild.
• Divers and snorkellers can identify them either by their “checkerboard” black-and-white form or by the vivid rainbow hues of dominant males, making them one of the more eye-catching wrasses on Indo-Pacific reefs.
• The species was named by Pieter Bleeker in the 19th century in honour of E. Jansen, a Dutch East Indies army officer and Bleeker’s friend, who supported his zoological work.