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By Brendan Ebner and James Donaldson, CSIRO

Flashes of iridescent green, pink and blue pulse through the crystal clear water of a rainforest stream as a male opal cling-goby signals to a passing female. Growing to a maximum size of only4cm, these little fish are just one example of some of the gems that are hidden away in the beautiful rainforest streams of the Australian Wet Tropics. Many of us have been to places such as Josephine Falls, Crystal Cascades and Emmagen Creek, but how many have actually put on a mask and snorkel to explore these underwater worlds?

A male opal cling-goby (Stiphodon semoni)

In addition to their stunning beauty (of the males at least!), these tiny fish lead an extraordinary life. Although they spend their entire adult life in swift-flowing freshwater streams, much of their early life is spent with the plankton in the warm oceanic waters of the Indo-Pacific. This pattern of migration, where the larvae are swept from freshwater streams into the ocean for several months before returning to freshwater, is quite common for freshwater fish species found on tropical islands, and is known as amphidromy. Real travelers, amphidromous species are able to disperse across ocean basins and colonise freshwater systems on oceanic islands. It is for this reason we think that many of the freshwater fish species found in the Wet Tropics are shared with places such as Vanuatu and French Polynesia.

In Australia, the opal cling-goby is only known from a handful of streams in the Wet Tropics, some of which are right on Cairns’ doorstep. These gobies were first recorded in Australia in 1987 from a single specimen. More recently, we have started to unlock some of the mysteries surrounding this species, although, many unanswered questions remain. As a result of its’ restricted distribution, low abundance, appeal to aquarists and the general lack of information on the species, the opal cling-goby was listed as critically endangered under the EPBC Act in 2011.

Unfortunately, these fish are often found in such small streamsthat these habitats are overlooked and remain unprotected. Critically important habitat for this species is threatened by sedimentation, over-extraction of water and waterway barriers such as pipe culverts.Raising public awarenessof this threatened species and it’s beautiful stream habitat is the purpose of this article. If you think that you can help in some way, contact James Donaldson or Ebb on james.donaldson@nulljcu.edu.au or brendan.ebner@nullcsiro.au.

And know that what the opal cling-goby lacks in size it makes up for with colour, charisma and an amazing travel story; a true gem worth saving.