Into the darkness – A look at the weird fish of Adelaide River

It is true that we know so little about the deep oceans. They’re dark and completely hostile to us, and even with the very best submarines we are still limited with what we can see and do. Closer to land is another mysterious marine environment we know little about, the turbid tidal freshwater rivers of northern Australia and southern New Guinea.

I’ve spent quite a bit of time on these rivers, and it’s amazing the diversity in the muddy, swirling waters. The water is extremely murky, you might normally only get 2cm visibility, at the most maybe 30cm. The water churns as it is forced up and down the rivers by the tide, stirring up sediment and food.

This article looks at the first few critters I found while conducting some casual surveys of the Adelaide River near Darwin, Australia. The fish were all photographed in a black tank as the water is pitch black in the river only a few centimeters down.

An unknown forktailed catfish from Adelaide River in Australia’s NT

In the murky waters it helps to have some sophisticated navigation equipment to find your way around and capture food. This unidentified Ariid catfish probes about with its sensitive barbels. Most catfish also have taste receptors all over their bodies to give them the best chance at finding something to eat. This catfish is extremely flexible and can quickly turn 180 degrees to snap up anything it finds. To communicate, many catfish like this one have the ability to make sounds by vibrating their swim bladder, producing croaks and grunts.

An unidentified freshwater anchovy from the Adelaide River in Australia’s NT

Food is abundant in the dirty waters, much of it is plankton. This Thryssa anchovy lives on the border of fresh and salt water, opening its huge mouth to strain the water for shrimp and fish larvae.

Possibly the first complete, high resolution image of a live nurseryfish. This unusual fish lives in murky estuarine waters of tropical Australia and New Guinea. Males have a hook like protuberance on their heads to hold and guard egg clusters. Almost nothing is known about this fish.

One of the strangest fish of all is this Nurseryfish (Kurtus gulliveri). These fish look and act like they belong in the deep seas. Their eyesight is poor, they are colourless in the wild, translucent and drift about in midwater, snapping up fish and shrimps that blunder too close. The cavernous mouth seems to be able to inhale a large volume of water, likely drawing in prey at the same time. They are weak swimmers, when netted seem to just flop about with no real vigor.

A female nurseryfish

The most remarkable thing about them is the method of raising young. The exact details of spawning are unknown but males end up with a bunch of eggs attached to a hook on their head. Skin grows to close the hook into a simple loop. This is where the eggs remain until they hatch and the young head off by themselves.

A male nurseryfish

A tiny juvenile nurseryfish

There is much more to see in these dark, mysterious waters. I’ll keep you posted on what else turns up…

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