405_April_24

MARINE BIOLOGY WITH MIKE SCOTLAND - FISH IDENTIFICATION

leatherjacket has four large spines on the caudal peduncle. The best known and most attractive is the Six Spined leatherjacket. The Dusky leatherjacket has no caudal spines at all. Leatherjackets have a large dorsal spine which is usually barbed. Many have a second smaller spine embedded into a groove along its dorsal surface. They are closely related to Triggerfish which have a similar arrangement of spines. These spines are a fantastic deterrent to predator trying to swallow a Leatherjacket. The locked in spine makes them very uncomfortable and quite dangerous inside the fish’s gullet. They have a good chance of being spat out to live another day. Scales are usually tiny and often covered with spikes. In some species, you can see the spikes clearly. I have had the experience of finding out how these spines work. Many years ago, on night dives one of my buddies had a special trick of capturing Leatherjackets bare handed just for fun. It was a very impressive trick to liven up a dive. His secret to this trick was to place his hand like a cup shaped open fist and then move closer to the fish so slowly that the fish was not disturbed. If it unwittingly moved between your fingers, he would slowly close his grip. Voila! He has a captive fish, which he could show off before releasing it. This takes a lot of skill and patience.

I taught some young guys to dive who were keen fishermen. After one dive, as we were debriefing and discussing the dive, one unzipped his wetsuit jacket and pulled out an edible sized Leatherjacket he had seen on the dive. He decided to take it home for dinner. The same guy did this trick with a large Flounder on another dive. One Leatherjacket that should never be handled is the Fan Bellied leatherjacket. Its spines are about two millimetres long. When it is gripped, I am told that the fish struggles and your grip naturally tightens as a reflex reaction. This drives dozens of tiny spines into your fingers painfully. It is a perfect torture for humans who like to play with fish, Leatherjacket style. The Leatherjacket Louse is a common parasite. This is a large species of parasitic isopod that occurs in Australia’s temperate marine waters. It burrows its way between the scales through the body wall of the fish. Then, it feeds on the fish tissue until it grows to the size of a fifty cent piece. The tail of the Isopod Louse usually can be seen through a slit in the body wall. The adult female Leatherjacket Louse remains inside the fish forever. When you see a fish with external parasites sucking on its blood, you can imagine the extreme pain that it has to live with. The Leatherjacket Louse

The strongly elevated dorsal spine of the Yellow Fin leatherjacket, Meuschenia trachylepis

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DIVE LOG Australasia #405 - April ‘24