Weird fish of the week: Razor fish

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This week's candidate for Weird fish of the week is another occasional aquarium subject, the Razor or Shrimp fish, (Aeoliscus strigatus).

There's little about these strange fish that isn't weird when compared to what most of us would think of as 'normal' fish anatomy.

The obvious oddity of the Razor fish is its swimming position - they swim in a vertical position with their heads pointing downward and their tails up.

This odd method of locomotion is made possible by some extreme body adaptation. Their dorsal fin is divided into two parts, the first of which is positioned on the end of the body where the caudal fin, (tail) would normally be, while the second dorsal and true caudal are positioned ventrally beside the anal fin.

This peculiar arrangement means the fish moves around using a sculling motion of its pectoral fins in combination with waving movements of the adapted dorsal, caudal and anal fins, sometimes described as 'Balistiform swimming' after triggerfish which employ this method themselves.

Razorfish are generally a shoaling species and groups of them achieve an impressive degree of synchronisation in their movement. Their cryptic swimming method make sense when they are seen in their natural environment, hidden among the spines of large Diadema sea urchins, sheltering among branching staghorn corals or gorgonians and hiding in sea grass beds. Here their vertical stance, along with a dark bar running from nose to tail means they can hide from both predators and potential prey such as copepods and zooplankton which they snap up in their tiny, toothless mouths.

They come from the order Syngnathiformes which includes the seahorses, pipefish and ghost pipefish with which they share another weird characteristic. Their bodies are covered with a series of transparent bony plates rather than scales which make them rigid and they taper to a sharp edge on their ventral surface. This gives rise to their common name as their hardened, laterally compressed bodies look rather like a cut-throat razor.

Growing to around 15cm/6in long they are found in the Indo West Pacific from Tanzania and the Seychelles to Southern Japan and New South Wales, Australia.

Why not check out some of our other Weird fish of the week features?

Tripod fish

Lumpsucker

Ocean sunfish

Two-headed arowana

Stargazers

Giant oarfish

Kroyer's deep sea anglerfish

Halimeda ghost pipefish