Weird fish of the week: Grunt sculpin

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This week's weird fish is the Grunt sculpin, Rhamphocottus richardsonii – a rather beautiful, almost dog-like, little marine fish that uses its fins to hop or walk along the seabed. Bob Mehen has the details – and check out the video!

The more I look at weird fish, the less weird they look to me. I'm getting used to huge teeth, distended jaws, bulbous eyes, capacious stomachs and the many other bizarre features seen to such an extent that it now seems to me that a cod is a bigger ocean oddity than a Goblin shark.

However there are still many strange looking species waiting to surprise us with their perplexing piscine peculiarity, and Rhamphocottus richardsonii, commonly known as the Grunt sculpin, is one of them.

Monotypic families are often a rich hunting ground when it comes to strange and unusual fish species, so it's no surprise that the Grunt sculpin is the only member of the Rhamphocottidae – clearly taxonomists agree it's a bit different to other Scorpaeniformes.

Growing to little more than 10cm/4" they are odd, hunch-backed little fish, with over half their length made up by their armoured heads.

Their entire body is covered in multi-spined plates, the tips of which protrude through the skin, making this otherwise tempting morsel of a fish a prickly mouthful for potential predators.

Their common name comes from the grunting noise they emit when frightened or removed from the water which is itself another useful deterrent.

They are very poor swimmers, preferring to hop or walk along the sea bed with the aid of specially adapted thickened pectoral fin rays, but this lack of Michael Phelps like aquatic propulsion is explained when you see the sculpin at home. They spend much of their time within the empty shells of the Giant acorn barnacle, Balanus nubilus.

Inside these armoured retreats their odd shape makes far more sense, with their bony head filling the opening and making the dead barnacle look like all the other closed barnacles in the colony – and if the fish dives into its shelter head first, its tail is adapted to look like the feeding fans of a living barnacle.

These empty shells provide an excellent place for the fish to lay and guard its eggs.

Observations of the breeding behaviour of captive specimens report the pugnacious females chasing and holing up prospective partners when ready to breed. The female then guards the fertilised eggs until they hatch.

In the absence of barnacles they will set up home in anything from suitably sized shells to tin cans and bottles.

They typically live in the inter-tidal temperate waters of the North Pacific, but are found at greater depths of up to 200m in their most southerly range as they prefer cooler water. Their diet consists of small crustaceans, zooplankton and fish larvae.