Ram’s horn: a relict from the past

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On these bleak winter days, few varieties of shell survive the white wrath of New Zealand’s West Coast. However, one that is com­monly picked out from among the wrack and foam is the delicately coiled ram’s horn, Spirula spir­ula—an object that has a number of claims on our interest.

Although the shell is common on exposed coasts in many tropical and subtropical seas, and has been known for eons, the animal that manufactures it was a mystery until com­paratively recently. The first shell with animal largely intact was de­scribed from Wellington in 1845, but was considered far too precious to dissect.

By 1900, five animals had been received worldwide, but none had been seen alive. Then, during the 1920s-’40s, a number of live specimens were encountered in the Atlantic during mid-water research trawling, and serious study of the crea­ture began at last.

That there is something peculiar about the shell of Spirula has been learnt by many a chubby-fingered youngster. Squeeze an ordinary snail shell and it will disintegrate into a disappointing assortment of sharp fragments. But a ram’s horn is quite differ­ent. It is a spiral train of pearly compartments, with a very fine calcareous tube running along the inside of the spiral. The chambers‑ and there can be up to 40—can be dismantled one by one. They contain no obnoxious residues of flesh, no water—in fact, nothing.

The only other vaguely similar living shell is produced by the chambered Nautilus, but tens of millions of years ago, the oceans teemed with their ancestors—ammonites and torpedo-like belemnites. Abundant fossils testify to the past success of the group. Some of the ammonites had massive heavy shells a metre or more in diameter. Both ammonites and Spirula are Cephalopods—that order of molluscs spe­cialised for fast movement. The main contemporary Cephalopods are squids and octopi, with Nautilus, Spirula and cuttlefish fascinating relicts from an ancient world.

The living Spirula  resembles a large fat thumb, with a cluster of eight short, solid tentacles and two somewhat longer ones at the head end. There are few clues that the animal contains a shell, since it is com­pletely internal, located near the rear end. Like many Cephalopods, Spirula lacks a radula (teeth) but has a squat bird-like beak that can administer a sharp bite. The animal feeds on small crustaceans, which are thought to be seized by the tentacles and chopped up by the beak.

During the day, Spirula lives in mid-water, at depths of 600-1000m; at night, it migrates up to 250m or less. The shell is important in this daily migration, acting as a flotation device. A muscu­lar animal usually weighs 4-5% of its weight in air when weighed in water, yet Spirula weighs nothing. The shell, with its water-tight compartments containing gas at something less than atmospheric pressure, gives  the animal neutral buoyancy and saves it energy as it rises and falls in the water column.

However, the shell also poses a risk. Water pres­sure-75 times atmos­pheric pressure at a depth of 700m—could crush the shell. Experiments suggest Spirula sails pretty close to the wind in this respect. Live specimens have been recovered from 1000m, which is enough to crush most test shells retrieved from beaches. Live shells must be a bit tougher.

Of course, it is the buoyancy of the shell that allows us to recover it. Almost alone among empty shells, Spirula floats—to be blown about by every gale, and finally dumped on our shores.

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