Spirula

Spirula spirula is a special kind of deep-water squid-like cephalopod mollusk. It is the only living member of its group, which includes the genus Spirula, the family Spirulidae, and the order Spirulida. Because of the unique shape of its internal shell, it is often called the ram's horn squid or the little post horn squid. One interesting feature of this squid is that it has a light-emitting organ, which sometimes earns it the name tail-light squid.

Because Spirula lives deep in the ocean, scientists and beach visitors very rarely see it alive. However, the small internal shell of this squid is well-known to many beachcombers. The shell is very light, floats easily, and can survive long journeys in the water. That is why it often washes ashore on tropical beaches around the world, and sometimes even on beaches in cooler climates. Shell collectors recognize this beautiful shell as the ram's horn shell or simply as Spirula.

Taxonomy

A famous Swedish scientist named Carl Linnaeus first described a sea creature called Nautilus spirula in his book Systema Naturae in 1758. Later, in 1799, a French scientist named Jean-Baptiste Lamarck gave it the name Spirula. Today, it is still called Spirula spirula. This creature is the only one in its group, called the genus Spirula.

Description

Spirula spirula is a small squid-like creature that is between 35 mm and 45 mm long. It has eight arms and two longer tentacles, all with suckers. These arms and tentacles can be pulled inside its mantle.

One special feature of Spirula spirula is its internal shell, which is shaped like a flat spiral. This shell helps the creature stay floating in the water. Unlike other similar creatures, Spirula spirula can use fluid in its shell for buoyancy.

Behaviour

S. spirula can glow with a green light from a special light organ at the tip of its body, between its ear-shaped fins. Scientists think this helps the squid hide in the deep sea by matching the light from above, keeping it less visible to animals below.

Habitat and distribution

The Spirula lives deep in the ocean during the day, going as deep as 1,000 meters. At night, it swims up between 100 and 300 meters. It likes water around 10°C and is often found near islands and the edges of continents.

This species is mostly seen in warm tropical waters, especially around the Canary Islands. Shells from Spirula are often found on the coasts of South Africa. In 2022, it was also seen in the Arabian Sea. Even in cooler places like the coasts of New Zealand, shells wash ashore, likely carried by ocean currents. In 2020, scientists first saw Spirula in its natural home using a special underwater camera near the northern Great Barrier Reef.

Evolutionary relationships

The order Spirulida includes two groups that no longer exist: Groenlandibelina and Belopterina.

The living species Spirula is thought to be closely related to two ancient groups called belemnites and aulacocerids. Together, these three groups share a close family connection with cuttlefish and true squids.