Spirula

The ram's horn squid, known scientifically as Spirula spirula, is a unique deep-water cephalopod.

This mollusk is the only living member of its genus, family, and order. Because of its distinct internal shell, people often call it the ram's horn squid or the little post horn squid. It is also sometimes called the tail-light squid due to its ability to glow. While the live animal is rarely seen in its natural habitat, its shell is a common find for beachcombers.

These shells are extremely light and buoyant, allowing them to float to shores worldwide.

The body of the squid is relatively small, measuring between 35 mm and 45 mm in length. As a decapod, it possesses eight arms and two longer tentacles, all of which feature suckers. These appendages can be pulled completely inside the mantle. Interestingly, this species lacks a radula, which is a specialized feeding organ found in many other mollusks.

The squid's most remarkable feature is its internal buoyancy organ. This is a chambered, coiled shell that sits inside the animal.

This internal shell is shaped like an open planispiral, which is a flat spiral where the coils do not touch. The shell consists of two prismatic layers and is mineralized. Mineralization is a trait shared only with cuttlefish and the nautilus among living cephalopods. The shell works through an osmotic process to control buoyancy. Unlike a nautilus, which only exchanges gas and liquid in its three most adoral chambers, the Spirula can move cameral fluid into all of its chambers.

This allows the animal to manage its position in the water column very effectively.

To protect itself, the squid uses a method called counter-illumination. It has a light-emitting organ called a photophore located at the tip of its mantle. This photophore sits between the animal's ear-shaped fins and emits a green light.

Observations show the squid often holds a vertical stance with its head up and the photophore pointing downward. This light likely helps it blend into the light coming from above, hiding it from predators below.

The history of naming this species involves two famous naturalists. In 1758, the Swedish botanist Carl Linnaeus first described the animal as Nautilus spirula in his book Systema Naturae. Later, in 1799, the French naturalist Jean-Baptiste Lamarck established the genus Spirula. He transferred the species into this new genus, creating the name Spirula spirula used today. A morphometric study from 2010 noted that shell characteristics can vary based on geography, though no new species were identified.

Spirula spirula lives a life dictated by the depth of the ocean. During the day, it inhabits deep waters reaching 1,000 m. At night, it rises to depths between 100 m and 300 m. The squid prefers water temperatures around 10 °C and lives near oceanic islands and continental shelves. While many consider it a tropical species, its shells wash up on temperate beaches, such as those in New Zealand.

These shells likely travel great distances via ocean currents due to their high buoyancy.

Scientists have found this species in many locations, including the subtropical seas near the Canary Islands. Their shells are also frequently found along the western coasts of South Africa. In 2022, records confirmed their presence in the Arabian Sea. Although they live in the deep, they occasionally enter the upper 10 m of the sea. This is evidenced by the fact that they are sometimes found in the guts of albatross birds.

In 2020, an ROV from the Schmidt Ocean Institute recorded the squid in its natural habitat near the northern Great Barrier Reef.

Evolutionary studies link the Spirula to ancient groups of cephalopods. The order Spirulida includes two extinct suborders: Groenlandibelina and Belopterina. Spirula is believed to be the closest living relative to the extinct belemnites and aulacocerids. These groups are all closely related to modern cuttlefish and true squids. This connection helps scientists understand how these complex marine animals have changed over millions of years.