The Modern Chambered Cephalopod – the Ram's Horn Squid

In ancient oceans, most cephalopods possessed large external shells. Some shells were tightly coiled, while others extended in long, straight forms. Over evolutionary time, however, most modern cephalopods lost these external structures. Today, the vast majority of living members of the subclass Coleoidea—including octopuses and squids—lack an external shell. The only well-known living cephalopod with a prominent external shell is the nautilus, whose appearance differs markedly from that of other modern cephalopods.

This raises an intriguing question: does any living coleoid retain a spiral shell reminiscent of its ancient relatives? Surprisingly, the answer is yes. In the dim waters of the deep ocean lives a small and rather mysterious animal with a rounded body and a faint glow at the tip of its mantle. This creature is the ram's horn squid.

Figure 1. Ecological reconstruction of the ram's horn squid. In reality, Spirula spirula swims in an inverted orientation（圖片來源：Rachel Caauwe，採用 CC BY-SA 3.0 授權）

The ram's horn squid, Spirula spirula, is the only species within the genus Spirula and belongs to the order Spirulida. Its overall body shape resembles that of a typical squid. These animals usually inhabit tropical and subtropical oceans and are commonly found in deeper waters of the mesopelagic zone. During daylight hours they typically remain at depths of roughly 550–1000 meters, while at night they migrate upward to about 100–300 meters to feed.

Compared with most of their squid relatives, ram's horn squids are quite small. The mantle length generally ranges from about 27 to 36 millimeters in collected specimens, though individuals may reach roughly 35–45 millimeters when measured without the tentacles. Like other decapodiform cephalopods, they possess ten appendages: eight arms and a pair of longer tentacles specialized for capturing prey.

Figure 2. Dorsal illustration of Spirula spirula, from the book Die Cephalopoden（圖片來源：Ewald Heinrich Rübsaamen，CC0 1.0 公共領域）

What makes this animal remarkable is its shell. Unlike most modern squids, Spirula spirula retains a tightly coiled shell. However, this shell is not external; it is hidden inside the mantle. The shell is composed of a series of gas-filled chambers arranged in a spiral, giving the species its name. Although small, this chambered structure plays an essential role in buoyancy control.

At first glance, the function of this shell may remind people of the nautilus. Yet the mechanisms used by these two animals differ fundamentally. Nautiluses regulate buoyancy by adjusting the liquid content of certain chambers through a siphuncle system, but only the most recently formed chambers remain actively regulated. Older chambers become sealed during growth. In contrast, the internal shell of the ram's horn squid remains functionally connected along its entire length, allowing the animal to regulate the fluid content of multiple chambers and thereby adjust its buoyancy more dynamically.

Figure 3. Horizontal internal anatomical diagram of Spirula spirula, from the book Die Cephalopoden（圖片來源：Ewald Heinrich Rübsaamen，CC0 1.0 公共領域）

Figure 4. Dorsal internal anatomical diagram of Spirula spirula, from the book Die Cephalopoden（圖片來源：Ewald Heinrich Rübsaamen，CC0 1.0 公共領域）

Beyond the shell, the ram's horn squid possesses several other unusual anatomical features. At the posterior end of the mantle lies a photophore—a specialized light-producing organ. This organ emits a steady glow in the dark ocean environment. When the squid orients itself vertically in the water column, the light points downward, helping to mask its silhouette from predators looking upward. Such counter-illumination is a common camouflage strategy among deep-sea organisms.

The species also shows distinctive morphological traits in its arms and tentacles. The fourth pair of arms is the longest, while the tentacles are slender and bear a club-like structure equipped with numerous small suckers used for grasping prey. Large eyes lacking a cornea and a cylindrical mantle further characterize its anatomy.

Despite being described scientifically as early as 1758 by Carl Linnaeus—who originally named it Nautilus spirula—the biology of this animal has long remained poorly understood. In 1799, the French naturalist Jean-Baptiste Lamarck established the genus Spirula and reassigned the species to its current name, Spirula spirula. For more than two centuries, most scientific knowledge about the species came from empty shells or dead individuals that washed ashore, leaving many aspects of its natural behavior unknown.

Figure 5. Internal shell of Spirula spirula, photographed from a dorsal view（圖片來源：Antonov，CC0 1.0 公共領域）

Figure 6. Internal shell of Spirula spirula, photographed from a lateral view（圖片來源：Antonov，CC0 1.0 公共領域）

Only recently have scientists begun to observe the animal alive in its natural habitat. In 2020, remotely operated vehicles exploring deep waters near Australia recorded the first in-situ footage of a living ram's horn squid at depths exceeding 800 meters. These observations revealed how the animal moves through the water and confirmed its vertical orientation while drifting in the deep sea.

Even with these discoveries, many aspects of the species remain enigmatic. Its reproductive behavior, population structure, and evolutionary relationships within cephalopods are still not fully resolved. Ongoing research continues to uncover new details about this small but fascinating inhabitant of the deep ocean.

Author: Rodrigo

Reference:

Sajikumar, K.K; Rajeeshkumar, M.P; Vellathi, Venkatesan (June 2022). "Rediscovery of Ram's horn squid, Spirula spirula (Cephalopoda: Spirulidae), from the Arabian Sea.