Spirula spirula, often called the ram’s horn squid or tail-light squid, is a deep-sea cephalopod. While live specimens are seldom observed due to their deep-ocean habitat, their distinct internal shells frequently wash ashore on tropical and subtropical beaches worldwide, even reaching temperate zones like New Zealand. These remarkably light, buoyant, and durable shells are a familiar sight to many beachcombers.
Its Distinctive Shell and Anatomy
The Spirula’s internal, coiled, chambered shell is its most distinctive feature. This shell functions as a buoyancy organ, composed of two prismatic layers and shaped as an open planispiral, meaning its coils do not touch. Unlike nautiluses, which can only exchange fluid in their three most anterior chambers, Spirula can regulate fluid in all of its chambers to control buoyancy. The shell is mineralized, a feature shared among extant cephalopods only with cuttlefish and nautiluses.
This cephalopod has a squid-like body, typically measuring 35 to 45 millimeters in length. It possesses eight arms and two longer tentacles, all equipped with suckers, which can be fully retracted into its mantle. A large bioluminescent organ, or photophore, is located at the posterior end of its mantle, between two small, round fins. This organ can emit a pale, yellowish-green light for several hours.
Life in the Deep Ocean
Spirula spirula inhabits the mesopelagic and bathypelagic zones of tropical and subtropical oceans, preferring temperatures around 10°C. During the day, it resides at depths between 550 to 1,000 meters. As night falls, it undertakes a vertical migration, ascending to shallower depths, between 100 to 300 meters.
This cephalopod is a nocturnal hunter, feeding on small crustaceans, fish, and other marine invertebrates. Its beaked mouth processes food. The Spirula maintains a characteristic “head-down, tail-up” orientation in the water, influenced by its buoyancy mechanism and the fluttering motion of its posterior fins. The upward-pointing photophore on its tail is thought to employ counter-illumination, a camouflage strategy where light is produced to blend its silhouette against the faint light filtering down from the surface, potentially masking the movement of its fins.
A Link to Ancient Cephalopods
Spirula spirula is the sole living member of the order Spirulida, making it a “living fossil.” It offers insights into the evolutionary transition from ancient shelled cephalopods to their modern soft-bodied relatives. Its internal, chambered shell is a remnant of its shelled ancestors, like ammonites and belemnites.
The fossil record reveals a progressive coiling of the conch in the lineage leading to Spirula, with forms such as Groenlandibelidae and Belopteridae preceding it. Spirula is considered the closest living relative to the extinct belemnites and aulacocerids. These groups are closely related to modern cuttlefish and true squids. Its unique shell structure and marginal siphuncle provide clues to the ancestry of the Coleoidea subclass from Paleozoic Bactritoidea.