Range: Subtropical and temperate marine waters worldwide

Habitat: Mesopelagic waters 100-1000 meters deep

Size: Up to 2 inches (5 cm)

Diet: Zooplankton

Threats: Large fish and sea birds

Lifespan: Up to twenty months

Transcript:

Today on Animal Fact Files we’re discussing the ram’s horn squid. Although they have the common name squid, these cephalopods aren't true squid. They have an internal, spiraling shell that looks a lot like the horn of a ram, hence the common name! Their shell makes them superficially similar to ammonites from fossil history, but these animals aren't that closely related. The ammonite shell was external while a ram’s horn squid’s shell is internal, encased in a muscular mantle. This invertebrate’s mantle is reddish brown with a light producing organ found at its apex, between the squid's fins. The light is pale green and may be used for communicating… or camouflage… or something else entirely; its purpose isn't fully understood.

While you may have seen a ram's horn squid's shell washed up on a beach shore, it's unlikely you've ever seen one alive. They're rarely observed. In fact, until 1922 they were only known by science from 13 individual specimens. This squid's coiled shell forms more than 30 chambers by adulthood and the coils don't touch. Of course, the shell isn't visible while they're alive. Some early drawings show it as visible near the top of the mantle, but these were based on specimens dragged in through trawling nets who were often damaged. Most observations of these animals come from these damaged specimens.

Ram's horn squid live in oceans around the world sticking to subtropical and temperate areas. They don't occur near the equator, so the southern hemisphere and northern hemisphere populations are separated. Although they're currently listed as a single species, this separation, as well as observations that groups from different regions are of differing sizes, could suggest there is more than one species alive today, but more research is needed. Most ram's horn squid have a mantle length no longer than 2 inches, 5 centimeters. That's not including the arms and tentacles which can essentially double that length. Still, they're small creatures.

A ram's horn squid has 8 arms with two longer tentacles. The arms are equipped with suction cups. Early descriptions of this cephalopod suggest they often swam with their head facing downwards and mantle facing upwards with their light organ aimed towards the sky, but more recent observations of living ram's horn squid suggest they instead rest the opposite direction with the mantle and light-producing organ facing downwards and the head and tentacles facing the sky. This could be a countershading measure. When viewed from below, the light would break up the squid's outline making it more difficult for predators to spot.

Predators to ram's horn squid include many fish species, such as swordfish, as well as marine birds– which suggests they, at least sometimes, travel towards the top of the water column. Like a nautilus, a ram's horn squid's shell provides buoyancy through gas and water exchange. This is what allows the animal to sink, float, or remain neutrally buoyant in the water column. Of course their fins help keep them stabilized as well. Ram's horn squid spend the day in deeper waters reaching 1,600 to 3,200 feet, 500 to 1,000 meters, deep. At night, they travel upwards to feed at depths of about 300 to 1,000 feet, 100 to 300 meters. These cephalopods eat zooplankton such as small fish and fish larvae as well as copepods and other small crustaceans.

Ram's horn squid are separated into males and females, but how they find each other to mate in the vast ocean is still a mystery. One hypothesis is that these animals actually form schools and travel in groups. Like many cephalopods, the male ram’s horn squid has a specialized arm for delivering sperm to the female. These are transferred directly by him placing this special arm into a specific spot inside the female's mantle. This squid’s eggs have not been observed floating in the water. They're believed to be laid in mud along the continental slope, usually even deeper than they typically swim during the day. Young ram's horns hatch with a tiny shell that has only two chambers. As the squid grows, more chambers are added. It's currently believed ram's horn squid reach reproductive maturity at just over an inch long– around 30 millimeters in length. This usually occurs when they're around 12 to 15 months old. Like many cephalopods, these invertebrates are semelparous, so they die shortly after mating and producing offspring. They're believed to live between one and two years old.

Aside from their camouflaging light, another way a ram's horn squid can protect itself is by releasing gray ink. This is similar to a lot of other members of their class. Something else they can do that's not so common among their relatives is pull their head and arms into their mantle. While in this position, they kind of look like a floating egg!

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