Spirula spirula

The distribution of Spirula spirula is poorly known. These mollusks are generally found in tropical waters, including the waters off the coasts of Indonesia, New Zealand, south Africa, northwestern Africa, the Canary Islands, and in the Gulf of Mexico. Nesis (1987) described this species as "tropical Atlantic and Indo-West Pacific mesopelagic nerito-oceanic." ("Ram's Horn Squid- Wikipedia, the free encyclopedia", 2005; Nesis, 1987; Turk, The Cornwall Trust for Nature, LTD; "Ram's Horn Squid- Wikipedia, the free encyclopedia", 2005; Nesis, 1987; Turk, The Cornwall Trust for Nature, LTD)

Spirula spirula is most often found on continental shelves at depths ranging between 500 and 1000 m during the day. Because of their hunting patterns, these cephalopods are found closer to the surface at night, at depths between 100 and 300 m. (Hanlon and Messenger, 1996; Moynihan, 1985; Nesis, 1987)

Members of the genus Spirula are decapods characterized by suckered appendages, including 8 arms and 2 longer tentacles. They are somewhat squid-like in appearance, and young individuals can completely withdraw the head and all extremities into the mantle. Adults measure 30 to 45 mm in length, and can only retract the cephalic area halfway into the mantle.

The skin is reddish-brown and smooth. Members of the genus Spirula have a large photophore (bioluminescent light organ) at the posterior end of the mantle which is surrounded by two small, round fins. The photophore can remain illuminated for several hours.

The shell of S. spirula in entirely enclosed in the mantle. It is divided into approximately 25 to 37 chambers connected by a siphuncle. This shell serves as a hydrostatic system, allowing and animal to control its buoyancy. The shell is located in the posterior half of the mantle, and its buoyancy pattern results in a characteristic "head down" positioning often observed in Spirula. (Cousteau and Diole, 1973; Hanlon and Messenger, 1996; "Ram's Horn Squid- Wikipedia, the free encyclopedia", 2005; Nesis, 1987; Turk, The Cornwall Trust for Nature, LTD; Ward, 1987)

Individuals measuring about 2 mm hatch from fertilized eggs. The young are independent, and no pelagic eggs have ever been identified. It is theorized that females lay eggs at the benthic layer. Capture of young hatchlings that closely resemble adult forms at this lower layer lends support to this idea.

Sexes, male and female, are spearate. The process for sex determination in Spirula has not been determined. (Hanlon and Messenger, 1996; "Ram's Horn Squid- Wikipedia, the free encyclopedia", 2005; Nesis, 1987)

Females are slightly larger than males, and as of yet no courtship rituals been identified in ram's horn squids. The breeding season for this species is unknown. The social structure in Spirula is also unknown.

Reproduction in Spirula is similar to reproduction in most cephalopods, where a modified tentacle on the male (the hectocotylus) is used to implant a sperm sac into the seminal receptacle in buccal membrane of the female's mantle during mating. The time until hatching is unknown for Spirula. (Hanlon and Messenger, 1996; "Ram's Horn Squid- Wikipedia, the free encyclopedia", 2005; Nesis, 1987)

The breeding patterns in S. spirula are unknown. ("Ram's Horn Squid- Wikipedia, the free encyclopedia", 2005; Nesis, 1987)

Females provide eggs with the nutrients needed to reach hatching successfully. Newly hatched offspring are independent. Any other details on the parental behavior of this species are lacking in the literature. (Hanlon and Messenger, 1996; "Ram's Horn Squid- Wikipedia, the free encyclopedia", 2005; Nesis, 1987)

Average life span for S. spirula is 1 to 1.5 years; very few specimens are captured and captivity life span is unknown. (Nesis, 1987)

Spirula spirula is a motile species. Depth distribution varies according to time of day, with peak activity and lowest depth range occuring at night.

These animals have a characteristic "head-down" swimming position caused by their buoyancy mechanisms. The swimming style for S. spirula also contributes to this head down position, as the fluttering motion of the posterior fins point the animal in a downward direction.

Spirula spirula is able to withdraw its head and appendages about halfway into its mantle, and has highly mobile irises as is characteristic of many decapods. (Hanlon and Messenger, 1996; "Ram's Horn Squid- Wikipedia, the free encyclopedia", 2005; Nesis, 1987; Turk, The Cornwall Trust for Nature, LTD)

The average home range for S. spirula is not well known, inasmuch as its total marine distribution is largely unknown. ("Ram's Horn Squid- Wikipedia, the free encyclopedia", 2005; Nesis, 1987)

Although specific means of communication are relatively unknown for this species, its light-emitting organ may be a key source of communication between these animals. At least during mating, some physical contact and communication must occur between males and females, as the male must inseminate the female. In addition to this, S. spirula may employ chemical signalling. (Durr, et al., 2003; Hanlon and Messenger, 1996; "Ram's Horn Squid- Wikipedia, the free encyclopedia", 2005; Nesis, 1987)

In general, the specific feeding habits of S. spirula resemble that of the family Sepulidae. Spirula spirula hunts nocturnally, probably consuming small fish and crustaceans. The feeding apparatus of S. spirula consists of a beaked mouth containing a radula, towards which food is propelled by the tentacles.

Members of the Sepulidae typically consume 30 to 60% of their body weight per day, so it can be assumed that the general food intake for S. spirula, while perhaps not being quite as high, is a sizeable amount. (Durr, et al., 2003; Hanlon and Messenger, 1996; "Ram's Horn Squid- Wikipedia, the free encyclopedia", 2005; Nesis, 1987; Turk, The Cornwall Trust for Nature, LTD)

Known predators for S. spirula include great-winged petrels, splendid alfoniso fish, and swordfish. Whales may also be one of the main predators of cephalopods, including S. spirula.

In other cephalopods, predator evasion mechanisms include photophores and bioluminescence, which could account for the presence of the bioluminescent organ in S. spirula. However, details of how this may be used are not available. (Durr, et al., 2003; Hanlon and Messenger, 1996)

Spirula spirula is a common food source for swordfish, and may also provide nourishment for marine animals such as whales and other carnivores. (Durr, et al., 2003; Hanlon and Messenger, 1996; "Ram's Horn Squid- Wikipedia, the free encyclopedia", 2005; Nesis, 1987; Turk, The Cornwall Trust for Nature, LTD)

This species is a common food source for swordfish (Xiphias gladius), so that it impacts swordfish populations and therefore the commercial swordfish market. (Durr, et al., 2003; "Ram's Horn Squid- Wikipedia, the free encyclopedia", 2005; Nesis, 1987)

There are no known adverse effects of S. spirula on humans. (Durr, et al., 2003; Hanlon and Messenger, 1996; "Ram's Horn Squid- Wikipedia, the free encyclopedia", 2005)

Spirula spirula is not listed under any of the databases for endangered species.