Idiosepius paradoxus

Geographic Range

Idiosepius paradoxus, the Japanese pygmy squid, is native to the western Pacific Ocean, near the waters of Japan, northern Australia, and South Korea. It can be found in the whole Indonesian Pacific region ranging from South Africa to Japan to Southern Australia. (Reid, 2005; Shigeno and Masamichi, 2002)


Idiosepius paradoxus is a benthic species living in shallow, inshore waters. (Reid, 2005)

Physical Description

Idiosepius paradoxus can grow up to 16 mm in mantle length. In one study off Japan, female size ranged from 4.2 mm to 18.8 mm in mantle length and 15 mg to 796 mg in wet body weight. The males ranged from 4.2 mm to 13.8 mm in mantle length and wet body weight ranged from 10 mg and 280 mg.

The smallest of the cephalopods, I. paradoxus varies in colors and size. Size and maturation varies with season as they produce two generations a year. They are sexually dimorphic, with the females larger than the males. (Kasugai and Segawa, 2005; Reid, 2005)

  • Sexual Dimorphism
  • female larger
  • Range mass
    0.010 to 0.796 g
    0.00 to 0.03 oz
  • Range length
    4.2 to 18.8 mm
    0.17 to 0.74 in
  • Average length
    16 mm
    0.63 in


Idiosepius paradoxus does not have a larval stage. Instead their development is direct. Having a toothed beak is a sign of pedomorphosis compared to other cephalopods that have toothed beaks in their para-larval forms. (Kasugai, et al., 2003)

  • Development - Life Cycle
  • neotenic/paedomorphic


Sexes are separate and fertilization is internal. One of the male’s arms is differentiated and contains a hectocotylus at the tip. This arm is then jammed into the female’s body cavity. Courting can occur and this is done in this species by means of changing color, body movements or the combinations of both. Male Idiosepius paradoxus mature faster than females in both the cool and warm seasons. (Kasugai and Segawa, 2005)

Males tend to copulate with females while they are laying eggs. Males choose copulation over courtship with females whenever the occasion arises. Since they are quick to act, a male sometimes mistakes other males as females and implants his spermatangia into a male. These pygmy squid become sexually mature after 1.5-2 months. For over a month the female lays 30-80 eggs every 2-7 days when kept in a lab setting. Spawning occurs late February to mid-May and from June to late September. In a natural setting, eggs are laid in a flat mass onto substrates. (Kasugai and Segawa, 2005; Mangold and Young, 2003; Shigeno and Masamichi, 2002)

  • Breeding interval
    twice every year
  • Breeding season
    late February to mid-May and from June to late September
  • Range number of offspring
    30 to 80
  • Range gestation period
    2 days to 7 days days
  • Average age at sexual or reproductive maturity (female)
    1.5 - 2 months
  • Average age at sexual or reproductive maturity (male)
    1.5 - 2 months
  • Parental Investment
  • pre-fertilization
    • provisioning


Idiosepius paradoxus has a life span of 150 days, likely the longest living Idiosepius species. The longer life span is possibly due to Idiosepius paradoxus having a slower growth rate in lower temperatures. They have two generations with differing sizes. In the warm season they become sexually mature faster but are smaller and in the cool season they grow larger over the winter, but take a longer time to become sexually mature. (Kasugai and Segawa, 2005; Sato, et al., 2008; Shigeno and Masamichi, 2002)

  • Average lifespan
    Status: captivity
    150 days


Idiosepius paradoxus stays near the coast and lives in the algae or beds of sea grass such as eelgrass. It adheres to the substrate using an adhesive organ that is located on the dorsal mantle. (Kasugai and Segawa, 2005; Kasugai, 2001)

Communication and Perception

No specific studies have been conducted for Idiosepius paradoxus, but in general, Decapodiformes can change color, body patterns, and texture. These changes can possibly be used to communicate with each other and are used in mating, camouflage, and eluding predators. To see color changes they need to have a well-developed eye and rely on visual sense to locate food. A highly advanced olfactory sense aids them in their benthic lifestyles in the sea grass. (Shigeno and Masamichi, 2002; Young, et al., 2008)

Food Habits

Idiosepius paradoxus prefers to feed on crustaceans, gammarids, grass shrimp, and mysids. Although an initial study concluded I. paradoxus does not attack fish, a later study showed the contrary. When attacking fish the pygmy squid usually only eats the muscle mass and leaves the bones intact, usually as complete skeletons. Idiosepius paradoxus cannot completely paralyze larger fish and ingests only part of the fish.

The feeding habits have been described in detail in the literature, with two phases: 1) attacking, which includes attention, positioning, and seizure, and 2) eating.

Once I. paradoxus sees its prey it approaches with arms facing the hard shell of the crustacean until it gets to an attacking distance of less than 1 cm. The Japanese pygmy squid attacks very fast, and captures the prey with tentacles grabbing at the junction between the crustacean's shell and its first abdominal segment pulling the crustacean into its arm crown.

Idiosepius paradoxus will attack prey up to twice its size. The pygmy squid paralyzes shrimp within one minute, using a cephalotoxin. The prey must be held in the right position otherwise it will not be paralyzed, and I. paradoxus must shift where it is grasping the prey. On occasion more than one pygmy squid will attack the same prey. Usually the first attacker will get the meal. After capturing the prey, I. paradoxus swims back to sea grass to attach while it eats.

After capturing a crustacean, I. paradoxus inserts its buccal mass into the exoskeleton. The squid elongates the buccal mass to about the same length of its arm, and wiggles the mass around in all directions inside the crustacean's exoskeleton. While doing this, I. paradoxus ingests the flesh of the crustacean, and then discards it, leaving the exoskeleton completely empty yet intact. The crustacean’s perfectly intact exoskeleton looks like the organism has simply molted. The exoskeleton is usually emptied in 15 minutes for mysids, while the larger prey are sometimes not finished, leaving flesh attached to the exoskeleton.

Idiosepius paradox may externally digest its food first. External digestion makes it easier for the toothed beak to assist in shredding the flesh of a crustacean, which is removed using the buccal mass and enzymatic action. This enzyme is injected into the flesh allowing I. paradoxus to suck up semi-digested flesh. The specialized outer lip seems to be the organ assisting in external digestion. The lip contains goblet glandular cells in the lip gland that produces a mucous secretion. While eating the beak is moving in the buccal mass but it never passes the lips so it does not bite into the flesh. (Kasugai, 2001; Kasugai, et al., 2003)

  • Animal Foods
  • mollusks
  • aquatic crustaceans
  • other marine invertebrates


Idiosepius paradoxus can be eaten by fish, birds, marine mammals, and other cephalopods. (Stowasser, et al., 2007)

  • Known Predators

Ecosystem Roles

Idiosepius paradoxus plays a role in being part of the food chain by being eaten and eating crustaceans and fish. (Kasugai, et al., 2003)

Economic Importance for Humans: Positive

Idiosepius paradoxus are easily harvested. The Idiosepiidae are good experimental animals because they have short life spans, are easily maintained, and readily reproduce in the lab. These animals are currently used to study reproduction and nervous systems but also have the potential for studies on age-related and/or hereditary problems. (Myers, 2007; Shigeno and Masamichi, 2002)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

There are no known adverse effects of Idiosepius paradoxus on humans.

Conservation Status

They are present in large quantities, and will breed in captivity. (Kasugai, et al., 2003; Myers, 2007)


Sirisha Bupathi (author), Rutgers University, Kaycee Coleman (author), Rutgers University, David V. Howe (editor), Rutgers University, Renee Mulcrone (editor), Special Projects.



Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map


Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


an animal that mainly eats meat


uses smells or other chemicals to communicate


the nearshore aquatic habitats near a coast, or shoreline.


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.


eats mollusks, members of Phylum Mollusca


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


Referring to a mating system in which a female mates with several males during one breeding season (compare polygynous).

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female


uses touch to communicate


uses sight to communicate


Kasugai, T., S. Shigeno, Y. Ikeda. 2003. Feeding and external digestion in the Japanese pygmy squid Idiosepius paradoxus. Journal of Molluscan Studies, 70: 231-236. Accessed December 22, 2010 at

Kasugai, T. 2001. Feeding behavior of the Japanese pygmy cuttlefish Idiosepius paradoxus (Cephalopoda: Idiosepiidae) in captivity: evidence for external digestion?. Mar. Biol. Ass. UK, 81: 979-981. Accessed December 22, 2010 at

Kasugai, T., S. Segawa. 2005. Life cycle of the Japanese pygmy squid Idiosepius paradoxus (Cephalopoda: Idiosepiidae) in the Zostera beds of the temperate coast of central Honshu, Japan. Phuket mar. biol. Cent. Res. Bull., 66: 249-258. Accessed December 22, 2010 at

Mangold, K., R. Young. 2003. "Idiosepiidae" (On-line). Accessed December 22, 2010 at

Myers, P. 2007. "Cephalopod development and evolution" (On-line). Accessed December 22, 2010 at

Reid, A. 2005. Family Idiosepiidae. In P. Jereb & C.F.E. Roper, eds., Cephalopods of the World. FAO Species Catalogue for Fishery Purposes, 1 (4): 208-210. Accessed February 28, 2011 at

Sato, N., T. Kasugai, H. Munehara. 2008. Estimated life span of the Japanese pygmy squid, Idiosepius paradoxus from statolith growth increments. Journal of the Marine Biological Association of the United Kingdom, 88: 391-394. Accessed February 28, 2011 at

Shigeno, S., Y. Masamichi. 2002. Organization of the nervous system in the pygmy cuttlefish, Idiosepius paradoxus Ortmann (Idiosepiidae, Cephalopoda). Journal of Morphology, 254: 65-80. Accessed January 08, 2011 at

Stowasser, G., G. Pierce, J. Wang, M. Santos. 2007. "An Overview of Cephalopods Relevant to the SEA 5 Area" (On-line). Accessed January 08, 2011 at

Young, R., M. Vecchione, K. Mangold. 2008. "Decapodiformes Leach, 1817. Squids, cuttlefishes and their relatives" (On-line). Tree of Life Project. Accessed February 28, 2011 at