Idiosepius

''Idiosepius'' is a genus of squids in the family Idiosepiidae. Most well known for their incredibly tiny size, these squids range from 10 to 18 mm (0.39 to 0.75 in) in adult specimens. Members of this genus are of no interest to commercial fisheries.Reid, A. 2005. Family Idiosepiidae. ''In:'' P. Jereb & C.F.E. Roper, eds. ''Cephalopods of the world. An annotated and illustrated catalogue of species known to date. Volume 1. Chambered nautiluses and sepioids (Nautilidae, Sepiidae, Sepiolidae, Sepiadariidae, Idiosepiidae and Spirulidae)''. FAO Species Catalogue for Fishery Purposes. No. 4, Vol. 1. Rome, FAO. pp. 208–210. ''Idiosepius'' are found primarily in tropical and temperate waters throughout the Indo-Pacific, often found in association with seagrass and mangrove roots.

Anatomy

''Idiosepius'' have 8 tentacles and 2 arms and have an adhesive organ located on the dorsal mantle side at the posterior end between the fins. Pygmy squid brain structures and nervous system are comparable to other squids and octopuses but they have a more developed optical tract. The beaks of ''Idiosepius'' show small toothlike structures, a genus specific feature which can be useful in distinguishing ''Idiosepius'' species.

Species

The following species are recognised in the genus ''Idiosepius:''
*'' Idiosepius hallami'' Reid & Strugnell, 2018
*'' Idiosepius kijimuna'' Reid, Sato, Jolly & Strugnell, 2023
*'' Idiosepius minimus'' (d'Orbigny in Férussac & d'Orbigny, 1835)Férussac, A. & d'Orbigny, A. (1835). ''Histoire naturelle générale et particulière des Céphalopodes acétabulifères vivants et fossiles''. Paris: Baillière.
*''Idiosepius paradoxus'' (Ortmann, 1888)Ortmann, A. (1888). Japanische Cephalopoden. ''Zoologische Jahrbücher, Systematik'', 3, 639–670
*'' Idiosepius picteti'' (Joubin, 1894)Joubin, L. (1894). Céphalopodes d'Amboine. ''Revue suisse de Zoologie et Annales du Musée d'Historie Naturelle le Genèva'', 2, 23–64.
*'' Idiosepius pygmaeus'' Steenstrup, 1881
*'' Idiosepius thailandicus'' Chotiyaputta, Okutani & Chaitiamvong, 1991

Phylogeny

The major morphological characteristic that separates ''Idiosepius'' from other cephalopods is its adhesive organ. ''Idiosepius'' phylogenetic placement is disputed with early analysis placing ''Idiosepius'' as sister taxa to ''Teuthida'' and recent analysis placing ''Idiosepius'' in Order Sepiolida. Species within ''Idiosepius'' have been morphologically categorized by the shape of the male hectocotylus and arrangement of tentacle suckers. Molecular analysis has separated ''Idiosepius'' species into 4 clades although there has been debate about the number of species of ''Idiosepius''. Molecular and morphological analysis supported the hypotheses that ''I. thailandicus'' and ''I. macrocheir'' were junior synonyms of ''I. biserialis'', however, ''I. biserialis'' and ''I. macrocheir'' are now considered junior synonyms of ''Idiosepius minimu''s.

Reproduction

Most reproductive research has been on female pygmy squids. They are able to spawn multiple batches of eggs without a decrease in quality as they are deposited, and have a preference for spawning in no or low light. A female has the ability to allocate more or less energy to reproductive tissues and the development of eggs depending on the season, which causes slower body growth but larger gonads during colder temperature spawning periods. Cross mating has been observed in ''I. minimus'' and ''I. thailandicus''. Male and female ''Idiosepius'' reproductive structures and organs are the same or similar to most other types of squids.

Reproductive behaviours

Pygmy squids do not display monogamous behaviors such as pairing up or protecting a mate. Instead, they will copulate with many other pygmy squids. Mating patterns vary between species and involve the orientation of the male and female squid (with one or both partners floating above a substrate or one or both partners adhering to a substrate), the posture of each partner (either vertical or horizontal), and the colour patterns of each partner. 4 unique patterns have been observed in ''I. thailandicus'' and 3 have been observed in ''I. minimus''. Idiosepius species differ in how males hold onto females and with which appendages they use to pass spermatophores during copulation. Females also have the ability to remove spermatophores received from males and exhibit a preference for smaller, faster copulating males for reproduction. There is less variation in spawning patterns among species, females either adhere upside down to a substrate to lay eggs, or float near a substrate and swim closer to lay eggs.

Growth and development

''Idiosepius'' eggs range from 0.87 to 0.91 mm (0.032 to 0.036 in) in length and 0.67 to 0.72 mm (0.026 to 0.028 in) in width and the amount of egg layers varies among species. Embryo development typically take less than a month depending on the species and their mortality and gestation period depend on water temperature. Warm temperatures of 30°C (86°F) had no mortalities and the shortest gestation periods, whereas colder temperatures of 20°C (68°F) and lower had higher mortality rates and the longest gestation periods. ''I. paradoxus'' has been found to survive at temperatures as low as 4.5°C (39.74°F). All species of ''Idiosepius'', except ''I. thailandicus'' and ''I. minimus,'' hatch with tentacles. Temperate ''Idiosepius'' species mature slower, grow larger, and have less generations a year compared to tropical species. Sexual dimorphism in ''Idiosepius'' is common and generally females are larger than males.

Behaviour

Adhesion strategy

Pygmy squids adhere to substrates using secretions from the adhesive organ, not using suckers on their appendages. They are thought to temporarily attach to substrates in a similar fashion to gastropods, however the mechanism involved with detaching from a substrate is currently unknown. Adhesion is important during mating, egg deposition, and is a mechanism for avoiding predation.

Habitat preference

''Idiosepius'' are all neritic marine species. They have been found in the Indo-Pacific region, specifically South Africa, Tasmania, Mozambique, Japan, China, Australia, Indonesia, and Thailand. Pygmy squids like to adhere to substrates in seagrass beds, mangroves, seashores, and estuaries. Individuals and eggs are assumed to migrate as the substrate they are adhered to is moved and may also disperse passively as they are caught in sea currents.

Predatory behaviour

Most research into ''Idiosepius'' feeding behaviour has mainly been studied in ''I. paradoxus'' and has been categorized into attacking and eating phases. Pygmy squids attack prey in a similar fashion to cuttlefish, and once their prey has been captured, the squid inserts the buccal mass into the exoskeleton or skin. Once the buccal mass has been inserted, the squid releases cephalotoxin to paralyze the prey, and enzymes to externally digest the flesh. Pygmy squids typically only predate on smaller prey like shrimp, as larger prey can not be paralyzed and are only partially digested. These squids are able to digest so much of the flesh of a shrimp, that what remains often resembles what is left after a moult. ''I. paradoxus'' has been observed to sometimes ink during predation, which may function as a type of smoke screen or distraction when catching active prey.

References

Further reading

* von Boletzky, S., ''et al.'' 2005.   ''Phuket mar. biol. Cent. Res. Bull.'' 66: 11–22.

External links

{{Taxonbar, from=Q15286550

Idiosepiidae
Cephalopod genera
Taxa named by Japetus Steenstrup