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Idiosepius paradoxus

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Species of mollusc
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Idiosepius paradoxus
Conservation status

Data Deficient  (IUCN 3.1)
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Order: Idiosepida
Family: Idiosepiidae
Genus: Idiosepius
Species: I. paradoxus
Binomial name
Idiosepius paradoxus
(Ortmann, 1888)
Synonyms
  • Microteuthis paradoxus
    Ortmann, 1888
  • Idiosepius pygmaeus paradoxus
    (Ortmann, 1888)

Idiosepius paradoxus, also known as the northern pygmy squid, is a species of pygmy squid native to the western Pacific Ocean. This species can be found inhabiting shallow, inshore waters around central China, South Korea, and Japan.

Appearance

This species is part of the smallest known squid genus, growing only to 16 mm (0.63 in) in mantle length. Females tend to be larger than males, and the presence of nidamental glands for secretion of egg jelly differentiates them from males. Males can be identified by the presence of a singular white testis found posteriorly in the body. Both sexes have a unique organ found on their dorsal mantle for binding themselves to a substrate, such as seagrass. The type specimen was collected off Kadsiyama in Tokyo Bay and is conserved at the Musee Zoologique in Strasbourg.

Distribution and habitat

I. paradoxus is found farthest North of all the species in genus Idiosepius. The distribution of this species includes the waters off South Korea and northern Australia, as well as the Japanese islands of Honshu, Kyushu, and southern Hokkaido. In these locations, this squid can be found in the demersal zone of the ocean in subtropical climates. It resides in algae, seagrass, and seaweed. Based on season within its distribution, there are two recorded life histories. Small type squid hatch between March and July and spawn between June and September, while large type squid hatch in the summer and spawn during the next spring, and have longer reproductive seasons.

Diet

This species feeds mainly on small fish, shrimp, and other organisms. They can prey on larger fish, but may not be able to fully digest all of the flesh. Just as they utilize external fertilization, these squid can externally digest their food as well. Anatomically, they possess both a beak and a buccal mass. The buccal mass is used to break through hard exoskeletons, secrete digestive fluids, and then remove the softened flesh of prey. With small prey, the beak is not needed for biting or severing any body parts. There is also evidence of a sex-specific cognitive bias in I. paradoxus, such that females more often than males overestimate the size of prey they will be able to successfully attack.

Reproduction and sexual selection

Copulation and spawning

I. paradoxus mates through a polyandrous system, in which females copulate with numerous males. The focus of existing research seems to be on female promiscuity, with little information on how many partners are acquired per male. Males do not exhibit precopulatory behaviors, such as male-male competition, so a male and female will mate upon finding one another. The male grasps the female in a head-to-head position using his right hectocotylus, which he also uses to point towards her arm crown externally. He then uses his left hectocotylus to grasp spermatophores containing sperm from his funnel, post ejaculation. The spermatophores become spermatangia through an eversion reaction involving the ejaculatory apparatus and cement body, and are then placed on the female's body directed by the groove on the right hectocotylus. The sperm become activated by seawater, and will swim to the seminal receptacle around the female's buccal mass on the ventral mantle, where they will be stored until spawning and fertilization. In this species, sperm form swarms when swimming from spermatangia to the seminal receptacle. The receptacle does not become full until about 8 copulations, after which no more sperm can be held until spawning has occurred.

The female can mate with multiple males and retain sperm from each before adhering to a substrate such as seagrass to spawn. Once ready, the female releases egg jelly from her nidamental gland, out her funnel, and into her arms, followed by a single egg that she attaches to the substrate. During attachment, she covers the egg with her buccal membrane so that sperm can be passed from the seminal receptacle to the egg through individualized external fertilization. This process is repeated for multiple rows of eggs. Each egg is wrapped in 8–10 gelatinous layers, and these may function in protection against small microorganisms. Females may spawn several times in this fashion from a full seminal receptacle.

Cryptic female choice

I. paradoxus has been used in the study of cryptic female choice due to male sperm transfer to an external location on the female's body, making this process more easily observable than in other species. Once the female has mated, either with one male or several, she can use her buccal mass to pull spermatangia off her body individually to get rid of as many as she chooses. By removing spermatangia, she is choosing which male(s) will have greater opportunity to sire her offspring. In this way, postcopulatory mechanisms in this species can act as sexual selection for certain traits in males.

There is evidence from multiple studies that smaller males are preferred by females, meaning females remove more spermatangia from larger males. However, whether females prefer longer or shorter copulations is debated. Some evidence points towards preference for longer copulating males, while other evidence shows preference for decreased copulation times. It has been observed that sperm transfer to the seminal receptacle from the spermatangium must occur within 24 hours, as almost all spermatangia in the species discharged their spermatozoa within 24 hours. In the beginning, rapid sperm discharge is observed, but after 5 minutes it becomes intermittent. A possible explanation for female preference of small and fast copulating males could be that predation risk is decreased with shorter time spent in copula and less attention drawn with smaller body size. Predation could actually act as a selection pressure for increased postcopulatory versus precopulatory behaviors in I. paradoxus, as postcopulatory behaviors like cryptic female choice may draw less attention and be easier to exhibit when also under threat of predation. In addition, there is evidence that cryptic female choice could be adaptive when comparing populations experiencing high versus low predation: the population more often exposed to predation may be able to carry out cryptic female choice as usual without decreasing the behavior, unlike in populations which are not used to predation.

While females are able to exhibit choice, males engage in behaviors to give their sperm the best chance of making it to the female's seminal receptacle. During copulation, males have been observed directing their spermatangia by the right hectocotylus to different locations on the female's body (such as different arm crown bases) per spermatophore ejaculation. This is thought to reduce the number of spermatangia removed by the female from a particular male, as their spermatangia are spread to multiple locations that the female may be unaware of.

References

  1. Barratt, I.; Allcock, L. (2012). "Idiosepius paradoxus". IUCN Red List of Threatened Species. 2012: e.T162657A937941. doi:10.2305/IUCN.UK.2012-1.RLTS.T162657A937941.en. Retrieved 19 November 2021.
  2. ^ Kasugai, Takashi; Segawa, Susumu (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 Marine Biological Center Research Bulletin. 66: 249–258.
  3. ^ 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.
  4. ^ Sato, Noriyosi; Yoshida, Masa-Aki; Fujiwara, Eiji; Kasugai, Takashi (2013). "High-speed camera observations of copulatory behaviour in Idiosepius paradoxus: function of the dimorphic hectocotyli". Journal of Molluscan Studies. 79 (2): 183–186. doi:10.1093/mollus/eyt005. hdl:10069/32938. Retrieved 2023-02-17.
  5. ^ Iwata, Yoko; Sato, Noriyosi; Hirohashi, Noritaka; Kasugai, Takashi; Watanabe, Yoshiro; Fujiwara, Eiji (2019-01-21). "How female squid inseminate their eggs with stored sperm". Current Biology. 29 (2): R48–R49. doi:10.1016/j.cub.2018.12.010. ISSN 0960-9822. PMID 30668945. S2CID 58949271.
  6. ^ Sato, Noriyosi; Yoshida, Masa-aki; Kasugai, Takashi (2016-11-17). "Impact of cryptic female choice on insemination success: Larger sized and longer copulating male squid ejaculate more, but females influence insemination success by removing spermatangia". Evolution. 71 (1): 111–120. doi:10.1111/evo.13108. ISSN 0014-3820. PMID 27805265. S2CID 8866473.
  7. Current Classification of Recent Cephalopoda
  8. Sato, Noriyosi; Awata, Satoshi; Munehara, Hiroyuki (2009-06-01). "Seasonal occurrence and sexual maturation of Japanese pygmy squid (Idiosepius paradoxus) at the northern limits of their distribution". ICES Journal of Marine Science. 66 (5): 811–815. doi:10.1093/icesjms/fsp145. hdl:2115/43087. ISSN 1095-9289.
  9. Sato, Noriyosi (2017-11-06). "Seasonal changes in reproductive traits and paternity in the Japanese pygmy squid Idiosepius paradoxus". Marine Ecology Progress Series. 582: 121–131. doi:10.3354/meps12338. ISSN 0171-8630.
  10. ^ KASUGAI, T. (2004-08-01). "Feeding and External Digestion in the Japanese Pygmy Squid Idiosepius Paradoxus (Cephalopoda: Idiosepiidae)". Journal of Molluscan Studies. 70 (3): 231–236. doi:10.1093/mollus/70.3.231. ISSN 1464-3766.
  11. Takeshita, Fumio; Sato, Noriyosi (2016). Herberstein, M. (ed.). "Adaptive Sex-Specific Cognitive Bias in Predation Behaviours of Japanese Pygmy Squid". Ethology. 122 (3): 236–244. doi:10.1111/eth.12464.
  12. ^ Sato, Noriyosi; Kasugai, Takashi; Munehara, Hiroyuki (2014-06-01). "Spermatangium formation and sperm discharge in the Japanese pygmy squid Idiosepius paradoxus". Zoology. 117 (3): 192–199. doi:10.1016/j.zool.2014.02.001. hdl:10069/34531. ISSN 0944-2006. PMID 24813852.
  13. ^ Sato, N.; Kasugai, T.; Ikeda, Y.; Munehara, H. (2010). "Structure of the seminal receptacle and sperm storage in the Japanese pygmy squid". Journal of Zoology. 282 (3): 151–156. doi:10.1111/j.1469-7998.2010.00733.x. ISSN 0952-8369.
  14. Hirohashi, Noritaka; Iida, Tomohiro; Sato, Noriyosi; Warwick, Sauer H. H.; Iwata, Yoko (2016-09-01). "Complex adaptive traits between mating behaviour and post-copulatory sperm behaviour in squids". Reviews in Fish Biology and Fisheries. 26 (3): 601–607. doi:10.1007/s11160-016-9434-1. ISSN 1573-5184. S2CID 254988171.
  15. Kasugai, Takashi; Ikeda, Yuzuru (2003). "Description of the Egg Mass of Pygmy Cuttlefish, Idiosepius paradoxus (Cephalopoda: Idiosepiidae), with Special Reference to its Multiple Gelatinous Layers". The Veliger. 46 (2): 105–110.
  16. Sato, Noriyosi; Kasugai, Takashi; Munehara, Hiroyuki (2013-03-01). "Sperm transfer or spermatangia removal: postcopulatory behaviour of picking up spermatangium by female Japanese pygmy squid". Marine Biology. 160 (3): 553–561. doi:10.1007/s00227-012-2112-5. hdl:10069/31698. ISSN 1432-1793. S2CID 253740276.
  17. ^ Sato, Noriyosi; Kasugai, Takashi; Munehara, Hiroyuki (2014-06-01). "Female Pygmy Squid Cryptically Favour Small Males and Fast Copulation as Observed by Removal of Spermatangia". Evolutionary Biology. 41 (2): 221–228. doi:10.1007/s11692-013-9261-4. hdl:10069/34511. ISSN 1934-2845. S2CID 255343132.
  18. Noriyosi Sato, Takashi Kasugai, Hiroyuki Munehara, Spermatangium formation and sperm discharge in the Japanese pygmy squid Idiosepius paradoxus, Zoology,Volume 117, Issue 3,2014, Pages 192-199,https://doi.org/10.1016/j.zool.2014.02.001.
  19. ^ Sato, Noriyosi; Uchida, Yu; Takegaki, Takeshi (2018-07-12). "The effect of predation risk on post-copulatory sexual selection in the Japanese pygmy squid". Behavioral Ecology and Sociobiology. 72 (8): 129. doi:10.1007/s00265-018-2540-4. hdl:10069/38504. ISSN 1432-0762. S2CID 253806315.

Further reading

External links

Taxon identifiers
Idiosepius paradoxus


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