Bigfin reef squid, Glitter squid, Oval squid
Sepioteuthis lessoniana, commonly known as the bigfin reef squid, glitter squid or oval squid, is a species of loliginid squid. It is one of the three currently recognized species belonging to the genus Sepioteuthis. Studies in 1993, however, have indicated that bigfin reef squids may comprise a cryptic species complex. The species is likely to include several very similar and closely related species.Show More
Bigfin reef squids are characterised by a large oval fin that extends throughout the margins of its mantle, giving them a superficial similarity to cuttlefish. They are small to medium-sized squids, averaging 3.8 to 33 centimetres (1.5 to 13.0 in) in length. They exhibit elaborate mating displays and usually spawn in May, but it can vary by location. The paralarvae resemble miniature adults and are remarkable for already having the capability to change body colouration upon hatching. Bigfin reef squids have the fastest recorded growth rates of any large marine invertebrate, reaching 600 g (1.3 lb) in only four months. They are a short-lived species, with a maximum recorded lifespan of 315 days.
The diet of bigfin reef squids comprises mainly crustaceans and small fish. They are found in the temperate and tropical waters of the Pacific and Indian Oceans, and have recently been introduced into the Mediterranean as a Lessepsian migrant. They are commonly found near the shoreline, near rocks, and coral reefs. They are fished extensively for human consumption in Asia. Because of their rapid growth rate, short life span, and tolerance to handling and captivity, bigfin reef squids are regarded as one of the most promising species for mariculture. They are also a valuable source of giant axons for medical research.Show Less
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Like other members of the genus Sepioteuthis, bigfin reef squids are easy to distinguish from other squids in that they possess thick and muscular oval fins that extend around almost the entire mantle. The fins extend about 83 to 97% of the mantle length and are 67 to 70% of the mantle length in width. Because of these fins, bigfin reef squids are sometimes mistaken for cuttlefish, a fact reflected by their scientific names. A narrow blue or white line is visible at the point of attachment of the fins to the mantle. A fleshy ridge is also present where the fins meet at the back of the squid.Show More
The mantles of bigfin reef squids are cylindrical, tapering to a blunt cone at the posterior. The mantle is usually 4 to 33 cm (1.6 to 13.0 in) long in males and 3.8 to 25.6 cm (1.5 to 10.1 in) long in females. Both sexes can reach a maximum mantle length of 38 cm (15 in). Adult males weigh 403.5 to 1,415 g (0.890 to 3.120 lb), while adult females are 165 to 1,046 g (0.364 to 2.306 lb). Both sexes can attain a maximum documented weight of 1.8 kg (4.0 lb). The forward margin of the mantle on the ventral side is concave.
Their eyes are large and covered entirely by a transparent secondary cornea. They are greenish at the base. A pair of prominent ridges (olfactory crests) are present on the ventral surface of the head at the rear edge of the eyes. The mouth area is supported by seven triangular flaps (buccal lappets), each with 0 to 7 suckers of less than 0.2 mm in diameter and 18 to 25 teeth. The strong, curved, and short beaks (rostra) are mostly black to dark brown. The radula has seven rows of teeth.
The spermatophores of males are about 4.5 mm (0.18 in) long and 0.15 mm wide. The ink sac is pear-shaped, with a silvery blue-green outer layer. The vane of the gladius (the rigid internal remnants of the mollusc shell) is oval-shaped and pointed at both ends (lanceolate). It has a broad midrib (rachis).
The eight arms are thick, tapering to a narrow point. They are unequal in length, with arm pair I the shortest, followed by arm pair II and arm pair IV, and arm pair III the longest. All of them possess two rows of suckers. Each sucker has a diameter less than 2 mm (0.08 in), decreasing distally, and a ring of 17 to 28 sharp acute teeth. The left arm of pair IV in males is modified into a sexual organ known as the hectocotylus. They bear long fleshy protrusions (papillae) with toothless suckers at the distal portion. The tentacles are thick and long, extending the length of the mantle when retracted. They are slightly compressed laterally. A prominent ridge (a keel) is present on the outer surface of each of the tentacle clubs (the wide tip of the tentacles). There are four rows of suckers on the manus (proximal part of the club) and the dactylus (distal part of the club). The larger suckers in the centre of the manus have 17 to 18 widely spaced teeth.
Large chromatophores densely cover the upper surfaces of the head, mantle and arms. They are distributed more sparsely on the ventral side. The fins do not possess chromatophores on the underside. Living specimens range in colour from translucent creamy white through pale yellow to brownish pink and brownish violet.
Like some other cephalopods, bigfin reef squids are capable of metachrosis – rapidly changing body colouration and patterns through voluntary control of chromatophores. They also possess iridophores (particularly in the head), a form of structural colouration that produces iridescent metallic greens and red when illuminated. They are also possibly one of two squid species with leucophores. Leucophores are a reflector-type structural colouration that reflects ambient light, such that they are white in white light, green in green light, and so on. Bigfin reef squids are remarkable for having the ability to produce complex body patterns from the moment they hatch. In comparison, other loliginid squid species do not produce complex body patterns at less than four months of age. The patterns produced by bigfin reef squids, however, are less diverse than those of the Caribbean reef squids.
Bigfin reef squids do not possess photophores, and are thus not truly bioluminescent.Show Less
The bigfin reef squid is a neritic warm water-dwelling squid. They are usually found 0 to 100 m (0 to 328 ft) below the water's surface. They tend to remain close to the shoreline, near rocks and reefs. They are slightly more active during the night and will move to deeper waters or find cover in daytime. Large numbers of juveniles can often be found hiding beneath floating driftwood.Show More
The bigfin reef squid is the most widespread species in the genus Sepioteuthis. It is found in temperate and tropical regions of the Indian Ocean and the western Pacific Ocean. Their original range extends east to the Hawaiian Islands, west to the Red Sea, north to Japan, and south to Australia and New Zealand (42°N to 42°S and 32°E to 154°W). The range has also expanded to include parts of the Mediterranean Sea. In 2002, bigfin reef squids were first documented in the Gulf of İskenderun of the southeastern Mediterranean Sea. They may have already existed in significant populations in the area prior to their discovery in 2002, as they have acquired a common name among the fishermen of the Aegean Sea – σουπιοκαλάμαρο (soupiocalamaro, literally "cuttlefish-like squid"). It is a Lessepsian migrant, reaching the Mediterranean Sea through the Suez Canal.Show Less
Bigfin reef squids are closely related to the Caribbean reef squid (Sepioteuthis sepioidea ), a species noted for its complex social interactions. Like Caribbean reef squids, bigfin reef squids also exhibit elaborate mating displays.Show More
Bigfin reef squids also exhibit both schooling and shoaling behaviours. Very young bigfin reef squids will also stay close together (shoaling), but do not swim together parallel to each other (schooling). Unlike most other squid species, bigfin reef squids are rarely cannibalistic. Shoals can include animals of different sizes without the threat of larger members attacking and consuming the smaller members. Whether bigfin reef squids recognise each other individually still remains unknown.Show Less
The bigfin reef squid eats a variety of different marine organisms. Its main prey are usually prawns and other crustaceans, and fish. Captive specimens were observed to consume one fish every 2 to 25 hours.Show More
Bigfin reef squids are, in turn, preyed upon by tuna, marlin, swordfish, and other predator fish and groundfish.Show Less
Bigfin reef squids exhibit two most common social body patterning and posturing behaviours related to mating.Show More
The first is dubbed "accentuated gonads", in which they will sometimes increase the visibility of their gonads while reducing the rest of their body colouration. This makes their reproductive organs appear bright white through the transparent mantle. It may indicate the reproductive condition of the signalling squid.
Another common behaviour, primarily seen in males, is dubbed "spread arms", in which the squid will slightly tilt its body forward, head down and arms spread widely and raised above. The mantle is darkened. This behaviour is exhibited mostly when the squids are chasing or following another individual. It is thought to be a signal of reproductive arousal or aggression, similar to the "zebra display" behaviour of Sepioteuthis sepioidea, the "intense zebra display" behaviour of Sepia officinalis, and the "lateral display" of Loligo plei. Females will also sometimes use this display to rebuff courting males.
There are three known courtship behaviours in bigfin reef squids, dubbed "male-upturned" mating, "male-parallel" mating, and "head-to-head" mating. Actual insertion in each position lasts for only a few seconds.
"Male-upturned" mating involves rapid back and forth swimming by the courting male beside a slower-swimming female. The male will then flip over so that he is swimming upside down and quickly lunge forward towards the female. He will quickly eject several spermatophores from his funnel into his hectocotylus and attempt to deposit them on the female's mouth funnel, then jet away from the female. "Head-to-head" mating is regarded as a variation of this tactic.
"Male-parallel" mating involves the male and female swimming side by side. The male will then raise one or two of his arm pair I upwards and swing them back and forth. He moves below the female and clasps the female's neck with his arms. In contrast to the previous behaviours, in this position the male actually inserts his hectoctylus into the mantle cavity of the female, attaching the spermatophores right at the opening of the oviduct rather than at the mouth. Possibly for this reason, it is usually more successful in fertilizing the female than other mating behaviours.
In addition to the above, males will often engage in "sneaking" behaviour. In this scenario, a smaller male will attach spermatophores to the female's mouth area while she is being courted by a larger male using the "male-upturned" behaviour. Even when successful, the male using this strategy is usually chased away by the larger male afterwards.
The spermatophores usually remain embedded near the mouth of the female. Mating usually occurs well before spawning, but may also happen on the spawning grounds themselves. In those cases, the male will stay near the female's side as she lays eggs.
Males have been observed to exhibit mating behaviours with other males. Some males have been found with numerous spermatophores embedded in their mouth funnels. Since bigfin reef squids distinguish sex by visual cues, this may be a form of deception. The smaller males (termed "female mimics" or "sneaker males") might have assumed body patterning typical of females in order to trick larger males. Believing they are females, they will then waste their spermatophores on them. This behaviour has also been observed in other cephalopods.
The main spawning season for bigfin reef squids usually begins in May, but they lay eggs all year round and spawning seasons can vary by location. A single female can spawn more than once in her lifetime. Females can release 20 to 1180 eggs per individual and will die soon afterwards.
The females spawn by passing eggs from their oviducts. These eggs are then coated in gelatinous substances from the nidamental glands and oviducal glands, forming an egg 'capsule'. The egg capsules of the bigfin reef squids contain two to nine eggs each. These are laid in single straight strands on rocks, corals, aquatic plants, submerged branches and other surfaces. At this point, the eggs are 3 mm (0.12 in) in diameter and the egg capsules about 58.2 mm (2.29 in) in length and 12.6 mm (0.50 in) in width, on average.
The capsules incubate for about 3 weeks, depending on temperature. In warmer Indonesia, the incubation period was recorded to be only 15 to 16 days, while in Thailand it takes around 20 to 22 days. They gradually enlarge by absorbing water, reaching around 82.4 mm (3.24 in) in length and 14.6 mm (0.57 in) in width. Unfertilised eggs remain milky white and do not develop further. Fertilised eggs undergo cell division reaching a diameter of 16 mm (0.63 in) with the developing embryo at 11 mm (0.43 in) on the day before hatching. Upon hatching, the paralarvae are 6 mm (0.24 in) in mantle length (excluding tentacles), with fully functioning fins and ink sacs. They resemble miniature adults and are already strong swimmers. They exhibit schooling behaviour two weeks after hatching.
Hatchlings are often cannibalistic. This is regarded as the main cause of death in young squids, particularly in dense populations. However, cannibalism usually happens only when eaten individuals were already weakened significantly or dead, so the actual cause of death may have been something else. Subadults are usually recognisable by their size, ranging from 20 to 60 mm (0.79 to 2.36 in) in length. They reach sexual maturity at less than 210 days in the wild. Males reach sexual maturity earlier than females. In captive populations, males mature 140 days after hatching at most. Females will begin spawning at around 156 to 196 days after hatching. Both males and females mature earlier in captivity than in the wild. Water temperature may play an important role in the earlier sexual maturation of captive specimens. High temperatures may induce shorter lifespans and smaller body sizes, while cooler temperatures favour longer lifespans and larger individuals.
Bigfin reef squids have one of the fastest recorded growth rates for any large marine invertebrate. They can reach 600 g (1.3 lb) in only four months. Nonetheless, size can not often be reliably correlated with age, as variations of body size within a generation is fairly common. In captivity, bigfin reef squids have a lifespan of 161 to 315 days for both sexes.Show Less
Social animals are those animals that interact highly with other animals, usually of their own species (conspecifics), to the point of having a rec...