Oratosquilla oratoria, the Japanese mantis shrimp, is a species of mantis shrimp found in the western Pacific. It is known as shako in Japan, where it is widely harvested and eaten as sushi. This species is an important commercial fishing target and widely consumed in many countries along the coast of the Northwest Pacific Ocean.
It serves as the secondary consumer in its natural habitats, preying upon small creatures and being eaten by large fish and Cephalopods. Like other members of its order it has a powerful spear, which it uses to hunt invertebrates and small fish. It grows to a length of , and lives at depths of .
It lives in burrows on the muddy seafloor and conducts most of its activities inside those burrows, including spawning and feeding, for predator protection. It also shows great temperature tolerance by altering metabolic pathways, allowing this species to occupy a wide range of habitats. As an invasive species, it was observed in Australia and New Zealand, where similar environmental conditions and habitats at muddy estuaries promote the establishment of new populations.
Similar to other Stomatopods, mantis shrimp possess highly specialized sensory systems to facilitate hunting and mating. Its complex compound eyes allow it to detect a broad spectrum of wavelengths, even ultraviolet and polarized light. This sensitivity of its visual system, however, makes this species potentially vulnerable to artificial polarized light pollution. It also relies on sensitive antennules with chemoreceptors to detect chemical cues dissolved in the water.
Its normal population distribution is influenced by density, since cannibalism is possible when other food sources are scarce. During the mating season of late spring and early summer, males and females perform intricate social behavior. Then, females lay and nurse their eggs in the burrows until the larvae hatch. Being pelagic instead of benthic, the larval stage of this species adopts a reflective camouflage called eyeshine as crypsis.
The overall color of the Japanese mantis shrimp is light grey to light brown with dark red grooves running down the thorax and abdomen; The color of its posterior tips is dark green.
Japanese mantis shrimp typically reside in a wide variety of habitats including the shore, coral reefs, and level substrates. It is native to the Northwestern Pacific in the oceans near Korea, Japan, Taiwan, China, and Vietnam. This species can be found in temperate, sub-tropical, and tropical regions, with water temperature ranging from âÂÂ1ðC in northern Japan during winter to >30ðC in the South China Sea during summer. Cold temperature can alter the physiological processes of this species at the cellular level, including metabolism, immune response, and reproduction. Low temperature will slow down lipid metabolism. The metabolic mode will shift from lipid oxidation to a carbohydrate-based metabolic pathway in order to maintain ATP production and support the survival of cells in a low-temperature environment. Cold temperature also imposes pressure on the immune system and induces expression changes of related genes. By enhancing antioxidant capacity, maintaining protein stability, and promoting immunity, the individual's tolerance to cold stress is further strengthened. In addition, cold temperatures can disrupt reproductive behaviors by reducing the expression of genes related to sex hormones and vision.
In recent years the Japanese mantis shrimp has been artificially introduced to oceans near Australia and New Zealand where it has become an invasive species. As an invasive species in New Zealand, the Japanese mantis shrimp was first observed in the Kaipara Harbour in 2009, and more observations later have confirmed its distribution expansion to both the east and west coasts of the North Island. Japanese mantis shrimpâÂÂs preference on seafloor with muddy sediments is consistent with the local environments of those invaded esturaries. Macrofaunal species diversity in those sites is low, but the Japanese mantis shrimp may still act as an active bioturbator through burrowing, predators of small fish and invertebrates, and prey of large fish. Anthropogenic sediment input into the harbor may provide more available habitats for Japanese mantis shrimp, and its temperature tolerance range is wider than the actual measured water temperatures around New Zealand.
Japanese mantis shrimp typically reside on the muddy seafloor composed of silt and clay at a depth of less than 40 meters. It occupies long, U-shaped burrows in soft sediments which it excavates with its maxillipeds. Similar to other species in the genus Oratosquilla, the Japanese mantis shrimp starts the construction of a burrow by removing sediments using its maxillipeds, positioning sediments below its abdomen, and dispersing them backward with its pleopods. When the hole is 3-4 centimeters deep, the Japanese mantis shrimp will dive in for further digging, where it will make a 90ð turn to build a horizontal tube. This causes its burrow to have a distinctive elongated part in the vertical direction below the entrance. When the construction is finished, the Japanese mantis shrimp will use a cap made of sediments and self-produced secretion, with a nearly invisible hole in the center, to cover both entrances. It stays tucked away within these burrows in the daylight hours for refuge and rarely emerges except in the night. Most of the day, it will stay at the entrance, with only its eyes and antennules protruding out. The horizontal section has diameters greater than the Japanese mantis shrimp itself, and its length is proportional to the size of the dweller as well. Most daily activities will be conducted here for convenience and safety reason. These burrows are used to spawn, lay eggs in, and The inner wall of the burrow is smooth, and the remains of the prey often scatter on the flat floor. A constricted area is found at the center of the horizontal tube that divides it into two segments. Since two individuals are not observed to share the same burrow, an extra chamber may serve as an additional refuge against predators. If the burrow is not of compatible size, egg laying is inhibited and as a result if the burrow becomes too small, the Japanese mantis shrimp will expand its burrow or create a more suitably sized one.
The Japanese mantis shrimp is regarded as a secondary consumer, feeding mainly on primary consumers. It is a predator that actively hunts and kills its prey. When it gets close to a prey and recognizes it as the target for attack, it suddenly and rapidly launches the attack. At night it emerges from its burrow to hunt prey which it then brings back to their burrow in order to feed. It is considered to be a type in between spearer types, which use their claws to pierce soft bodied prey, and smasher types, which use a club to smash hard bodied prey. Such raptorial appendages allow it to capture a wide range of prey.
It feeds on an assortmentàof organisms varying with habitat including crustaceans, mollusks, small rays, small fishes, worms and algae. Some researchers believe that the occasional presence of algae found in their stomach contents might have been ingested during the process of hunting. Similar to other Oratosquilla species, the Japanese mantis shrimp has shorter raptorial appendages and smaller eyes compared to other mantis shrimp species that are more specialized for hunting fish. In experimental tanks, the Oratosquilla species were observed to have less successful hunting for fish, and even if they catch fish, their appendages can hardly fix the prey for feeding. Gobiid fish was found in the stomach contents of wild Japanese mantis shrimp, indicating the mantis shrimpâÂÂs ability to capture and consume fish. However, its hunting success often depends on whether its prey can respond and whether the prey can escape in time. Fish and shrimps have strong mobility, followed by brachiopods and crabs, while mollusks have the poorest mobility and are thus more likely to be eaten by the mantis shrimps. Besides catching, fish have bony spines, and other crustaceans have a hard shell, which make them hard for the mantis shrimp to break using its mandibles. Although mollusks also have shells, large Japanese mantis shrimps were observed in the aquaria smashing the thick shell of Bivalvia and consume soft tissue inside. When encountering thin-shelled Bivalvia, it will directly break them up with its mandibles and consume the meat with fragments of shells. Juveniles and small individuals canâÂÂt break those hard shells as efficiently as large adults. Fish and Polychaeta also appear more frequently in large individualsâ stomach. Despite this potential preference, the exact prey choice will be habitat- and environment-dependent.
Before spawning, large females will switch their diets to a more mollusk-based and reduce hunting for crustaceans. Nevertheless, the feeding intensity will remain the same during the gonads. Instead of relying on active energy intake from ingestion for gametogenesis, female Japanese mantis shrimp is considered to be conservative, where much of the energy for gametogenesis would come from relocation of energy from muscles and hepatopancreas to gonads. During the period of sexual maturation, the amount of lipid and protein in muscles and hepatopancreas will decrease, consistent with the highest value of lipid and protein content in gonads.
It has also been observed to cannibalize smaller members of their species in times of food scarcity. Though cannibalism plays an important part in the life of Japanese mantis shrimp, it saves energy to capture and feed on other smaller prey rather than conducting cannibalism because other mantis shrimps not only have a harder exoskeleton but also possess defensive weapons (predatory appendages), which makes them hard to subdue.
Reproduction for Japanese mantis shrimp occurs between mature inter molt pairs. Female Japanese mantis shrimp achieve reproductive maturity by late spring to early summer lasting until fall while males have year round reproductive maturity. There are several behavioral components in its courtship. One mating behavior that it demonstrates is attenuation between male and female where they face each other and mutually stroke antennas for several seconds. They have also been found to engage in pair walking, where mates walk closely together side by side, and engage in brushing where the male brushes the female with its appendages.
After fertilization the females spawn in the burrow and the female subsequently nurses the eggs until they hatch. As the ovarian shape of the ventral side of the telson turns into an isosceles triangle, females would spawn the egg, and experimental manipulation has shown that increasing water temperature will facilitate the maturation of the ovary. During the spawning, females will adopt a supine posture, using the dorsal parts of the first to fourth abdominal segments and both exopods of the uropods to attach themselves to the inner wall of the burrow tightly. They can remain in that posture motionlessly, except slightly elevated pleopods, until the spawning is completed after 125-217 minutes. Eggs will be released from the central genital opening on the sixth abdominal segment and freely emerge onto the abdominal segments of the thorax. These eggs are connected with each other by a sticky substance, forming an egg string and, eventually, into an egg mass with an uncertain shape. Females will stretch their bodies after completing the spawning and start to press, through the rotational movement of maxillipeds, the egg masses into a spherical shape. Then, females will alternately stretch and rotate the egg mass to transform it into a shape resembling a thick disc, and eventually shape it into a thin disc with a slightly raised edge, and it will only have a slight degree of stickiness. All these procedures usually occur and finish within 24 hours of spawning.
Soon afterwards, females begin nursing the egg mass and will perform the following actions alternately using the maxillipeds: pushing and withdrawing the egg masses, along with unfolding and folding the egg masses. This nurturing behavior continues until the eggs hatch. However, the frequency gradually decreases, so the egg masses gradually become loose and gradually become semi-transparent as the hatching progresses. Females devote significant periods of time to nursing their eggs and as a result rarely leave their burrows unless to feed. When emerging from their burrow the female carries her eggs in her maxillipeds for safekeeping. In the period shortly before the eggs hatch the female has been observed to neglect the eggs and feed more frequently. The abandoned egg masses are usually eaten by other individuals or gradually decay, and thus cannot hatch. The typical embryonic life of an egg is about 14 days. The larvae of the Japanese mantis shrimp follows 11 larval stages with active swimming and feeding beginning in the third larval stage. The larval stage ends in around 32 to 51 days and the juvenile stage begins.
The Japanese mantis shrimp population in Bohai is known to perform seasonal migration from shallow water at depths of less than 10 meters to deep water at about 30 meters in winter. In Tokyo Bay, due to the occurrence of oxygen deficiency in the lower water layer of the northern bay during the summer, its distribution will shrink to the southern bay. Its spatial distribution generally depends on population density; high density leads to habitat enlargement, while low density shrinks habitat size. Such sensitivity to density is likely due to the presence of cannibalism among individuals. High population density in an artificial breeding environment can lead to gonadal degeneration and other stress responses.
Large predatory fish are the major predators of adult Japanese mantis shrimp. The specific species will vary depending on regions, including Japanese sea bass and red seabream in the western Pacific, and smooth-hounds and Australasian snapper in the southern Pacific, where the Japanese mantis shrimp was introduced. Besides fish, large cephalopods that prey on crustaceans, like the giant Pacific octopus, are also considered potential predators of this species. Juvenile Japanese mantis shrimps will be preyed upon by yellowfin goby, Mimika bobtail squids, and crabs, but predation frequency is relatively low compared to other, more abundant prey items.
Adult Japanese mantis shrimps are vulnerable to attacks from other individuals and predators when they expose themselves on the flat seafloor. Therefore, they rely on the burrow to avoid predation, especially when focusing on feeding or spawning. For example, there was no observation for these behavior being conducted outside the burrow in either the wild or artificial environments, and females preparing to lay eggs will immediately stop spawning after being dragged out of their burrows. The elongated horizontal section of their burrows, though it requires more energy to construct, also provides extra survival benefits against various crustacean predators.
For the more plegic larval stage, the most visible part to predators is the compound eyes due to the presence of opaque pigments. Similar to other Stomatopods, Japanese mantis shrimp larvae respond to that predation pressure by making the remaining body mostly transparent, but the screening opaque pigments are needed for photoreceptors to maintain the resolution of the visual scene. Although the specific structure is unknown, their eyes possess a function called eyeshine, which can effectively reflect green or blue wavelengths of light. It serves as a reflective camouflage, by reducing the visual contrast between the eyes and the surrounding water background, to decrease the chance of larvae being detected. Moreover, this eyeshine can significantly reduce the visibility of the eyes under different observation angles, depths, and natural light conditions, especially in the deeper water layers where the larvae stay during the daytime.
Geographic separation and glacial-interglacial climate change during the mid-Pliocene created two divergent lineages of Japanese mantis shrimp. One lineage has adapted to the cold water temperature of the Bohai, Yellow Sea, and the northern Japan Sea, facilitated by the China Coastal Current and the Korean Coastal Current. Another southern lineage has adapted to the warmer water in the East and South China Sea, along with the Kuroshio Current. These two lineages expanded their distributions in the late Pleistocene. Rising temperature and sea level during the interglacial period had created many inshore and shallow water habitats suitable for Japanese mantis shrimp. Following the reopening of the Tsushima Strait and reconnection between the South and East China Seas, both lineages started a series of colonization, with the northern lineages entering the northern China coast, and the southern lineages reoccupying the East China Sea. Hybridization exists between these two lineages since they share habitats in the southern coast of Japan and the Yellow Sea, but without major morphological divergence, its extent is hard to assess.
The Japanese mantis shrimp has a complex visual system to facilitate hunting and mating. It can detect a wide range of wavelengths from ultraviolet to polarized light, so it is considered to have better color vision than many other animals and humans. The diversity of opsins in the eyes may partially explain why it has such excellent and broad color vision. On the other hand, polarized light detected by the eyes can help this species contrast between the target and background effectively in the dark seafloor. Its compound eyes contain thousands of light-sensing units, and each one of them is independent, allowing the mantis shrimp to capture slight changes in the surrounding environment. The compound eye can be divided into three parts: the dorsal hemisphere, the ventral hemisphere, and a band of small eyes in the middle. This unique structure gives this species a trinocular vision.
Besides vision, the Japanese mantis shrimp has a pair of sensitive antennules as well. It senses prey and mates with the help of ionotropic receptors located on the antennules, which enable it to detect dissolved chemical cues in the surrounding water.
In Japan, the species is called shako, and commonly eaten.
Its consumption is mostly limited to Japan as the price and availability of it limits its popularity abroad. It is most tasty during the period of spring as it is their breeding season and is occasionally also eaten with its roe. Its texture and flavor is said to be somewhere in between eel and shrimp.
The appearance of shako in the form of sushi began in the 1950s where it was commonly brushed with nitsume (eel sauce) and presented as nigirizushi. Shako was originally prepared by boiling it in a sugar syrup, but is now typically prepared through a slow simmer allowing its freshness to last longer.
Due to its taste and abundance, this species has experienced an intense and unregulated fishery along the coast of the Northwest Pacific Ocean. As a natural resource, the Japanese mantis shrimp population has been overexploited in many Asian countries. Reduction in the number of large individuals due to overfishing causes smaller individuals to become breeding parents, and the spawning season was observed to be delayed to late May and even September.
The Japanese mantis shrimpâÂÂs compound eyes are sensitive to linearly polarized light (LPL), so artificial LPL sources from human architecture may create an ecological trap for it. According to comparative transcriptomics, vital genes involved in phototransduction are down-regulated in the LPL condition. This can negatively affect the normal development of retinal cells, the transmission of light signals, and the transformation of light signals into neural electrical signals, which all impair it visual ability. Regarding reproduction, several genes related to sperm production, maturation, and sex hormone release are also down-regulated in the LPL scenario. Therefore, LPL pollution may reduce the reproductive efficiency by inhibiting the proliferation of reproductive cells, hormone release, and the process of gamete combination. Finally, researchers have observed down-regulation of immune-related genes when the LPL pollution is present, including inflammatory responses, antioxidant defense, phagocytosis, and wound repair. This indicates that continual LPL stimulus will expend extra energy and result in malfunction of the immune system, eventually leading to reduced individual fitness.