''Phronima sedentaria'' is a species of amphipod

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can ...

s found in

ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...

s at a depth of up to . They are large in size relative to other members of the family

Phronimidae The Phronimidae are a family of amphipod crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods ...

. Individuals may be found inside barrel-like homes, created most commonly from the tunics of select species of

tunicate A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ...

, where they rear their young. ''P. sedentaria'' is known to employ multiple feeding strategies and other interesting behaviors, including daily vertical migration. The species is also known by the more common names “pram bug” and “barrel shrimp.”

Description

''Phronima sedentaria'' is the largest and most abundant species in the family Phronimidae.

Sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...

is reflected between male and female members in the more extended and prominent antennae of the males relative to the short, reduced ones of females. More obviously, the size discrepancy between males and females distinguishes them further. Females measure up to 42 mm (1.7 in) long, while males are only 15 mm (0.6 in) long. This species also possesses a complex optical system which involves the use of two sets of compound eyes. Both sets use bundles of crystalline cones to process visual information: one set faces ventrally and one set faces dorsally. Phronima sedentaria eyes

Distribution

''Phronima sedentaria'' is found in temperate, subtropical, and tropical waters of all the world's oceans, including the

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ...

. It is usually found in midwater

pelagic The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or w ...

habitats, but can be found migrating all the way to the surface.

Ecology

''Phronima sedentaria'' most commonly exhibits a symbiotic relationship with tunicates of the genera ''Pyrosoma spp.'', ''Doliolum spp.'', and ''Salpa'' ''spp''. It is not entirely certain how to classify the symbiotic relationship between ''P. sedentaria'' and its hosts, as instances of

commensalism Commensalism is a long-term biological interaction ( symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fr ...

, parasitism, and

predation Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

have all been observed. However, some research has suggested that the majority of suborder Hyperiidea members exhibit parasitic behavior. Females of ''P. sedentaria'' live in the barrel-like bodies of

salp A salp (plural salps, also known colloquially as “sea grape”) or salpa (plural salpae or salpas) is a barrel-shaped, planktic tunicate. It moves by contracting, thereby pumping water through its gelatinous body, one of the most efficient ...

s, pyrosomes, and

cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that ...

ns, and use their strong pleopods to propel their homes through the water. They can somersault rapidly in their barrels, thus quickly changing direction. However, speed is reduced by a factor of three to four when swimming while inside a barrel compared to swimming without one. The shape of the barrel is generally asymmetrical, with one opening three times larger than that of the other. In order to construct their barrels, females first locate a suitable salp, pyrosome, or cnidarian host and either cut into the host or enter through an existing opening. Once inside, the female consumes the organism within and carve out the gelatinous inside, leaving nothing but the tunic. The cells on the tunic layer may serve various functions for the ''P. sedentaria,'' including protection from UV light, storage of acid, and defense against microorganisms, like bacteria. In laboratory experiments with restricted access to potential hosts, females have additionally displayed competition for barrels. Phronima sedentaria 3684544

While certain prey (salps, pyrosomes, and cnidaria) have additional uses for ''P. sedentaria'' in hosting their young and providing feeding platforms, the species is also carnivorous on

zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...

krill Krill are small crustaceans of the order Euphausiacea, and are found in all the world's oceans. The name "krill" comes from the Norwegian word ', meaning "small fry of fish", which is also often attributed to species of fish. Krill are consi ...

, arrowworms, and other crustaceans. ''P. sedentaria'' use different feeding techniques depending on the food source, but the leading sets of pereiopods (front legs) are primarily used in all cases. Mouth pieces, such as the mandible, maxillipeds, and maxillae, manipulate the food into small pieces which are then able to fit through the esophagus. Feeding preferentially occurs at nighttime when members of this species undergo a vertical migration of around 200–350 meters to the ocean's surface. Research has shown this species is susceptible to temperature fluctuations outside of the range 8-25 degrees Celsius (46-77 degrees Fahrenheit), which explains the desire for cooler deep water (300–600 meters deep) throughout the day and warmer shallow waters (0–25 meters deep) at night. ''P. sedentaria'' typically migrate to hypoxic areas (such as the

Oxygen Minimum Zone The oxygen minimum zone (OMZ), sometimes referred to as the shadow zone, is the zone in which oxygen saturation in seawater in the ocean is at its lowest. This zone occurs at depths of about , depending on local circumstances. OMZs are found worl ...

) during the day, causing low metabolic rates and physical activity. Known predators of ''P. sedentaria'' include the longnose lancetfish, European flying squid, Pacific pomfret,

albacore The albacore (''Thunnus alalunga''), known also as the longfin tuna, is a species of tuna of the order Perciformes. It is found in temperate and tropical waters across the globe in the epipelagic and mesopelagic zones. There are six distinct s ...

, and skipjack tuna.

Life cycle & development

Female ''Phronima sedentaria'' are capable of producing up to 600 eggs at a time. Juveniles spend their early development within the mother in a specialized pouch called the marsupium. After finding a suitable host, the female begins to transform the barrel into a nursery for its young. She uses her pleopods and anterior pereiopods to remove offspring from the marsupium, while her posterior pereiopods maintain stability and grasp onto the barrel. Once inside, the young organize themselves into a medial ring around the interior of the barrel. This shape is maintained until the mother delivers food, which the offspring then feed on and return to formation afterwards. Young ''P. sedentaria'' use the barrel as another food source. The offspring of ''P. sedentaria'' develop within their barrel homes until reaching prematurity, after which they are able to feed and survive independently. Development is characterized by growth stages in which molting occurs. Each molt adds a new segment to the sets of pleopods in the rear. The emergence of sexual dimorphism occurs soon after prematurity.

References

External links