Peracarida

The superorder Peracarida^[a] is a large group of malacostracan crustaceans, having members in marine, freshwater, and terrestrial habitats. They are chiefly defined by the presence of a marsupium (the "brood pouch"), formed from thin flattened plates (oostegites) borne on the basalmost segments of the legs.

Peracarida Temporal range: upper Devonian–present, 365–0 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Gammarid amphipods from the Gulf of Naples
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Malacostraca
Subclass:	Eumalacostraca
Superorder:	Peracarida Calman, 1904 [1]

Peracarida is one of the largest crustacean taxa and includes about 12,000 species. Most members are less than 2 cm (0.8 in) in length,^[3] but the largest can be quite sizeable, such as the giant isopod Bathynomus giganteus which can reach 76 cm (30 in) in length, and the giant amphipod Alicella gigantea (34 cm (13 in) long). The earliest known peracaridian was Oxyuropoda ligioides, a fossil taxon dated to the Late Devonian of Ireland (more than 360 mya).^[4]

Orders

There is some disagreement as to which orders should be included within Peracarida.^{[clarification needed]} The World Register of Marine Species WoRMS (2023) includes the following 13 orders:^[5]

• Amphipoda Latreille, 1816 (scuds, sandhoppers, skeleton shrimp, whale lice, etc.)
• Bochusacea Gutu & Iliffe, 1998
• Cumacea Krøyer, 1846
• Ingolfiellida Hansen, 1903
• Isopoda Latreille, 1817 (woodlice, gribbles, tongue lice, etc.)
• Lophogastrida Sars, 1870
• Mictacea Bowman et al., 1985
• Mysida Haworth, 1825 (opossum shrimp)
• †Pygocephalomorpha Beurlen, 1930
• Spelaeogriphacea Gordon, 1957
• Stygiomysida Tchindonova, 1981
• Tanaidacea Dana, 1849
• Thermosbaenacea Monod, 1927

Of these Bochusacea, Ingolfiellida and Stygiomysida were added to the classification of Martin & Davies (2001).^[6] Ruppert et al. (2004) excluded Thermosbaenacea and placed it in a separate order, Pancarida. They also still kept Lophogastrida, Mysida and Pygocephalomorpha in a single order Mysidacea,^[3] an arrangement that was disputed by Meland and Willassen (2007) since molecular data shows that the three orders are not closely related (they do not form a clade).^[7]

Description

Generalized peracarid mandibula^[b]

The defining characteristics of the order includes the possession of a single pair of maxillipeds (rarely 2–3), mandibles with an articulated accessory process between the molar and incisor "teeth" in the adults (called the lacinia mobilis), and of carapace which is often reduced in size and is not fused with the posterior thoracic somites.^[8] In some orders, the young hatch at a post-larval, prejuvenile stage called a manca which lacks the last pair of legs.^[8]

Marsupium

Livoneca redmanii; Cymothoidae, young in brood pouch

The marsupium of female peracaridans is a characteristic unique to this group (though males lack this part of their anatomy).^[9][10] It is a space on the ventral surface of the animal which functions as a brood pouch,^[11] and is enclosed by the large, flexible oostergites, which are bristly flaps (sometimes defined as lamellae) which extend from the basal segments of the pereiopods (the "hip" or coxae),^[12] which form the floor of a chamber "roofed" by the animal's sternum.^[13] This chamber is where the eggs are brooded; females lay their eggs directly into the brood chamber, and the young will develop there, undergoing several moults before emerging as miniature adults referred to as mancae; the larval development is direct in most cases.^[3]

In the underground order Thermosbaenacea, there are no oostergites and the carapace of the female is expanded to form a dorsal marsupium.^[3]

Manca

Mancae (sg. manca)^[c] are the post-larval stage or juvenile of the clade Mancoida which comprises all the members of the Peracarida except the Mysidacea and the Amphipoda.^[15] Mancae closely resemble the adult form, though have not yet developed the last pair of pereiopods.^[15] In some isopods, specifically the family Gnathiidae, the manca stage is a parasite of fish, and is also known as the praniza.^[16]