CHAPTER 56 ORDER AMPHIPODA LATREILLE, 18161) BY DENISE BELLAN-SANTINI Contents. – Introduction. External morphology – General habitus – Tagmata of the body. Internal anatomy – Integument and colour – Musculature – Nervous system – Sense organs – Circulatory system – Respiratory system – Digestive system – Excretory system – Various other glands and organs – Genital apparatus – Chromosome complement – Molecular genetic markers. Reproduction and development – Reproduction – Growth – Life cycle and reproductive periods – Determination of sex. Physiology – Respiration – Osmoregulation – Calcium metabolism. Ethology – Behaviour – Feeding. Ecology – The influence of environmental factors – Interrelationships with other species – Distribution and biogeography – Importance of amphipods in biocoenoses. Origin and phylogeny of the Amphipoda – Palaeontological origin – Phylogenetic classification. Amphipods and man. Systematics – The present state of amphipod taxonomy – Suborder Gammaridea Latreille, 1802 – Suborder Corophiidea Leach, 1814 – Suborder Hyperiidea H. Milne Edwards, 1830. Acknowledgements. Appendix. Bibliography. Note added in proof. INTRODUCTION The order Amphipoda comprises approx. 10 000 described species. In 1991, Barnard & Karaman listed 5733 species for Gammaridea alone, subsequently Vader (2005) counted 6950 gammarideans without the Talitridae, which Serejo (2004) had earlier estimated at including about 400 species. Recently, Ahyong et al. (2011) calculated a total of 9896 spp. for Amphipoda as a whole. The representatives of this order occur in all permanent aquatic habitats and even in some terrestrial biotopes. In the marine realm they are found from the littoral zones down into the hadal trenches (Halice subquarta, at 10 500 m depth). Some live on land, on 1) The original edition of 1999 by D. Bellan-Santini was updated by the author October 2012; latest additions February 2015. © Koninklijke Brill NV, Leiden, 2015 Crustacea 5 (56): 93-248 94 D. BELLAN-SANTINI and in the soil, though they are sparsely represented there, whereas they frequently occur in almost all continental waterbodies, fresh and brackish. Their occurrence up to high altitudes in the mountains (Hyalella curvispina, at 4100 m a.s.l. in the Andes) is well documented. Hardly any ecological niches have not been occupied by amphipods: there are pelagic species, benthic forms as well as benthopelagic representatives, while there also are species that thrive in forest litter, and a few live as commensals or parasites. The order Amphipoda was, in its classical concept, divided into four suborders: Gammaridea, Hyperiidea, Caprellidea, and Ingolfiellidea. Dahl (1977) and subsequently Bowman & Abele (1982) reduced this complement to three suborders by including Ingolfiellidea in Gammaridea. Myers & Lowry (2003) revised the Corophiidea and reunited these with the Caprellidea thus creating a suborder Corophiidea. Hence, currently the Amphipoda are to be divided into three suborders, i.e., Gammaridea, Corophiidea and Hyperiidea. (However, see also the Note added in Proof at the end of this chapter.) Amphipods are classified in the superorder Peracarida Calman, 1904, i.e., among forms that are conceived as Malacostraca Latreille, 1806, deprived of a carapace.The head is fused with one or two thoracic somites, the eyes are sessile if present at all, the antennules are often equipped with an accessory (second) flagellum, the antennae are uniramous. The pereiopods have no exopodite, while some bear a gill. The anterior pereiopods are often modified as prehensile organs, the first pair always being transformed into maxillipeds (though in descriptive diagnoses these are never considered pereiopods but instead as ranking among the mouthparts). The second and third thoracic appendages are typically subcheliform, but may be cheliform as well; these are denoted as gnathopods. The five succeeding pairs are true pereiopods that serve locomotion. The first three pairs of pleopods are adapted to swimming but these appendages can be reduced in some species. The three following pairs are denoted as uropods. The telson is typically free. In most species of Amphipoda the development is direct and takes place in the brood pouch of the female. EXTERNAL MORPHOLOGY General habitus Size.–In general, amphipods are small species of crustaceans. By far the majority has a body length hardly above 1 cm, while the smaller ones measure approx. 1 mm. As a giant representative, often Alicella gigantea has been cited with no less than 14 cm in total length. However, observations made with the aid of submersible cameras at bathyal depths, allowed Hessler et al. (1971) to produce photographs of an amphipod reaching 28.2 cm in length, while Schulenberger & Hessler (1974) report Amphipoda of over 30 cm. At present, amphipods of these sizes have been collected already many times: their generalized attribution to the species Alicella gigantea, however, remains doubtful..