Amphipoda
General morphology

Unfortunately, amphipods (especially the gammarids) have a bad reputation among planktologists and are often poorly identified, frequently referred to in the literature only as undifferentiated "Amphipoda". However, despite their remarkable diversification, the identification of planktonic amphipods is not all that complicated. With a little practice major families and many genera may be easily distinguished, and species identifications are not insurmountable.

Pelagic amphipods vary in length from 2 to 150 mm. The overall shape of the animals is globular to needle-like, commonly elongate. In many parasitic hyperiids (especially infraorder Physosomata and infraorder Physocephalata) and a few gammarids (also parasitic, like genus Danaella or genus Thoriella) the male body is elongate and compressed, whereas females are globular (Archaeoscina steenstrupi 1, Mimonectes sphaericus 1). Females are sometimes larger than males. Rarely (especially in gammarids) the amphipod integument bears sharp or blunt outgrowths, either isolated (spines) or as ridges (carinae). Such outgrowths are usually on the dorsal (very rarely lateral) side of the animal, sometimes only on the posterior body (see, for example, Eusirus perdentatus 1). In genus Calamorhynchus (Hyperiidea) two horizontal carina are on the lateral surface of the head.

The amphipod head has 5 pairs of appendages, and sessile eyes (if any). The thorax has 8 pairs of appendages. The first thoracic segment, bearing the maxillipeds, is fused with the head, and other segments have uniramous legs. The abdomen has 6 pairs of biramous legs. Lastly, there is a terminal supra-anal telson. Amphipods are an order of the Peracarida lacking a carapace covering the thorax, so seven distinct thoracic segments (pereonites) are visible. Only in the caprellids is one of these seven segments fused to the head. In some hyperiids, 2 to 5 of the first pereonites may be fused to each other (especially in females). Amphipods differ from all other Malacostraca in having pereonal legs arranged in two distinct groups: the first four pairs directed forward (with the dactyls directed backwards), the last three pairs backwards (dactyls forward) (Diagram of hyperiid amphipod). Their abdominal appendages comprise 3 pairs of biramous swimming legs (pleopods) and 3 pairs of thrusting legs (uropods). The last character is unique - all other similar groups of crustaceans have 5 pairs of pleopods and 1 pair of uropods. In caprellids the abdomen and its appendages are strongly reduced.

The head of an amphipod bears two pairs of antennae. The first 3 articles of the first pair, and the first 5 articles of the second pair are known as the peduncles. In hyperiids some of the peduncular articles may be absent, especially in antennae II, but the two first articles of the peduncle of antenna II are always present and fused to the head. The remaining smaller articles of the antennae form the flagellum. In many gammarids there may be an accessory flagellum forking from the end of the third peduncular article of antenna I, either well developed (Orchomenella abyssorum 2) or reduced. Hyperiids never possess this accessory flagellum. Sometimes the antennae (especially in gammarid males) have calceoli - tiny globular sense organs attached to the antenna (absent in hyperiids).

In suborder Hyperiidea, flagellae of the antennae are often more or less reduced. In many species only the proximal conical article is developed (or even hypertrophic), although a few rudimentary distal articles may also be present (Lanceola loveni 4). In Hyperiidea Phronimoidea and Platysceloidea antennae are strongly reduced in females but well-developed in males. In superfamily Phronimoidea both pairs have multi-articulate flagellae. In superfamily Platysceloidea antenna II (Diagrams of male heads, Fig. D) is long, irregular, 5-articulated (3 elongate peduncular articles + 2 elongate flagellar articles). In family Lycaeopsidae, antennae are short in both sexes, but in females antennae II are absent. In Hyperiidea infraorder Physosomata both antennae may be reduced in different combinations, but morphotypes with gigantic proximal articles of the flagellum of antennae I also occur.

The gland cone (or antennal gland) is located in the proximal part of the peduncle of antennae II; its shape is an important taxonomic trait in some hyperiids (Lestrigonus schizogeneios 2). A triangular, spine-like or needle-like (in Rhabdosoma brevicaudatum 1), rarely rounded or hood-like process termed rostrum is sometimes present on the anterodorsal margin between the peduncles of antennae I.

—Eyes
Eyes, when present, are very variable both in shape (round, oval, elongate, irregular) and in size (from very large, to a few separate ommatidia, to a simple pigmented spot). In hyperiids the eyes can cover the entire head (in infraorder Physocephalata). Sometimes hyperiid eyes are strongly modified, having lost not only the pigment (as in family Cystisomatidae), but even the refracting elements (genus Scypholanceola). In many Physocephalata the eyes are double structures, with a specialized upward-pointing region covering a narrow field of view (Land, 1989). In some cases eyes may be partly reduced, inconspicuous externally, and partly covered by the integument; in this case they are termed "glandular", referring to their gland-like appearance. The presence/absence and characteristics of the eyes are more important as a diagnostic characters in hyperiids than in gammarids.

—Mouth parts
The mouth parts are highly variable and their morphology is important for classification. All mouth parts are concentrated in a buccal mass beneath the head. They include the following structures (lower lip, maxilliped, mandible (Amphipoda), maxilla I, maxilla II, maxilliped):
—Upper lip
A single lobe or flap anterior to the mouth. The frontal part of the head above the upper lip is termed the epistome; normally quite indistinct, but occasionally protruding anteriorly as a laminar rounded lobe.
—Lower lip (lower lip)
A bilaterally symmetrical structure forming a partition behind the mouth. It consists of one pair of lateral or outer lobes, provided with lateral projections, and one pair of inner lobes, which are often reduced or fused. Lips, especially the lower one, are important taxonomic characters for the gammarids, but are of little use in hyperiids.
—Mandibles (mandible (Amphipoda) )
A pair of appendages attached laterally to the mouth. The mandibles have powerful muscles and often solid anterodistal processes called incisors which are often strongly serrate. Right next to the distal "teeth" may be an articulated serrate accessory plate, the lacinia mobilis, which may be present on only one of the mandibles. A molar, with or without a grinding surface, often occurs on the medio-ventral surface of the mandible, but it may be reduced or absent. Most amphipods have a 3-articulated palp attached to the dorsolateral surface of the mandible. In Hyperiidea this palp is often absent, or may be present in the male and absent (or, more rarely, reduced to 1 or 2 articles) in the female of the same species.
—Maxilla I (maxilla I)
In Gammaridea this pair of appendages is small, each bearing an inner lobe (=plate) (sometimes with setae), an outer lobe with spines, and attached to the latter, a palp (reduced or absent in some cases) comprising 1 or 2 articles. In Hyperiidea Physosomata (but not in the Physocephalata), the maxilla I has an inner lobe, and the palp, when present, is formed by a single article. In some Physocephalata (Lycaeidae, Oxycephalidae) the maxilla I can be rudimentary or absent.
—Maxilla II (maxilla II)
This pair of appendages bears simple outer and inner lobes (=plates). In gammarids a row of setae on the dorsal face of the inner lobe (or their absence) is important for systematics. In hyperiids the maxilla II may be rudimentary or absent (Lycaeidae, Oxycephalidae).
—Maxillipeds (maxilliped)
Maxillipeds are formed by an inner and an outer lobe (=plate). In gammarids the outer lobe has a 4-articulated palp (rarely reduced), whereas the palp is always absent in hyperiids. In the latter the inner lobes of the maxillipeds are partly or completely separated in most of the Physosomata, but completely fused into a single median lobe in other hyperiids, or reduced. Outer lobes may be broad or narrow, in some cases fused into a single plate (Paraphronimidae). Maxillipeds were originally the first pair of thoracic legs, but they (with their body's segment) have become completely incorporated into the cephalic complex and are therefore included in the mouth parts, rather than among the legs of the pereon.

—Pereon (or thorax)
This section bears seven pairs of uniramous legs called pereopods I-VII. Pereopods I and II also are termed gnathopods I and II. Pereopods are 7-articulated: the first article, attached to the body, is termed coxa (or coxal plate), article 2 - basis, article 3 - ischium, article 4 - merus, article 5 - carpus, article 6 - propus or metacarpus, and article 7 - dactyl. However, coxa and dactyl are often (including this review) the only terms used, other articles being referred to by their numbers.

The coxa is an immovable flat plate inserted into the lateral surface of the pereonite (pereon segment), and sometimes fused to it (in hyperiids). In gammarids the coxa may be enlarged, forming a lateral shield (Bathystegocephalus globosus 1), while in other suborders it is always small. Other articles may broaden in different ways or be elongated, more or less narrow and with parallel margins. Articles 5-7 or 6-7 sometimes form the chela or subchela, especially on the gnathopods, but sometimes on other pereopods as well. However, prehensile pereopods are not always present, and all pereopods are simple in many amphipods. Prehension is usually accomplished in hyperiids by pressing article 6 (provided with a small dactyl) against an expanded article 5; whereas in gammarids (and in some hyperiid cases as well) the strong dactyl presses against an expanded article 6. In chelate pereopods the palm, the part of surface or margin of the article on which subsequent article(s) close(s) for the purpose of prehension, is located on the distally produced carpal process (Oxycephalus clausi 2). In subchelate pereopods the carpal process is lacking, and the palm located on the distal or posterior margin of the article (Stenopleura atlantica 2, Primno brevidens 2).

When the carpal process is small, and it is difficult to decide whether the pereopod is chelate or subchelate, the term weakly chelate is used (Orchomenella gerulicorbis 3). Weakly subchelate means that the palm of the subchela is not distinct. Sometimes it is impossible to draw a clear line between simple and weakly subchelate pereopods. In hyperiid families Lanceolidae and Chuneolidae (the latter not recorded in the South Atlantic Ocean) the apex of article 6 is expanded in some pereopods into a hood surrounding the base of the dactyl. The dactyl may be flexed posteriorly so that it lies within the spoon-shaped cavity within the hood (also called retractive dactyl; Scypholanceola aestiva 3).

—Gnathopods (pereopods) I and II
These are commonly smaller and more delicate than other pereopods, more or less prehensile, and rarely simple. Often the gnathopods are hidden behind other pereopods. The shape of the distal articles of the gnathopod is an important taxonomic character. In gammarids, gnathopods II are usually sexually dimorphic, with the chela or subchela strongly enlarged in males. Such dimorphism is especially noticeable in benthic species, whereas in the planktic ones it is weak, or absent altogether. In hyperiids only Phronimopsis has sexually dimorphic gnathopods II, its chela larger in females than in males.

—Pereopods III-VII
Pereopods III and IV are usually slender and their dactyls are directed posteriorly.
Pereopods V-VII are often more robust and elongate, and their dactyls directed anteriorly. Article 2 of these pereopods is often inflated. In Hyperiidea the distal articles of pereopods VI and VII may be reduced (or completely absent in pereopod VII). Pereopod VII may be much smaller than the rest. Pereopods III through VII are usually simple, but a few may be prehensile in some cases. In Phronimidae and Phrosinidae (Hyperiidea) pereopods V are extremely subchelate (even almost chelate in Phronima). In Phronima the subchelae of pereopod V are sexually dimorphic. In some species pereopod articles may bear conspicuous glands, in which case they are termed "glandular" in the keys and diagnoses.

—Gills
Hollow sacs inserted into the medial surfaces of the coxae. Typically there are 6 pairs of gills in gammarids, 5 in hyperiids, and 3 in caprellids, but some pairs may be reduced. Gills are always lacking on the first pereopods.

—Oostegites
Thin plates which overlap to form a pouch beneath the pereon (marsupium) in which the eggs are hatched and the young develop (amphipods lack planktonic larvae and new-borns resemble miniature adults). These structures are located on pereopods 2-5, medial to the gills, and present only in mature females. In Gammaridea oostegites have marginal setae in the latest stages of development, whereas in Hyperiidea these setae are absent. In Rhabdosoma (Hyperiidea) the oostegites are reduced in size, and their function taken over by the gills.

—Abdomen
The abdomen comprises 6 segments, 3 segments bearing pleopods corresponding to the pleon sensu stricto, and 3 segments of the urosome (bearing uropods). In hyperiids urosomal segments (urosomites) II and III are fused. In caprellids the abdomen is reduced. Ventrolateral plate-like extensions of the pleon segments are called epimera or epimeral plates. The shape of the free margin of the epimera III and II is important in gammarid taxonomy.

—Pleopods
This are paired biramous swimming appendages on the segments of the pleon (first three abdominal segments). The rami are multisegmented and strongly setose.

—Uropods
Uropods are paired biramous appendages attached laterally to the last three abdominal segments of the body. They comprise the basipodite and two branches or rami: an exopodite (outer) and endopodite (inner). The rami are uni-articulate, except in gammarids, where the exopodite of uropod III may be 2-articulated. In hyperiids the endopodite is sometimes fused with the basipodite, and the exopodite reduced (Cystisomatidae, Oxycephalidae, Scinidae). In other cases, parts of the uropod are completely fused or reduced, and the animals have only one petal-like article, rather than a "normal" uropod (Phrosinidae).

—Telson
A flap attached to the last urosomal segment, above the anus. In Gammaridea it may be divided (= cleft) (Scopelocheiropsis abyssalis 1, Synopia scheelana 3) or undivided (= entire) (Stenopleura atlantica 3, Parandania boecki 2); whereas in Hyperiidea it is always undivided (Hyperia crassa 3, Platyscelus serratulus 4). It may be longer or shorter than the basipodites of uropods III. In hyperiids it is sometimes fused with the urosome.

—Setae
All appendages (from mouth parts to uropods) may bear setae. The number and distribution of these setae (especially on the mouth parts) are important taxonomic characters, especially in gammarids, where they may be pinnate or simple. In Hyperiidea they are always simple.

For more detailed description of amphipod morphology see Gurjanova (1951); Bowman and Gruner (1973); Lincoln (1979), Vinogradov et al. (1982), and Barnard and Karaman (1991).

Abbreviations used in the keys: [l][textfile]amphipoda-abbreviations.txtAmphipoda-abbreviations[/textfile][/l]