Invertebrate Anatomy OnLine
Crangonyx ©
Freshwater Amphipods
26jun2006
Copyright 2001 by
Richard Fox
Lander University
Preface
This is one of many exercises available from Invertebrate Anatomy OnLine , an Internet laboratory manual for courses in Invertebrate Zoology. Additional exercises can be accessed by clicking on the links on the left. A glossary and chapters on supplies and laboratory techniques are also available. Terminology and phylogeny used in these exercises correspond to usage in the Invertebrate Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated figure callouts refer to figures in the textbook. Callouts that are not hyphenated refer to figures embedded in the exercise. The glossary includes terms from this textbook as well as the laboratory exercises.
Systematics
Arthropoda P, Mandibulata, Crustacea sP, Eucrustacea, Thoracopoda, Phyllopodomorpha, Ostraca, Malacostraca C, Eumalacostraca, Caridoida, Xenommacarida, Nomen nominandum, Neocarida, Peracarida SO, Amphipoda O, Gammaridea sO, Crangonyctidae F (Fig 16-15, 19-67, 19-90)
Arthropoda P
Arthropoda, by far the largest and most diverse animal taxon, includes chelicerates, insects, myriapods, and crustaceans as well as many extinct taxa such as Trilobitomorpha. The segmented body primitively bears a pair of jointed appendages on each segment. The epidermis secretes a complex cuticular exoskeleton which must be molted to permit increase in size. Extant arthropods exhibit regional specialization in the structure and function of segments and appendages but the ancestor probably had similar appendages on all segments. The body is typically divided into a head and trunk, of which the trunk is often further divided into thorax and abdomen.
The gut consists of foregut, midgut, and hindgut and extends the length of the body from anterior mouth to posterior anus. Foregut and hindgut are epidermal invaginations, being derived from the embryonic stomodeum and proctodeum respectively, and are lined by cuticle, as are all epidermal surfaces of arthropods. The midgut is endodermal and is responsible for most enzyme secretion, hydrolysis, and absorption.
The coelom is reduced to small spaces associated with the gonads and kidney. The functional body cavity is a spacious hemocoel divided by a horizontal diaphragm into a dorsal pericardial sinus and a much larger perivisceral sinus. Sometimes there is a small ventral perineural sinus surrounding the ventral nerve cord.
The hemal system includes a dorsal, contractile, tubular, ostiate heart that pumps blood to the hemocoel. Excretory organs vary with taxon and include Malpighian tubules, saccate nephridia, and nephrocytes. Respiratory organs also vary with taxon and include many types of gills, book lungs, and tracheae.
The nervous system consists of a dorsal, anterior brain of two or three pairs of ganglia, circumenteric connectives, and a paired ventral nerve cord with segmental ganglia and segmental peripheral nerves. Various degrees of condensation and cephalization are found in different taxa.
Development is derived with centrolecithal eggs and superficial cleavage. There is frequently a larva although development is direct in many. Juveniles pass through a series of instars separated by molts until reaching the adult size and reproductive condition. At this time molting and growth may cease or continue, depending on taxon.
Mandibulata
Mandibulata is the sister taxon of Chelicerata and in contrast has antennae on the first head segment, mandibles on the third, and maxillae on the fourth. The brain is a syncerebrum with three pairs of ganglia rather than the two of chelicerates. The ancestral mandibulate probably had biramous appendages and a J-shaped gut, posterior-facing mouth, and a ventral food groove. The two highest level mandibulate taxa are Crustacea and Tracheata.
Crustacea sP
Crustacea is the sister taxon of Tracheata and is different in having antennae on the second head segment resulting in a total of 2 pairs, which is unique. The original crustacean appendages were biramous but uniramous limbs are common in derived taxa. The original tagmata were head but this has been replaced by head, thorax, and abdomen or cephalothorax and abdomen in many taxa. Excretion is via one, sometimes two, pairs of saccate nephridia and respiration is accomplished by a wide variety of gills, sometimes by the body surface. The nauplius is the earliest hatching stage and the naupliar eye consists of three or four median ocelli.
Eucrustacea
Eucrustacea includes all Recent crustaceans except the remipedes. The taxon is characterized by a primary tagmosis consisting of heat, thorax, and abdomen although the derived condition of cephalothorax and abdomen is more common. Eight is the maximum number of thoracic segments.
Thoracopoda
In the ancestral thoracopod the thoracic appendages were turgor appendages used for suspension feeding in conjunction with a ventral food groove. Such appendages and feeding persist in several Recent taxa but have been modified in many others.
Phyllopodomorpha
The compound eyes are stalked primitively although derived sessile eyes occur in many taxa.
Malacostraca C
Malacostraca includes most of the large and familiar crustaceans such as crabs, shrimps, lobsters, crayfish, isopods, and amphipods. Primitively the trunk consists of 15 segments, eight in the thorax and seven in the abdomen but in most Recent species the abdomen has only six segments. The female gonopore is on the eighth thoracic segment and the male on the sixth.
Peracarida SO
Peracarida is an important taxon of relatively small malacostracans including the isopods and amphipods. During the reproductive season female peracarids have a ventral brood pouch, or marsupium, made of setose plates extending medially from the thoracic appendages. This pouch is ventral to the thorax and is formed of large, thin setose plates attached to some of the thoracic appendages. The plates project medially and together form a basket into which the eggs are released and the young are brooded. The mandible bears a movable tooth, the lacinia mobilis, between the molar and incisor.
Amphipoda O
Amphipoda is the largest peracarid taxon with about 6000, mostly marine, species. Most amphipods are laterally compressed in contrast with the depressed isopods. The gills and heart are thoracic. Large lateral epimeral plates and expanded coxae enclose the branchial chamber. The first thoracic segment is fused with the head and its appendages are maxillipeds. There is no carapace. The first two pereopods are prehensile gnathopods. The abdominal appendages are three pairs of pereopods and three uropods.
Laboratory Specimens
Most of the approximately 6000 species of amphipod crustaceans are found in marine habitats but about 150 species from a handful of families occur in freshwater in North America. Most of these are found in surface waters but a substantial number are characteristic of subterranean habitats, especially in areas of karst topography. Four taxa, Pontoporeiidae, Crangonyctidae, Gammaridae, and Hyalellidae occur in North American fresh waters. The exercise is written specifically for Crangonyx but representatives of any of these taxa can be used.
External Anatomy
Study a living or preserved freshwater amphipod of either sex but do not use an ovigerous female for this initial study. A living specimen should be in 5 % non-denatured ethanol to anesthetize it. A preserved specimen should be in water. Conduct the study in a small dish with a dissecting microscope.
Examine the specimen at about 15X but be willing to change magnification when appropriate. Note the general shape of the animal. Freshwater amphipods are laterally compressed, that is, flattened from side to side, and this should be readily apparent.
Tagmata
Amphipods, like many Crustacea, have modified the primitive head-thorax-abdomen tagmosis and instead have the body divided into a cephalothorax, pereon, and abdomen (or pereon) (Fig 1, 19-54). In the ancestral malacostracan the head consisted of five segments, the thorax eight, and the abdomen seven plus the telson. In amphipods the cephalothorax comprises six segments, the pereon seven, and the abdomen six plus the telson. The anteriormost thoracic segment has fused with the head to form the cephalothorax and the abdomen has lost a segment. The pereon is the thorax minus its first segment.
Figure 1. A generalized freshwater gammaridean amphipod. Redrawn from Holsinger (1972). Amphipod49L.gif
Cephalothorax
The cephalothorax is the anteriormost tagma and consists of the original five-segmented crustacean head plus the first thoracic segment. It bears the original five pairs of appendages of the head plus the single pair of appendages of the first thoracic segment. There is no carapace.
The cephalothorax usually has a black compound eye (on each side) in most epigean (surface-dwelling) amphipods, but hypogean (subterranean) amphipods are often blind (Fig 1, 19-54). The individual facets of the compound eye are visible with magnification.
The anteriormost appendages of the cephalothorax are the first antennae. These arise at the end of the cephalothorax and are slender and whiplike. They are composed of a proximalpeduncle consisting of three long articles and a long flagellum consisting of many small articles. A short accessory flagellum may be present arising from the distal end of the third peduncular article. The accessory flagellum usually has 1-7 articles.
The second antennae arise from the head just ventral to the first and are also composed of a peduncle (of five articles) and a flagellum. The second antenna has no accessory flagellum.
The remaining appendages of the cephalothorax are three pairs of mouthparts and the maxilliped. The mouthparts consist of a pair of mandibles on the third head segment beside the mouth. Posterior to the mandibles are the first maxillae on the fourth head segment and the second maxillae on the last head segment. These are small and require dissection if you wish to examine them closely. Together the mouthparts form the oral cone ventral to the cephalothorax. The amphipod maxilliped is the fused appendages of the first thoracic segment and it covers the oral cone, obscuring the mouthparts.
Pereon
The middle region of the body is the pereon consisting of seven segments, each of which bears a pair of uniramous appendages (Fig 1). The pereon is the thorax without its first segment, which is fused with the head to form the cephalothorax. The segments of the original thorax are thoracomeres and their appendages are thoracopods. The seven thoracomeres that contribute to the pereon are pereomeres and their appendages are pereopods. Look at the dorsum of the pereon and find the seven segments, or pereomeres, that contribute to it. Begin counting with the first segment posterior to the cephalothorax. The cephalothorax, remember, is not segmented.
Examine one of the pereopods. The generalized malacostracan thoracopod consists of seven articles (Fig 1, 19-54). In amphipods the thoracopods are the maxilliped of first thoracomere and the seven pereopods of the pereonites. These eight pairs of appendages exhibit the sequence of articles of the generalized malacostracan limb. These are, from proximal to distal, the coxa, basis, ischium, merus, carpus, propodus, and dactyl. The pereopods are uniramous.
All appendages of the pereon are pereopods but in common usage the first two pairs are known as gnathopods and only the remaining five pairs are called pereopods. The propodus of each gnathopod is enlarged to form a “ hand” and the dactyl is a finger that closes against the palm of the hand to form a pincer. Such an arrangement of movable finger against a palm is referred to as subchelate and it is typical of amphipod gnathopods. In the remaining five pereopods there is no subchelate grasping mechanism and the dactyl does not close against the propodus. Appendages in which there is no prehensile structure are said to be simple.
The space below the pereon and abdomen is enclosed laterally by side walls composed of coxae, bases, and abdominal epimera. The coxae of the anterior four pereopods are expanded to form large coxal plates that contribute to the anterior lateral wall (Fig 19-52). The bases of the posterior three pereopods are similarly expanded to form basal plates which extend the lateral walls posteriorly. The side plates, or epimera, of the first three abdominal segments complete the side wall. These walls partially enclose a space, the branchial chamber, under the animal where the gills (both sexes) and brood pouch (females only) are located (Fig 19-52). The walls channel the ventilating current anteriorly from the abdominal appendages over the gills and developing embryos in the branchial chamber. In this regard the walls are analogous to the carapace of other crustaceans such as cladocerans or decapods.
Hold the specimen with your forceps so the ventral surface is up and you can look down into the branchial chamber between the coxal and basal plates. Here you can see oval platelike gills in both sexes. In females you may also see large, transparent oostegites, or brood plates, with long setae on their margins. The gills and oostegites are epipods extending medially from the appendage (Fig 19-52). The brood plates form the marsupium in which eggs are brooded. You may get an opportunity later to see a marsupium full of eggs or embryos. The rhythmic beating of the abdominal appendages creates a respiratory current over the gills and the marsupium.
Abdomen
The posterior tagma is the abdomen. As in other malacostracans it consists of six segments which can be counted back from the last thoracic segment.
The abdomen is divided into two regions of three segments each. The first three segments form the pleon and bear biramous appendages known as pleopods (Fig 1, 19-54). The pleopods generate the respiratory and swimming current. The three pairs of biramous pleopods are visible from the side. The pleopods consist of a proximal protopod from which arise two similar, setose rami.
The posterior three abdominal segments together form the urosome and each bears a uropod. These appendages are also biramous appendages but are much stiffer than the pleopods and are used for kicking. Each is composed of a basal protopod and two rigid rami, one the lateral exopod, the other the medial endopod. The uropods extend posteriorly from their respective urosome segments. They are large anteriorly and get progressively smaller posteriorly. Uropod 1 is much larger than uropod 3.
The exoskeleton of the lateral body wall of the three pleon segments extend ventrally to form three large side plates, or epimera, on each side. These three plates form the posterior part of the lateral wall and enclose the pleopods. Because of the wall, the current generated by the beating pleopods is channeled anteriorly over the gills, and embryos if any are present.
The dorsal surface of the third (last) urosome segment bears the telson. This is a small plate attached to the segment. The anus lies under the base of the telson.
Ovigerous Female
Study an ovigerous, or egg bearing, female. Hold the specimen ventral side up with your forceps and look into the space ventral to the pereon, between the pereopods (Fig 19-52). The space will be filled with small, opaque, spherical eggs that are retained by large, oval, also opaque, oostegites, or brood plates. The oostegites bear a fringe of long setae and they may be inflated so they look like balloons. Together, the oostegites create the marsupium, or brood pouch. Eggs are released from the female gonopores on thoracomere eight into the marsupium, fertilized by sperm deposited there by the male, and retained until embryonic development is completed and miniature replicas of the adults, with the adult complement of appendages, hatch. Development, then, is direct. Amphipods have no manca larva such as occurs in isopods and hatchlings have a full complement of appendages.
References
Couch AP, Thorp JW . 1991. Crustacea: Introduction and Peracarida in Thorp, J.W. & A. P. Covich (eds). Ecology and classification of North American freshwater invertebrates. Academic Press, San Diego. 911pp.
Holsinger, JR. 1972. The freshwater amphipod crustaceans (Gammaridae) of North America. Biota of freshwater ecosystems Ident. Manual 5:1-89. US Environmental Protection Agency Proj. 18050, contract 14-12-894.
Pennak RW. 1989. Freshwater invertebrates of the United States, 3 rd ed. Wiley.
Ruppert EE, Fox RS, Barnes RB. 2004. Invertebrate Zoology, A functional evolutionary approach, 7 th ed. Brooks Cole Thomson, Belmont CA. 963 pp.
Supplies
Dissecting microscope
8-cm culture dish
Living or preserved freshwater amphipod, non-reproductive male or female and an ovigerous female
Dissecting set with microdissecting tools
80% ethanol for a preservative or
5 % non-denatured ethanol for an anesthetic