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LAB # 8. THE ARTHROPODA 1. Overview The Arthropods are the largest single phylum of multicellular animals. They are segmented protostomes with a molting exoskeleton and jointed appendages. There is an estimated 1 to 20 million species of arthropod living today. The phylum is found in all major habitats, but most are found in the terrestrial environment. When researchers talk of the ‘biodiversity’ crisis they often use the example that 1 species of arthropod goes extinct per day in the Brazilian rain forest. The greatest diversity of form is found in the relatively small Sub-phylum Crustacea, which dominates the marine environment. Since the crustacea show the greatest diversity of form and also include the most primitive of the animals constituting the ‘arthropod body plan’, we will spend most time on them, rather than the structurally simpler insects and spiders. The immense size of this phylum forces us to only briefly consider their major features. There are a seemingly infinite number of new terms associated with the Arthropod body plan. I’ve tried to keep these to a minimum. All arthropods are characterized by a jointed skeleton comprised of chitin and protein. This impervious skeleton means that they are well adapted to the terrestrial environmnent, where dessication is a major selective pressure. Another universal feature is that they are all metameric (recall annelids, tardigrades etc.), consisting of a variable number of segments. In addition, arthropods are essentially acoelomate animals, with open circulation systems. They also abandoned the use of cilia and mucus. • • 2. Essential components of the lab Arthropod diversity - be able to place the specimens at the side bench into the classification system described below (which follows the scheme outlined in your text) crayfish dissection 3. Classification of the Arthropods Up to now, I haven’t required you to devote too much time to general classification and taxonomy. However, the Arthropods require a different approach. You are required to be able to recognize the various groups and their distinguishing characteristics (e.g. mites vs. ticks; crabs vs. shrimp; copepods vs. amphipods etc.) You should also aim to familiarize yourself with their feeding and locomotory adaptations, their life-histories, and their various modes of reproduction. SUBPHYLUM TRILOBITA All members of this primitive group of arthropods are extinct. Approximately 15,000 species have been described. They were all marine and were most common during the Cambrian and Ordovician periods. Although they are all extinct, they share many common characteristics with living arthropods. The name comes from the three longitudinal lobes running the length of the body (note the fossil specimen on the bench). The entire thorax was flexible and capable of ‘enrollment’ - an anti-predator strategy similar to living pillbugs (Isopoda). 40 SUBPHYLUM CHELICERATA This group includes those arthropods that don’t possess antennae. Instead, the first 2 head appendages are the chelicerae (clawlike) and the pedipalps. These are followed by 4 pairs of walking legs. Class Merostomata (Horseshoe crabs) Most species of these primitive arthropods are extinct; only 4 species remain and these are all marine. Limulus is the common species found along the Eastern US coast and in the Gulf of Mexico. Interestingly, the body architecture resembles the extinct Trilobites. Sexes are separate. Males cling to a female as she comes to shore. She deposits eggs as the males release sperm into the sand. Look at the ventral and dorsal features of a specimen of Limulus. The prosoma is large and covered by a dorsal carapace. The opisthoma is extended into a long telson (what is it’s function?). The first 2 pairs of appendages are the chelicerae and pedipalps, and are morphologically similar to the next 4 pairs. Chilicerae are present as modified chewing appendages. Note the unique book gills. Note also the spines on the proximal ends of the walking legs. What might these be for? Note also the bizarre morphology of the last pair of legs. What is their function? Class Arachnida There are more than 60,000 species in this immensely important group. The class is characterized by a body divided into 2 tagmata, the prosoma and the opisthoma. Chelicerae and pedipalps are obvious. The prosoma has 4 pairs of legs and the opisthoma lacks appendages. We will look at the 3 common orders (of at least 12!) in this class. Order Scorpionida Scorpions are considered the most primitive of arachnids. All are terrestrial; found primarily in tropical or warm temperate climates where they are predaceous on other arthropods (and on Clint Eastwood’s toes). All have the distinctive poisonous sacs associated with the stinger on the tail. Note the external morphology of the preserved Centruroides. Note that the chelicerae are small, but the pedipalps are large and adapted for grasping prey (they are also used in courtship). Some fossil species were up to 0.5 m in length. Scorpions are known from areas south of Lethbridge, near the Milk River Ridge. Order Araneae (spiders) These are the familiar spiders, with over 30, 000 spp. The order is characterized by a‘waist’ which connects the opisthoma and the prosoma. There is almost always a carapace over the prosoma with its eight anterior eyes. The pedipalps look like legs and are tactile in function (they also serve to transport sperm to the female). Examine preserved specimens of Tarantula to locate these structures. Note also the grinding structures at the base of the pedipalps; these surround the mouth and are used to grind food. In males, the distal end of the pedipalp is uniquely adapted for sperm transfer. Between the pedipalps are the two jointed chelicerae; the distal end is formed into a large, nasty fang. Connected to each fang is the venom duct. Tarantulas are somewhat unusual in that they can articulate their fangs to puncture their host in a 41 direction parallel to their bodies (i.e. spears!). Most other spiders puncture their prey at right angles to their bodies. Turn the spider over to its’ ventral side. Posterior to the pedical is the transverse epigastric furrow, where the gonopores are located. In females, the openings to the seminal receptacles are found here (for sperm storage after mating). Lateral to the pores are the opening to the book lungs. Lastly, note the obvious spinnerets that lead to the internal silk glands. Order Acarina (mites and ticks) The mites and ticks are the most ubiquitous, diverse and speciose group of arachnids. Such diversity is mirrored in diversity of body form, making the Acarina difficult to characterize. Both mites and ticks have the two major body sections reduced from typical chelicerates (very simply, into a head region and body region which bears the legs). Mites are usually tiny, delicate and soft-bodied, showing considerable variation of body form. They occupy various aquatic and terrestrial habitats, and may be scavengers, predators, or animal or plant parasites. Some researchers believe they rival the nematodes as the most abundant metazoans. The mites which form the common galls on our local cottonwoods are highly specialized forms, barely resembling a ‘typical’ mite. The two species that are looking down from your forehead (Demodex spp.), reading this script (the notorious ‘forehead mites’), look very similar to these bizarre gall formers. Can you imagine how this body form might be an adaptation for parasitism? Ticks are usually larger and are all bloodsucking parasites of vertebrates. They are extremely important as vectors of important human diseases (e.g. Rocky mountain spotted fever, Lyme disease, tularemia and tick paralysis. Observe specimens of Dermacentor and compare their basic body plan to the mites and other arthropods. How are the ticks adapted for parasitism? Class Pycnogonida – the sea spiders The minute body and enlarged legs are characteristic of this marine group. There is a cephalon at the anterior end; the remainder of the body is a trunk. 4-6 prs of elongate legs are present. Sometimes you can see eggs on the tips of the legs of the males. Unlike spiders, they do not have specialized respiratory or circulatory systems. The juveniles of most sea spiders are actually parasitic or commensal on/in cnidarians and bivalves. SUBPHYLUM MANDIBULATA Class Crustacea Crustaceans are of immense ecological and evolutionary significance. They are the most morphologically diverse group of arthropods and are the dominant arthropods found in aquatic habitats, anywhere on the planet. The group is most easily defined as arthropods with 2 pairs of antennae. No other feature is shared by all the crustaceans. The primitive crustacean was probably one with a linear arrangement of similar segments, each bearing a pair of biramous (double branched) appendages (imagine Nereis with a thick cuticle). In many groups, this primitive condition has been modified such that one of the 2 branches in the appendages has been lost (now uniramous). 42 The characteristic larval form is the nauplius, which often makes up a major part of marine plankton. We will see live examples, using Artemia. Under the dissecting scope, note the three pairs of appendages and the naupliar eye. Compare the morphology of the larvae, with that of an adult. Sub-class Branchiopoda Class Cladocera Most members of this class are found in freshwater. They all have leaf-like appendages. The most common around Lethbridge are the water fleas, such as Daphnia (see fig. 14.27 in your text). These are microscopic and have laterally compressed bodies and their heads are always distinctly separated from the rest of the body. Unique to the cladocerans is the very large, single compound eye. Obtain a living specimen and refer to the figure in your text to aid in understanding its functional morphology. In the live specimens, you will be able to observe the heart beating. Blood enters through the ostia and is pumped anteriorly. There are no blood vessels and blood simply empties into the hemocoel. Observe the locomotion of Daphnia. You will also observe developing embryos in the brood pouch. Daphnia is well-known to undergo cyclomorphosis (the production of alternative morphs due to predation and other stresses) and also parthenogenesis (similar to the rotifer example we took in class). Both are adaptations to deal with the temporary nature of their temperate, freshwater habitats. Class Anostraca Artemia, the brine shrimp, is a familiar example. It is typically found in high-saline environments, such as our temporary prairie sloughs. It is actually an endangered species on the prairies, primarily because of the introduction of rainbow trout for sport fisheries. Class Notostraca This is another interesting group from our prairie perspective. These represent Alberta’s largest nartive crustaceans (crayfish are not native to Alberta), but they are also among the most rarely seen. Like the Anostracans, they prefer saline sloughs and temporary ponds. They look like freshwater versions of horseshoe crabs, but are only very distantly related. Observe the specimens on the side bench for form and function. These specimens were collected by a U of L student in temporary ponds near Beauvais Lake Provincial Park. Class Copepoda This group includes the largest and most abundant class of marine and freshwater plankton. Planktonic copepods are ecologically significant as they are the vital link between phytoplankton and higher trophic levels in aquatic food webs. The long laterally projecting first antennae is typical of most free-living copepods. Some are suspension feeders, using their second maxillae for filter feeding. Others are predatory carnivores (e.g. Cyclops). Free-living forms are generally small (<2mm), lack a carapace, lack compound eyes and have greatly enlarged antennae. Obtain a living specimen of Cyclops, and compare its general morphology to Daphnia and other crustaceans. Parasitism has evolved many times in the Copepoda. Some parasitic species are so highly modified that it can be difficult to determine that they are even arthropods. Some species, such as 43 the salmon louse can be major economic pests. Lake trout in northern Alberta are often heavily infected with a similar species. Note the bizarre specimen that lives in the left eyeball of some species of sole on the west Coast. The protruding sacs are eggs which are released from the adult copepod, hatch and then reattach to the eye of another host. Within the eye, the rest of the copepod body has evolved as a fungus-like anchor. This pathogenic parasite is known to cause severe disorientation and ultimately blindness. Observe examples of some of the bizarre parasitic copepods, collected from various fish. Observe the gradation of adaptations as you move from free-living forms (e.g. Cyclops), to forms that clasp onto host gills (e.g. Ergasilus from whitefish in Northern Alberta) and onto forms that live permanently attached to their hosts (e.g. Haemobaphes from the hearts of tidepool sculpins and Cecrops from the gills of ocean sunfish). For the permanent forms, where females live attached to their hosts, how do imagine they attract males and reproduce? Class Cirripedia (barnacles) These crustaceans have a carapace covered with calcareous plates and all possess special appendages (cirri) which function to collect suspended plankton from the water. Two of their most distinguishing characteristics are 1) they are sessile as adults and 2) they are always hermaphroditic. Examine the living specimens in the marine tank and note their adaptations for a sessile, largely intertidal lifestyle and their unique means of suspension feeding. Note the differences between the stalked acorn barnacles and the stalkless species. Species of barnacles, as for the copepods, also demonstrate a clear gradation of adaptations from strictly free-living species (suspension feeders), to commensal species (such as some bizarre species which attach to whales), to bizarre parasitic species. In fact, two of the four orders of barnacles are exclusively parasitic. One order, the rhizocephalans have body structures resembling fungi, not arthropods ! These are severe pathogens of crabs. We will go over the bizarre life-cycles of these ‘weirdos’ in class. The most conspicuous internal structures are the 6 pairs of biramous cirri. The extended penis is located between the 6th pr of cirri. Although all barnacles are hermaphrodites, cross-fertilization is the rule, hence the extremely long penis. Barnacles often brood fertilized eggs to the nauplius stage. Sub-class Malacastraca The basic body plan of members of this group is fairly uniform. The body is almost always composed of 19 segments in three tagmata: cephalan, thorax and abdomen. Often, the carapace covers the first two segments, giving the impression they only have two sections. The eyes are usually stalked and compound. Appendages associated with the thorax are called pereopods; those with the abdomen are called pleopods. The first few pereopods are modified as maxillipeds to manipulate food and another is usually modified as a cheliped (the claws of crayfish). The remaining pereopods are usually modified for walking or swimming. The uropod is usually distinctive and is modified last pleopod. This is an immense and complicated group from a phylogenetic viewpoint - we will only be scratching the surface. 44 Class Mysidacea These are shrimplike crustaceans, although only 2 are freshwater. The opossum shrimp, Mysis relicta, is fairly common in some northern and alpine lakes, and is assumed to be a glacial relict. They are an important group for us, primarily because of their significance to sport fisheries. Mysids are preyed upon heavily by salmonid fish and are known to dramatically increase their growth rates. Lakes in the interior of BC (e.g. Kokanee and Okanagon) were stocked with Mysids in the 1950’s to improve the fishery for Kokanee salmon. Unfortunately, the attempt has been a disaster, primarily because the Mysids compete with larval trout for food. The specimens we have on display came from colleagues in Kelowna, where Mysid shrimp have drastically altered the sports fishery. Class Isopoda The isopods are somewhat unusual in that they are one of the few groups that has been successful at colonizing both marine and terrestrial habitats. Many marine forms have secondarily adapted to a parasitic existence. Isopods have dorsoventrally flattened bodies (very different from amphipods). The first antennae are reduced but the second antennae are large. Each of the seven segments bear one pair of uniramous walking appendages (this is where the name isopod comes from). We should have live examples of our common terrestrial isopods, commonly known as pill pugs (see Figure for general body structure). Class Amphipoda These have laterally compressed bodies. Most are marine but this group also includes our familiar scuds (Gammarus and Hyallela). They make up critical components of the ‘shredder’ community in freshwater habitats and are thus important as decomposers. Obtain a living specimen and observe it under the scope. Like the isopods, they lack a carapace. The first thoracic segment is fused to the head, which bears prominent eyes and a large, conspicuous antennae. Gammarus and Hyallela provide good examples of sexual dimorphism, with males being larger. We will discuss their reproductive life histories in class. Class Euphausiacea These are the well-known ‘krill’. There are fewer than 100 species, but they are extraordinarily abundant in the worlds oceans. They are a major food source for many animals, especially the baleen whales. Krill are often considered one of the last major unexploited sources of protein for human populations. They are often well known for their luminescent organs, used to attract mates. The preserved krill are from collections made by government scientists in the Gulf of St. Lawrence. Class Decapoda This is the largest and most diverse of the crustacean orders. All decapods have a distinct carapace that fully encloses the thoracic gills and the first three thoracic appendages are modified as maxillipeds. The remaining 5 prs of thoracic appendages are modified as walking/swimming legs and are uniramous (i.e. decapoda!). The first 5 prs of abdominal appendages are usually similar and biramous; the 6th is often fused together as a terminal uropod. Many decapods have numerous larval forms. Although the nauplius larvae is the basic type in the lower crustaceans, most decapods have more complex larvae called zoea. This larvae has more segments than the 45 nauplius as well as prominent eyes, a carapace with long spines and an abdomen. This type of larvae is typically followed by one or two types of larvae which tend to be even more complex. There are 4 general subgroups 1) swimming macrurans - shrimp, prawns - legs adapted for swimming 2) walking macrurans - lobsters, crayfish - legs highly developed, first pair formed as chelipeds 3) brachyurans - crabs - bodies flattened, laterally extended - abdomens reduced and carried permanently folded under thorax 4) anomurans - hermit crabs - bodies asymmetrical and elongate, adapted for living within gastropod shells SUBPHYLUM UNIRAMIA We will only skim over this vast group. The key point to remember is that despite the tremendous diversity, there is very little diversity in body form and function. Most insects are very similar in basic anatomy. The diagnostic features of these primarily terrestrial arthropods are a pair of mandibles and one pair of antennae. They have unbranched appendages. There are three broad groupings: Diplopoda - these are largely herbivorous with mouthparts adapted to feed on decomposing vegetation. They have a rounded cross-section and 2 pairs of appendages per segment. These are the millipedes. Chilopoda - These are the centipedes - with one pair of legs per segment. They are largely carnivorous with anterior pincer-like mandibles. They have a relatively flattened cross-section. Insecta - These ‘Terrestrial machines’ are by far the most successful animals in terms of both number of species and individuals, with over 3/4 million described species (more than all other animal groups combined). They are of immense economic significance as important components of food chains, as pollinators of flowering plants, and as vectors of human diseases. Many are agriculturally significant pests. Diagnostic features are the presence of 3 pairs of thoracic legs; adults usually have 2 pairs of wings on the thorax. They typically also have single antennae and a pair of compound eyes. I am only concerned that you understand the adaptations which contribute to insect success in the terrestrial environment. The study of insects provides an excellent opportunity to understand adaptability of structure and correlation of structure with function. Go over you notes from 1020 on the external and internal anatomy of the crayfish. You are responsible for knowing the characteristic features of the four most diverse insect orders: Lepidoptera (moths and butterflies), Coleoptera (beetles), Diptera (flies) and Hymenoptera (ants, wasps etc.). 46 4. Crayfish dissection The aim of this dissection is to assist you in the understanding of the functional morphology of a representative Decapod. Although the specimens we have were freshwater organisms, the same general morphology applies to the more typical marine species (crabs, lobsters or shrimps) External morphology: Note the well-developed chitinous exoskeleton. The body is divisable into two principal regions, an anterior cephalothorax and a posterior, segmented abdomen. A cervical groove divides the cephalothorax into an anterior head and a posterior thorax. Anteriorly, the cephalothorax is extended as a slender rostrum occupying the space between the two stalked eyes. Note the bilaterally-arranged appendages. Both embryologically and phylogenetically these are believed to have differentiated from an originally biramous (Y-shaped) appendage. Beginning at the posterior end of the crayfish, carefully remove all of the appendages on one side of the organism (use blunt forceps and gentle shake them back and forth). Aim to remove entire appendages intact. As you remove each appendage, try to identify its function. With few exceptions, it is possible to identify each appendage in isolation from the others. When you have finished, you should have all of the appendages, in sequence and in approximately the same orientation. The crayfish has exactly 19 appendages. Listed from anterior to posterior, they are: Head: antenna, antennule, mandible, maxilla I, maxilla II; Thorax: maxillipeds I – III, chelaped, periopods II – V; Abdomen: pleopods I – V, uropod. Note that the periopods have gills attached to them. Note further that male crayfish may be distinguished from females by the presence of enlarged and modified first and second swimmerettes (pleopods). These are used to transfer sperm to females during copulation. The male genital pore is at the base of the 5th pereipod; the female pore is at the base of the 3rd pereipod. Internal anatomy Remove the dorsal part of the exoskeleton of the cephalothorax with a scalpel. If done carefully, you should be able to remove the exoskeleton but to leave intact the inner tissue which secreted it. Beginning at the anterior end, carefully peel back the hypodermis and locate the dorsal, muscular, shield-shaped heart (near the posterior end of the cephalothorax). In well-preserved specimens, you should be able to see the paired ostia which allow blood to enter the heart. There are three arteries leaving the heart towards the anterior end. Try to trace these as far forward as possible. There is a dorsal abdominal artery and if you move the heart to one side, a ventral abdominal artery. The lateral part of the cephalothorax covers a gill chamber above the bases of the periopods. Clip away the exoskeleton to see the exposed gills. Beneath the heart (which may be removed now) is the gonad. Depending on the season, it may appear plump and y-shaped. Ducts (either vas deferens or oviducts) lead to the base of the appropriate (third or fifth) pair of pereiopods. The stomach is at the anterior end of the cephalothorax. It is divided into a large, anterior cardiac stomach and smaller, posterior pyloric stomach. By pushing the pyloric section aside, you can see the underlying digestive gland. Trace the intestine posterior to the abdomen, where it may be followed later. By cutting the esophagus and the intestine, you can remove the stomach and place it under a dissecting microscope. Note the median and lateral teeth of the gastric mill (function?) and the hairlike setae at the entrance to the pyloric stomach. The 47 excretory organs of the crayfish are the green glands, and are located lateral and anterior to the esophagus; their opening to the exterior may be seen as prominant white circular elevations on the basal segment of the antennae. 48