Crustacea

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Sally Lightfoot Crab. Source: Victor Burolla

Crustaceans are invertebrates belonging to the phylum Arthropoda and include such familiar groups as barnacles, crabs, crayfish, lobster, water fleas and pill bugs. Crustaceans are key players in marine and freshwater food webs. The majority of zooplankton in freshwater is composed of cladocerans and copepods, and in the oceans copepods, who are the major consumers of phytoplankton. Benthic crustaceans are often both scavengers and consumers of plant life found on lake bottoms and the seabed. Collectively, these crustaceans serve as a key food source for fishes, especially during juvenile stages. Aside from their role in food webs, the largest species of crustaceans are of considerable economic importance. Lobster, shrimp and even freshwater crayfish each support important fishing industries. They are also increasingly important in aquaculture. In fact, the value of crustaceans produced in aquaculture is already as great as that of fish!

Adults of the smallest species are less then 0.1 mm in length and weigh less than 1 mg. By comparison, the heaviest crustacean is the mud crab which reaches a peak weight of 40 kg. The Japanese spider crab is the largest living arthropod, with a leg span of 4 m.

Most crustaceans employ standard sexual reproduction. Other crustaceans, such as water fleas that live in temporarey ponds, reproduce by cyclic or obligate parthenogenesis, where males are unknown or rare. Females in parthenogenetic species produce eggs which do not require fertilization to develop. Aside from this variation in mating systems, many freshwater crustaceans produce two types of eggs: one which develops immediately, while the other which may diapause for up to several hundred years.

There are more than 40,000 different species of crustaceans. Some 4,000 of these species occur in freshwater and nearly 200 species are found in the North American Great Lakes.

Crustaceans show extraordinary diversity in body shape and form, bearing anywhere from 3 to 50 pairs of limbs. However, crustaceans also share common features such as jointed, paired appendages, and two pairs of antennae. All crustaceans are enclosed in a protective exoskeleton made of chitin, which must be shed (or "moulted") to accommodate growth.

Most crustaceans are carnivores or scavengers, though herbivores and detritivores are also common, and some crustacean groups (e.g., the bopyrid idopods) are parasites hardly recognizable as crustaceans. In some species cannibalism can occur at high densities, or when individuals have just moulted and are vulnerable to attack. Food is taken into the mouth and passed to the gastric mill where it is ground into small particles. Digestion occurs in the midgut and waste is passed out of the hindgut.

All crustaceans have an open circulatory system and employ either haemoglobin or haemocyanin as a respiratory pigment. Most crustaceans have a dorsal heart, but some smaller crustaceans simply circulate their hemolymph with body movements. Crustaceans osmoregulate in freshwater by producing copious amount of urine. Most freshwater crustaceans have thoracic and abdominal gills with which they exchange gases while the rest simply diffuse gases across their body integument.

Crustaceans have developed a complex tripartite brain and paired, ganglionated ventral nerve cords. They often possess both compound eyes and a array of simple eyes. Zooplankton show particular sensitivity to light as they undergo daily migrations up and down the water column to stay in the best light conditions and to avoid visually hunting predators. Chemosensory systems allow them to locate food and mates while avoiding predators.

Amphipoda

Introduction

Amphipods, aslo known as side-swimmers or scuds, are ubiquitous in marine and freshwater environments. They are small, laterally compressed crustaceans with a shrimp-like appearance, but without a carapace. Amphipods occur in vegetation, under rocks, on sand and burrowing deep in profundal sediments. Most of the 6000 known species of amphipods live in the oceans, but many are also found in freshwater and terrestrial environments. A spectacular evolutionary radiation of amphipods has occurred in Lake Baikal, in Siberia, where there are hundresd of species of amphipods and these animals.

Morphology

Amphipods are many-segmented crustaceans. The body is divided into three parts; the cephalothorax, the thorax, and the abdomen. The first thoracic leg is modified into a feeding appendage ("mouth part"), the maxilliped, while the other seven pairs of thoracic legs are used for movement. The first two pairs of thoracic legs are modified for grasping food and are also used by male amphipods to hold onto the female during copulation. The two pairs of antennae are elongate and curved ventrally. Females have a marsupium in which they brood their young until they are ready to be released into the environment.

Reproduction

The timing of reproduction varies among amphipods. Some species have an extended breeding season, and produce multiple broods, while others have a discrete breeding season and produce only a single brood. Before mating, males of some species such as Gammarus spp. and Hyalella azteca seize females with their gnathopods and hold on for up to a week, waiting for her to moult and be ready for mating. During this time, the pair is said to be in precopula. Copulation consists of the male wrapping the posterior part of his body around the female's ventral side, bringing his uropods in close proximity to her marsupium. He then releases sperm which she sweeps into her marsupium by vibrating her pleopods. After mating is complete, the female releases eggs into her marsupium where fertilization takes place. The incubation period varies with species, locality and time of year.

The newly hatched young amphipods stay in the marsupium until the female undergoes a post-copulatory moult. The total number of instars (developmental stages) undergone by most amphipod species is poorly known. Amphipods usually only live for one year, though some species, such as Diporeia hoyi, may live for more than two years.

 

Ecology

Amphipods can be found in marine and freshwater habitats from shallow, densely vegetated areas, to the deepest ocean depths, sometimes at densities of 10,000 per square metre.

Amphipods are more active at night than during the day. Because many species are scavengers and consume a wide variety of organic detritus, they form an important trophic link within both freshwater and marine ecosystems, recycling organic material which is then passed back up through the food chain. Amphipods are an extremely important food source for numerous species of fishes, as well as for the opossum shrimp Mysis relicta. Due to their importance in aquatic food webs, scientists use amphipods to study the effects of chemical contaminants like PCB's and DDT on aquatic ecosystems.

Idiosyncratic Inverts

The swimming speciality of amphipods is a rapid escape response where the abdomen flicks the animal away after the uropods are dug into the ground.

Anostraca

Introduction

Fairy shrimp are easy to distinguish from all other branchiopods because they swim upside down with their legs extending upwards. They occur in temporary freshwater ponds from the arctic circle to southernmost Canada.

 

Morphology

Distinguishing characteristics of fairy shrimp are the absence of a carapace, an elongate body and distinct head region. They have a pair of stalked compound eyes and 11, 17 or 19 pairs of thoracic legs. The largest fairy shrimp are 100 mm long, though most species are between 7 and 20 mm. Anostracans are sexually dimorphic; the males have a pair of ventral penes on their genital segments, while the females have a ventral, medial brood pouch to hold their eggs.

Reproduction

Anostracans usually reproduce sexually, though parthenogenetic reproduction occurs in some populations of Artemia salina, the brine shrimp. Mating usually occurs just after the female has moulted, so nearly mature females are often attended by several males. After mating, the female retains the eggs in her brood pouch until she dies, and the eggs settle to the bottom. The eggs are resistant to dessication, freezing, and ingestion by birds. The eggs hatch into nauplius or metanauplius larvae which undergo several moults before reaching maturity. Species that live in temporary ponds typically only have one generation per year.

Ecology

Fairy shrimp are restricted to temporary pools of freshwater during the cool months of the year. They are not usually found in habitats that contain fish. Algae, bacteria, protozoans, rotifers, and bits of detritus constitute most of their diet, but large species, such as Branchinecta gigas, are predators.

 

 

Idiosyncratic Inverts

Many species of fairy shrimp compete intensely for mates. Since mating usually occurs just after the female moults, males often grasp her and are towed around while waiting for her to moult. Some females have a chain of these attached males, including some which have died while awaiting her moult!

 

 

 

Branchiura

Introduction

Branchiurans are parasites once considered to be a group within the copepods, but recent phylogenetic studies have elevated the group to its own subclass. Their common name "fish lice" comes about because they attach themselves to the outside of fish.

Morphology

Branchiurans are highly modified to suit their parasitic life style. They are dorsoventrally flattened and their carapace has been widened to cover most of their appendages. Two compound eyes make them easily distinguishable from parasitic copepods. Their mandibles have been modified into a proboscis used to suck food out of their prey. Large suckers are actually modified maxillules and are used to help 'stick' to their host. They have eight legs and a small tail that acts like a rudder when swimming. A hollow spine is used to pierce the skin of the host and access the nutrients inside. Argulus sp. can grow to 10 mm in length, but most are only 7 mm.

 

 

Reproduction

Male and females branchiurans drop off their host to look for mates. They swim (or somersault) through the water until a mate is encountered. After mating the females search for an egg-laying site where they attach their eggs in rows to rocks and other submerged objects. The free swimming larvae attach themselves in the gill chamber, mouth or on the outer surface of host fish with hooks that are modified antennae and maxillules. In 4 to 5 weeks the larvae become adults.

 

Ecology

Branchiurans feed on the blood or body fluids of their host. They attach to their host using modified hooks and sucker, but can also detach and swim through the water when looking for mates. They have many host species including the common carp, Cyprinus carpio, and the white sucker, Catostomus commersoni. Heavy infestations of branchiurans can cause mass mortalities in fish populations. Argulus sp. are also suspected to spread viruses.

Idiosyncratic Inverts

Many parasites eat blood, but branchiurans also feed on the mucous of their hosts.

Cladocera

Introduction

In the early spring, the water in many lakes and ponds teems with pale specks moving through the water column. Cladocerans, or water fleas as they are commonly known, are small crustaceans (0.2-3.0 mm). They have successfully invaded a wide variety of aquatic habitats, ranging from hot springs to polar ponds, from large lakes to temporary pools. Cladocerans are an important component of aquatic food-webs; they feed on phytoplankton and are themselves consumed by fish. Cladocerans have attracted much scientific interest, partly because they are easy to culture in the laboratory. They are often used in studies of animal behaviour, functional morphology, evolution, speciation and community ecology.

Morphology

One of the most obvious features of a cladoceran is its single, large compound eye. It is especially evident in living specimens, because it is constantly rotated by three pairs of muscles. The heart lies behind the head and is a simple bag-like structure. The rostrum of cladocerans is the beak-like termination of the head. The first antennae are inconspicuous and are used in olfaction. The second antennae, which are used in swimming, are large, branched appendages powered by three pairs of muscles. Cladocerans have five or six pairs of lobed thoracic legs covered with fine projections called setae. The postabdomen bears two long abdominal setae and two terminal claws. It is used primarily for cleaning debris from the thoracic legs, but may also be used for locomotion. The brood chamber is located dorsally in the carapace and is used to hold the eggs until they hatch.

Reproduction

Most cladocerans reproduce both parthenogenetically and sexually, employing a breeding system termed cyclic parthenogenesis. For much of the year, populations consist entirely of females reproducing by parthenogenesis. The females deposit clutches of 1 to 100 eggs in the brood chamber, which undergo direct development, meaning that they develop into miniature versions of their parent before being released. Parthenogenesis occurs until adverse conditions are encountered; females then produce both males and resting eggs which cannot develop without fertilization. These resting eggs are then enclosed in a thick, protective case called an ephippium. A few species have given up sex entirely, and reproduce only parthenogenetically. In all species, the resting eggs play a key role in the colonization of new habitats, because of their resistance to freezing and drying. Eggs of some zooplankton species have remained viable for up to 300 years in the absence of water.

Ecology

Cladocerans are primarily freshwater organisms. They are abundant everywhere in these habitats except in grossly polluted or rapidly flowing water. The highest diversity of species occurs in the littoral zone of lakes where the most common inhabitants are chydorids and macrothricids. The open-water areas of the lakes are dominated by other genera such as Bosmina, Daphnia, and Holopedium. Rooted aquatic plants actually have a repellent effect on cladocerans.

Most cladocerans are filter feeders which consume algae, protozoans, bacteria and organic detritus. The movement of their thoracic legs creates water currents which draw food particles to them. Tiny projections on the legs filter the particles out of the water. Once transferred to the mouth, food is crushed by the mandibles. A few Cladocera are predatory. They use modified thoracic limbs to seize their prey, which include protozoans, rotifers and other small crustaceans.

Idiosyncratic Inverts

Many cladocerans show dramatic changes in their body shape over the year, so individuals in midsummer look very different from those of the same species in winter. These changes are known as cyclomorphosis. Daphnia retrocurva, a common cladoceran in the Great Lakes of North America, has a rounded head from fall to early spring. As summer advances, the females's head becomes greatly elongated. By winter, the head returns to its original size.

Conchostraca

Introduction

Conchostracans have the flattened leaf-like legs of branchiopods, but they are completely encased by their carapace. They can usually be found swimming along the bottom amid vegetation.

Morphology

Conchostracans are fairly small (2 to 16 mm long) animals, with a carapace that wraps around their entire body. There are two groups of conchostracans that can be divided by their shell morphology; the clam-like conchostracans (Apinicaudata) and the globular conchostracans (Laevicaudata). They can extend their second antennae out of the carapace to swim. The head has a pair of close-set compound eyes that are fused in one genus (Cyclestheria) and 10 to 32 pairs of thoracic legs.

 

 

Reproduction

Most conchostracans reproduce sexually, but other species reproduce parthenogenetically. Male conchostracans have specialized hooks on the first and/or second pairs of thoracic legs which grasp the female during mating. The females often swim about with the male holding onto her carapace. Conchostracans hatch from resting eggs as nauplii and don't develop a carapace until their third naupliar stage. As they moult, juveniles become more and more adult-like until they become sexually mature. Species that live in temporary ponds only have one generation per year.

Ecology

Conchostracans occur in transient ponds throughout much of Canada except the high arctic. They feed on detritus or on plankton by drawing water into the carapace and removing the food particles with their phyllopods.

Idiosyncratic Inverts

Some conchostracans look remarkably like a tiny clam - until they start swimming!

Copepoda

Introduction

The Copepoda is one of the largest groups of crustaceans. Approximately 12,000 species have been described, but this may be as little as 15% of the total number of species! Copepods are abundant in both marine and freshwater systems, but are much more diverse in marine environments.

 

 

 

Morphology

Copepods have long first antennae and a single, median eye located on the anterior (front) portion of the head. They have two pairs of antennae and biramous feeding appendages. The first pair of antennae is often sexually dimorphic and bears chemoreceptors and mechanoreceptors. Antennules (first antennae) are used for locomotion, feeding and reproduction. Each segment of the thorax bears a pair of swimming appendages. The last abdominal segment has two caudal rami which have an unknown function, but are useful in taxonomy for distinguishing between groups of copepods. The largest copepods attain lengths of 3 mm, although most species are less than 1.0 mm.

Reproduction

All species of freshwater copepods reproduce sexually. After mating, the eggs undergo brief embryonic development before they are released into the water as nauplius larvae. The nauplius undergoes five moults before it transforms into a copepodite stage that more closely resembles the adult. If unfavourable environmental conditions, such as declines in temperature, oxygen or food availability, occur, the copepodite larvae of some freshwater species can enter a diapause stage until conditions improve.

Ecology

Copepods occur in oceans, estuaries, lakes, ponds, groundwater, wet moss and even in pools of water that collect in the leaves of terrestrial plants. They are an important food source for larval fish and other invertebrates in aquatic systems. Most are omnivores, filtering algae and bacteria from the water as well as preying upon other small invertebrates. As a result they are an important link between the primary producers (algae) and the larger secondary consumers (fish larvae). Almost half of all known copepod species are either parasitic or commensal with other organisms. Parasitic copepods often cling to the outside of their host, but others are more intrusive, having invaded their gills, nostrils, or mouths of their hosts. Copepods are the intermediate hosts of other parasites, such as the nematode Dracunculus medinensis and the tapeworm Diphyllobothrium latum which can be passed on to humans.

Idiosyncratic Inverts

Although copepods are extremely small and generally harmless, some species can be dangerous. Copepods carry many parasites and diseases that can cause death in humans - and all you have to do is eat or drink the copepod.

Decapoda

Introduction

The decapods include such well-known marine species as lobsters, crabs, and shrimp. Many species have also invaded freshwater, the most familiar of which are the crayfish, which are represented by more than 250 species in North America. Several groups of shrimp have also entered freshwater, particularly in tropical regions.

 

 

Morphology

Decapod morphology may be illustrated by that of the crayfish. The cephalothorax has a pair of stalked eyes and 12 pairs of appendages. The first and second pairs of antennae are used as sense organs to probe the substrate. The base of the first antennae houses a statocyst which is used for balance. The next five pairs of appendages on the cephalothorax are used in food handling, while the last five pairs of appendages are the walking legs. The first 2 or 3 pairs of these legs are clawed and are used for food handling, defence and digging into the substrate. Their abdomen bears pleopods (swimming legs) on the first five segments. Decapods are sexually dimorphic. The first two pleopods of males are modified to transfer sperm to the female, while females have a calcified area between the fourth pair of periopods used to receive the male's spermatophore during copulation. The typical tail fan is composed of expanded uropods on the end of the abdomen.

Reproduction

Hatching in Canadian crayfish occurs in two to twenty weeks. After they hatch, the larvae cling to the stalk still attached to their mother. In a few days they moult and lose the stalked connection with their mother, but they remain attached to her swimming legs with their chelae for up to two weeks. Third instar larvae begin to make excursions away from their mother and eventually leave permanently. Maturity is reached in six to ten moults.

Ecology

Decapods are enormously diverse ecologically, occuring throughout the world's oceans and freshwaters, from surface plankron to abyssal depths. Some species even live on land, including various terrestrial crabs. The decapods include a wide variety of herbivores, omnivores, scavengers, and predators. In temperate freshwaters, decapods are most commonly encountered along rocky or weedy shorelines of quiet bays in water less than 2 m deep, though they occur in water up to 30 m deep. Crayfish can be burrowers, benthic or semi-pelagic. They are mainly nocturnal foragers; their diet consists mainly of macrophytes and debris, but they also eat small insects and crustaceans as well as dead animal matter. Decapods are important prey items for many fish, waterfowl, and aquatic mammals.

Idiosyncratic Inverts

Male crayfish know when they have to toughen up. After their last juvenile moult, males enter their sexual form with larger chela, increased length, sharper spines and increased sclerotization. This form seeks out females and is ready to do battle with other males. At the end of the mating season, males moult back to a form that resembles the juvenile stage.

Decapods have a breaking point at the base of each leg. If they are grabbed or agitated, the muscles contract, severing the leg at this point. If the leg is broken at this point, it can be regenerated later.

 

Insecta

Introduction

Insects are arthropods in the class Insecta. There were more than 750,000 described species as of 1996 and more are being added to this total daily. The membership of Insecta is three times larger than that of any other animal order. The insects have successfully colonized the entire planet with the exception of the subtidal zone of the oceans.

Insects are characterized by three pairs of legs, two pairs of wings, a single pair of antennae, and a pair of compound eyes. Their bodies are protected by an exoskeleton of sclerotized cuticle and is divided into three regions: head, thorax and abdomen.

Insects have a ventral nerve cord and a dorsally located heart which pumps blood through an open circulation system. The blood is usually colourless or green and is rarely needed for oxygen circulation in the body. Tracheae regulate air exchange, and free amino acids are the major players in osmoregulation. Malphigian tubules rid the body of metabolic wastes.

The sexes are separate in all species, and copulation or the production of spermatophores are often the rule. Eggs are laid, but brooding by one parent is a specialization found in a few families. Development can be ametabolous, paurometabolous, or holometabolous.

The importance of insects is almost unfathomable. They are critical links in many food webs world wide. Two thirds of the world's flowering plants use insects as pollinators. Many insects are either parasitic or vectors of diseases, such as malaria and sleeping sickness, that cause human mortality. And they make great pets. The Insecta includes the following orders:

Isopoda

Introduction

With over 10,000 species, the order Isopoda is the second most diverse group of crustaceans. Isopods can be found in all types of habitats, but most species live in the sea or on land. Looking under garden rocks or rotting logs, you will often discover them. They are the small, hard-shelled creatures which resemble miniature armadillos, that roll into a tiny ball when disturbed. You might know them as rolly-pollys, or sow bugs. The most striking characteristic of isopods is their dorsoventral compression.

Morphology

The body of isopods is divided into three sections; the cephalothorax, the thorax and the abdomen. The compound eyes of isopods are dorsal and unstalked. Isopods are negatively phototactic (reathey ct to light by moving away from it), and usually remain hidden under rocks and debris. Their first antennae are short, while the second set is much longer and reach ahead of the isopod to "feel' for signs of food sources as well as potential danger. The first pair of legs are called gnathopods ("jaw-feet") because they are modified for grasping and tearing food. The other thoracic segments each bear a pair of walking legs. The last abdominal segment is a result of the fusion of the last four abdominal segments to form a posterior shield in all North American species. Uropods are the last pair of abdominal appendages that are biramous and extend beyond the end of the abdomen. Mature females have large, inwardly directed, platelike oostegites at the inner base of their anterior (front) legs. They form a shallow chamber on the ventral surface of the thorax called a marsupium. It serves to hold and protect developing eggs or young.

Reproduction

Isopods reproduce sexually and breeding occurs throughout the year. Thus, egg-bearing females may be found year-round, but most are found during the spring and summer months. During mating, males grasp females and carry them under their bodies using their periopods. They may remain attached for months. The male presses his ventral surface against the side of the female, placing his genital pores close to one of the female's genital pores. Sperm are released while the male pleopods vibrate rapidly. The male then moves to the other side of the female and repeats the process with the other genital pore. After 1 to 10 months, the eggs are fertilized and pass into the marsupium. The incubating eggs and newly hatched young remain in the marsupium for up to one month. The newly hatched first instar young look much like miniature versions of the adult. The number of instars in unknown, but there are thought to be at least 15 before maturity is reached. Most freshwater isopods have a lifespan of approximately one year.

Ecology

Freshwater isopods are primarily benthic and occur in the littoral zone of ponds, lakes, streams and springs, but they have been recorded at depths of as much as 55 m. They are the poorest swimmers of all freshwater crustaceans. In fact, they can barely swim at all and locomotion is usually restricted to a slow crawl. For this reason, they are exclusively associated with the substrate, or aquatic macrophytes where they live a somewhat secretive existence. Most species are scavengers, feeding primarily on dead and injured organisms, as well as green, decaying vegetation. As a result they form an important link in food chains by recycling dead and decaying material back into living tissue. Isopods have also been shown to be an important food item in the diet of many stream fishes.

Idiosyncratic Inverts

The tongue-eating isopod, Cymothoa exigua, lives in the mouth of the rose snapper (Lutjanus guttatus) and causes tongue stub and the floor of the fish's mouth and resembles the missing tongue!

Mysidacea

Introduction

Superficially, mysidaceans look much like small shrimp, and since they have a ventral marsupium, they are often called opossum shrimp. However, they are not true shrimp (order Decapoda), but belong to the order Mysidacea, a large, almost exclusively marine order. Mysidaceans are easily distinguished from decapods because they have 6 pairs of legs, while decapods have only 5. Mysis relicta is the most important and most abundant member of this order in North American freshwater systems, though it is seldom seen by humans except in the stomachs of fish! The name relicta refers to the fact that this species has a distribution which is restricted to areas of past marine inundation.

Morphology

As with many crustaceans, the first thoracic segment is fused with the head to form the cephalothorax in mysids. The head has two pairs of antennae and a pair of eyes. The eyes of Mysis are very large and are attached to stalks. The thorax has eight segments of which the first six are covered by carapace (segment 1 is fused with the head). The thorax of mature females bears oostegites, which project from the base of the thoracic legs. They are large and platelike, and form the marsupium, in which the eggs and developing young are protected. Because of this pouch-like structure, Mysis is sometimes called the opossum shrimp. Maxillipeds are found on the first two thoracic segments and are used to filter plankton and particulate matter from the water for food. The remaining six pairs of thoracic appendages are used for swimming, as well as for conveying water to the maxillipeds for filtering. The abdomen has 6 segments, of which the first five bear a pair of pleopods. On males, the fourth pleopod is long and specialized for mating. The last abdominal segment bears a pair of uropods that bear gravity-sensing statocysts that help the animal orient itself in the water column.

Reproduction

Mysis relicta reproduces sexually. Breeding occurs in the early autumn in Lake Ontario, but some individuals also reproduce in the spring. By contrast, studies in Michigan have identified four breeding seasons: mid-spring, late summer, mid-autumn and late winter. The difference may be due to differing water temperature in these two lakes. No one has ever observed mating, but males are thought to die shortly after copulation. Up to 40 developing embryos are carried by the female in its brood pouch, and pass through seven embryonic stages before their release after approximately five months. At this point, they are three to four millimetres long, and often migrate into shallow water. The entire life cycle of Mysis is completed in about two years.

Ecology

Mysis relicta occurs naturally in deep lakes in the glaciated regions of North America and Europe. It is usually absent from water less than 25 metres deep and its abundance increases with depth up to approximately 200 metres. During the day, Mysis remains very close to the lake bottom where it feeds on benthic organisms. At night, when it is closer to the surface, Mysis feeds on both zooplankton and phytoplankton.

 

 

Idiosyncratic Inverts

Fisheries biologists have long recognised that Mysis forms an important part of the diet of many fishes. In an attempt to boost fish populations, Mysis has been introduced into many lakes in North America. However, interfering with a natural food web can be a dangerous business! In 1983, Mysis was stocked in Lake Tahoe. The assumption was that it would provide food for young salmon, speeding their growth. Instead, Mysis ate the zooplankton that all the smallest fish (too small to eat Mysis) had depended on. The young salmon starved! In 1970, before the introduction of Mysis relicta, 40,000 kokanee salmon spawned in Lake Tahoe. Afterward, the spawning population was reduced to a few hundred individuals.

Notostraca

Introduction

Tadpole shrimp are large branchiopod crustaceans that occur in arctic Canada and on the prairies.

Morphology

Tadpole shrimp have a large, flattened carapace that covers their head and thorax. The rest of their body is elongate terminating in a pair of filamentous cercopods. Adults are from 10 to 58 mm long and have 35 to 70 pairs of thoracic legs that are often partially hidden by the carapace. The first pair of legs is used for swimming, while the rest are used for walking, digging and handling food as well as swimming.

Reproduction

Most notostracans have separate males and females (gonochoristic), but self-fertilizing hermaphrodites are not uncommon. Females have a specialized brood pouch on their eleventh pair of legs which carries their eggs. The eggs are carried in the brood pouch until they hatch into either nauplius or metanauplius larvae. These larvae undergo around 12 moults before becoming mature in 2 to 3 weeks. Adults moult throughout their life.

Ecology

Most species of notostracans occur in very temporary ponds, though one species Lepidurus arcticus is also found in large arctic lakes. Within Canada, notostracans occur in the north and the west. Notostracans feed on detritus and on anostracans, conchostracans and other benthic invertebrates.

Idiosyncratic Inverts

Because of their resemblance to primitive arthropods, such as the trilobites, the tadpole shrimp are celebrities. Their dried eggs are sold in tiny boxes and you can grow your own!

Ostracoda

Introduction

Ostracods are small crustaceans enclosed in a bivalved carapace. They are the oldest known crustaceans in the fossil record because their shells preserve well. They occur in nearly every aquatic habitat from polar lakes to tropical oceans, and they are important microfossils for dating ancient sediments.

 

 

Morphology

Ostracods are small animals, ranging in size from 0.3 to 30.0 mm, although most freshwater species range between 0.5 and 2.5 mm. Their bodies are completely enclosed in a calcified, bivalved carapace which is hinged dorsally. The surface of the shell may be smooth or pitted, punctuated, wrinkled, or reticulated. Several large muscles are directly attached to the carapace, and these points of attachment leave scars that are useful in discriminating some groups of ostracods.

Adult ostracods have 6 to 7 pairs of appendages. Although distinct segmentation is absent, the head possesses four pairs of appendages including the antennules, antennae, mandibles and maxillae, while the thorax has 3 pairs of legs.

Reproduction

Freshwater ostracods go through eight moult stages before they mature. Their life span varies from one month in temporary ponds to a year or longer in larger water basins. Reproduction may be either sexual or asexual. Some lineages are exclusively parthenogenetic , but other species reproduce sexually.

Freshwater ostracods are remarkable for their high frequency of species which have abandoned sexual reproduction. While transitions to parthenogenesis are rare in most other animal groups, about one-third of all freshwater ostracods have given up sex. The breeding system employed by any population is easily determined, as asexual populations contain only females, while sexual populations include both males and females.

Very few ostracod species incubate their eggs within their body; most lay their eggs either singly or in groups on sediment or aquatic vegetation. These eggs may hatch soon after they are laid or remain undeveloped for a year or longer. The ability of eggs, larvae, and adults to withstand freezing and desiccation is a very important biological characteristic which aids the dispersal of ostracods from one body of water to another.

Ecology

Freshwater ostracods are free-living except for members of one group which are commensal on the gills of crayfish. Most free-living forms are benthic, though pelagic forms also occur.

Idiosyncratic Inverts

Ostracods possess the largest sperm in the animal kingdom in both relative and absolute terms. Ostracod sperm can be up to ten times the length of the male's body! Some male ostracodes need a special organ (Zenker's organ) to aid in sperm transport.

Glossary

Citation

(2014). Crustacea. Retrieved from http://www.eoearth.org/view/article/151549

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