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GENERAL
CHARACTERISTICS OF CRUSTACEANS The Subphylum: Crustacea is a group of mostly aquatic, and
marine, animals. Some terrestrial
forms are restricted to damp sites.
They may be identified by having the following characteristics: Head
Appendages. -- Two pairs of antennae located
on the 1st and 2nd segments. One pair
of mandibles used for
crushing. Two pair of jaw-like
structures called maxillae on
the 1st and 2nd segments. Origin
of Appendages. --The origin of appendages is
from primitively biramous structures:
protopod, exppod and endopod.
Cephalothorax. -- The head and thorax are fused in many cases to produce
the cephalothorax. Carapace. --This structure covers the head and one or more thoracic
segments. It is variable in size and
attached only to the head and some thoracic segments. It is frequently very conspicuous. Excretory
System. -- One pair of nephridia are
involved in excretion. They occur
either at the base of the second maxilla or at the base of the second
antenna. They are often called "green glands." Circulation. -- The heart and some vessels may occur in the haemocoel
to circulate the blood. Blood is
colorless and may contain haemocyanin or haemoglobin. The blood also is able to carry much
oxygen in contrast to the terrestrial arthropods where blood carries very
little oxygen. Larval
Stage. --A Nauplius larva is present.
Respiration. -- This usually involves gills. ------------------------------------ The Class: Branchiopoda, whose name means "gill-feet" includes primarily freshwater species. Some are able to swim by using their
antennae. They are fee living with
compound eyes and usually a carapace.
The mandibular palp is rarely present. They show both primitive and specialized characters. See Inv128 for example.
The Order Phyllopoda includes the fairy shrimp,
tadpole shrimp and brine shrimp. These are primarily
small (5 cm.) freshwater species with gills on their legs. They differ from other crustaceans by the
absence of the 2nd maxilla. Some
species display up to three reproductive strategies: bisexual, parthenogenesis and
hermaphroditic. The elongated tails
structures are frequently as long as the remaining body. The Order Cladocera includes Daphnia species. They are free-swimming crustaceans that tend to swim with their
dorsal side up. They have a two-valve
carapace that covers most of the body save for the appendages. In some members the carapace covers only
the brood pouch. The head is
generally separated from the body by a deep indentation. It projects forward as a beak or rostrum. On the front of the head there is a single
compound eye, which is derived from two fused eyes. Most species also have a simple naupiliar eye. ------------------------------------ Please see following plate for
Example Structures of the Branchiopoda: Plate 49 = Phylum: Arthropoda:
Crustacea: Branchiopoda, Copepoda & Ostracoda ------------------------------------ The Class Remipedia include a group of
blind crustaceans that occur in coastal aquifers containing saline
groundwater, with members found in almost every ocean basin. The first
described remipede was the fossil Tesnusocaris
goldichi (Lower Pennsylvanian), but, since 1979, at least
seventeen living species have been identified with global distribution
throughout the Neotropics. Their size is variable from 10–40
millimetres long and include a head and an elongated trunk of up to forty-two
similar body segments. Swimming
appendages are lateral on each segment, and theyswim on their backs. They are
slow-moving. They have fangs connected to secretory glands; it is still
unknown whether these glands secrete digestive juices or poisonous venom, or
whether remipedes feed primarily on detritus or on living organisms. They
have a primitive body plan for crustaceans, and have been regarded as an
ancestral crustacean group. However, least one species, Godzilliognomus frondosus, has a highly
organised and well-differentiated brain, with a large olfactory area that is
a common feature for species that live in dark environments. The size and
complexity of the brain suggests that Remipedia might be the sister taxon to
Malacostraca, regarded as the most advanced of the crustaceans ------------------------------------ Class: Cephalocarida – horseshoe shrimp are small, 2-4 mm. crustaceans that have
an elongated body and a large head, the posterior edge of which covers the
1st thoracic segment. Eyes are absent
and the 2nd pair of antennae is located posterior to the mouth, which is
unique for the Crustacea. The mouth
is behind the upper lip and mandibles occur on either side. The 1st pair of maxillae is very small and
the 2nd pair has the same makeup as the following thoracic legs. A large basal area has outgrowths on the
inner side that are used for locomotion.
A forked inner branch and two outer lobes, or pseudoepipod and exopod,
are present. Maxillae are
unspecialized. There are 10 thoracic segments and the abdomen bears a
telson but no other appendages. ------------------------------------ Class:
Maxillopoda – barnacles, fish lice, etc. Subclass: Cirripedia are the barnacles. They are the only Crustacea, which are
frequently sessile. They are
typically hermaphroditic. A carapace
encloses the entire body and is very heavy by being impregnated with calcium
salts. All species are found in the
marine environment, and some are parasitic.
See Inv127 for examples
Subclass: Copepoda includes the genus Cyclops. They are primarily marine organisms bud some species also occur
in freshwater. They have a single
median eye and swim by means of their antennae. Parasitic forms are common. See Inv125 for example.
------------------------------------ Please see following plate for Example Structures of the Copepoda: Plate 49 = Phylum: Arthropoda:
Crustacea: Branchiopoda, Copepoda & Ostracoda ------------------------------------ Class: Ostracoda – seed shrimp. They
have the appearance of clams because their carapace is bivalved. They are very tine and live in marine and
freshwater environments. In northeastern North America they are some of the
first animals to appear in ponds in springtime. They differ little in form of
the body but have a great variety of appendages. Their cephalic limbs are well developed and complex. There is frequently a gastric mill and
usually a pair of hepatic caeca.
Antennal and maxillary glands are present. Parthenogenesis is common in some species males are
unknown. See Inv126 for example
------------------------------------ Please see following plate for Example Structures of the Ostracoda:
Plate 49 =
Phylum: Arthropoda: Crustacea: Branchiopoda, Copepoda & Ostracoda ------------------------------------ The Class: Malacostraca – have abdominal appendages and a definite number of
segments: Head = 6, Thorax = 8 and
Abdomen = 6. The eggs hatch into more
advanced larvae (e.g., Mysis
larvae) Eight
principal groups are shown as follows: Syncarida A superorder of
Melacostraca called "mountain
shrimps." They lack a
carapace and oostegites but have exopodites on all thoracic limbs. Eyes are sessile or stalked or completely
absent. The exopod is in the shape of
a whip.. They live in freshwater
lakes and streams. They appear to be
confined to the Southern Hemisphere. See Inv134 for example.
Order: Mysidacea. -- Primarily marine animals. They have a carapace, which covers most of the thoracic
somites. They have stalked eyes. These are small normally pelagic
animals. See Inv132 for example.
Order:
Cumacea is a small group of marine or brackish water organisms
often called "hooded shrimps." They have a uniform body plan that
distinguishes them from other Crustacea.
There is an enlarged carapace, a slender abdomen and forked tail. They are small ranging up to 10 mm long. They have two eyes that are often fused in
some species. They have a short life
span living in benthic mud and debris.
They are frequently observed swarming at the water surface. See Inv135 for example.
Order:
Leptostraca are marine organisms that are
principally filter feeders. Some
authorities consider them to be the most primitive members of the
Malacostraca. They are small, ranging
to about 16 mm long. They have eight
abdominal segments instead of the usual six.
Their compound eyes are stalked.
The carapace is very large and covers the head and thorax. Eggs hatch to produce a Manca stage immature individual. See Inv133 for example.
Order: Isopoda. -- The pill bugs or sow bugs are flattened
dorso-ventrallay. Abdominal
appendages are modified into gills.
Both terrestrial and marine species occur. Terrestrial species are widespread and found under rocks and
garden wastes where they often abound. See Inv129 for example.
Order: Amphipoda. -- Primarily marine.
They do not have a carapace.
The eyes are sessile and the body is usually depressed. There is much variety exhibited in the
group. See Inv130 for example.
Order: Stomatopoda. -- Includes the mantis
shrimp, a burrowing form. They
have a shallow carapace that is fused with three thoracic somites. The 2nd thoracic limb has a large, raptorial subchela. They are all marine and live primarily in
burrows. See Inv131 for example.
Order: Decapoda. -- The order is divided into three suborders which are
(1) Macrura = "Big
Tail" includes lobsters and crayfish, (2) Anomura = Peculiar tail" includes the hermit crab, and (3) Brachyura = "Short Tail"
includes the crabs. The lobster, Homarus americanus,
is an example animal for the Malacostraca: Decapoda: Macrura. It is a marine animal that burrows along
rocky ocean shores (See Plate 55)
Body Plan. -- The cephalothorax is covered by one carapace. The anterior tip bears the rostrum, which protects the eyes and
other sense organs in the region.
Abdominal segmentation is very conspicuous. Body
Wall. -- The exoskeleton is chitinous
and covers the entire body. It also
has high protein content and the calcium salts that impregnate it give it
hardness. The epidermis is derived from the hypodermis. Connective tissue does not form a thick layer. Muscles are not in a continuous tube or
sheath, but are broken up into a particular segmental pattern. The haemocoel serves for circulation of the blood, and functions by a
sloshing about of the blood. Head
Appendages. -- There are six segments in the
head, the most anterior of which does not bear appendages. The 1st antennae are small, not biramous, and sensory to tactile and
chemical stimuli. Statocysts
in the base contain grains of sand. The 2nd antennae are large and biramous.
The exopod is scale-like while the endopod is a long major
branch. It is tactile and chemical
sensory. The excretory pore, or green
gland, lies at the base. The mandibles function as crushing structures. The maxillae are accessory mouthparts that function in the
manipulation of food. They are
sensory also. The 2nd maxilla bears a
gill bailer, which bails water
out of the front end of the gill chamber. Thorax
Appendages. -- There are eight segments in
the thorax. The 1st, 2nd and 3rd are maxillipeds. These are leg-like appendages that are
modified into extra mouthparts. They
function to manipulate the food and are partially sensory. Five pairs of pareiopods are the "walking
legs." The first pair is
large pincers called chelipeds;
the second and third pair is small chelate pincers, and the fourth and fifth
pair is nonchelate pincers. The legs are modified into structures for defense and offense. They are pincer-like at the end (=
chelate). The third and fifth pair is
referred to as the coxopodite, the 3rd pair being the female genital pore and
the 5th pair the male genital pore.
The exopods are lost during development. Gills are borne on the bases of the thoracic appendages. Abdomen
Appendages. -- The first pair is modified
into a copulatory organ in the male, while in the female these are reduced to
a vestige. [Note: in the crayfish these are the first two
pairs] The second through fifth pair of abdominal appendages are pleopods or gills that create a water current. The sixth pair are uropods, which form the tail fan.
Food
& Digestion. -- Fish and some clams and snails
constitute the food. These are
captured by chelipeds and food is passed on to the mouthparts and to the
mouth. The mouth is ventrally located
near the mandibles. The esophagus opens into the cardiac
stomach. This is a gastric mill with three teeth that
is modified to chew food. The teeth
are shed at molting. The midgut is a tube that contains digestive glands or a liver. The intestine ends in the rectum with a rectal gland and anus.
Digestion is intracellular and extracellular, with a major portion
occurring in the digestive gland or liver. Circulation. -- The heart, located dorsally, has three pair of ostia, which admit the blood. Seven arteries lead out away from the
heart. They are (1) ophthalmic artery
to the head, (2) antennary artery to
the base of antennae, (3) hepatic
artery to the digestive gland, (4) sternal artery to ventral parts of the animal, which passes
between a split in the ventral nerve cord and then divides into two branches,
(5) dorsal abdominal artery,
(6) ventral thoracic artery,
and (7) ventral abdominal artery. The arteries simply open out into the spaces of the haemocoel.
Blood runs from the haemocoel to the gills and back to the
haemocoel again via ostia of the heart.
The blood contains haemocyanin and has colorless corpuscles that are
believed to function in coagulation. Respiration. -- The gills are outgrowths of the body wall and covered
by a lateral flap of the carapace.
The early recognition that gills are part of the exoskeleton provided
evidence for their being molted. It
was much later also recognized that the counterparts of gills, the tracheoles
of insects, were similarly involved in the molt. The swimmerets circulate water around the gills, and the gill bailer pulls
water out the front end through the 2nd maxilla. Excretion. -- The green glands located at the base of the 2nd
antennae are involved in the excretion process. These glands are similar to nephridia but they are bathed in a
haemocoel and not a coelom as is true of nephridia. Support
& Protection. -- The exoskeleton is very hard
and rigid, while the pincers are formidable defense weapons. Locomotion. -- Homarus
can only walk; there is no swimming of adults as the body is too heavy. The abdomen pushes the animal along backwards. Sense
Organs. -- There are sense organs located
on the 1st and 2nd antennae, as well as on the mouthparts. The compound eyes are well developed and consist of a large number
of ommatidia with square
ends. Each ommatidium consists of a
cornea, pigmented cells, a lens and a nerve element. Statocysts are hollow cavities with sensory cells at the base of the
1st antennae. They are an inpocketing
from the surface (ectoderm and cuticle on the inside). Molting results in a loss of statoliths (sand grains), which must
be reacquired after each molt. Nervous
System. -- The system is very similar to
that of the Annelida. There is a
dorsal brain, circumesophageal connectives, and a double, solid, ganglionated
ventral nerve cord. The brain is larger and more subdivided, however. Fusion of ganglia is in the anterior part
of the body. Reproduction. -- The sexes are separate in Crustacea. Genital openings are located on the 3rd
peripod in the female and the 5th peripod in the male. The annulus ventralis is the seminal receptacle of the female
located on the posterior part of the 5th periopod, and is not connected with
the genital tract. In the male a sperm guide is located
on the first pair of pleopods, which have become modified. Fig. Inv123 shows the relative positions of the
heart, testes and gonoduct.
Sperm is stored in the seminal receptacle of the female, and
they are liberated at egg laying time.
Eggs are attached by a sticky secretion to the swimmerets of the
female and undergo most of their development there. Most larval stages are in the egg. Mysis
larvae occur that
swim until after the third molt. ------------------------------------ CRUSTACEANS
OF PRIMARY MEDICAL IMPORTANCE Most crustaceans are aquatic, being dominant in the sea but
also occurring in lakes, ponds and streams.
A few species such as pill and sow bugs are terrestrial in damp
environments. Terrestrial crustacea
fluoresce with various colors under ultra-violet light. Shrimp, lobsters and crawfish that include
most of the group, and their habits in the aquatic environment are similar to
that of insects on land. Most species
are free-living. They prey as
scavengers on animals that inhabit aquatic plants. Many of the species occur in very large numbers. The smaller species are important food for
larger aquatic animals. However, only
a few species of crustaceans are important in transmitting human parasites
(Matheson 1950). Two groups, the Malacostracea and
Entomostraca, have members that are intermediate hosts of human parasites. These are the
larger crustaceans such as crabs, lobsters and crayfish. Among the several orders, only the
Decapoda act as intermediate hosts of human parasites. Included here
are very large numbers of small fresh water and marine species, some of which
have a parasitic life. Their importance
as the main food supply of larger aquatic animals such as fishes is
great. Among the four orders,
Cladocera, Phyllopoda, Ostracoda, only Copepoda, only the Copepods are
intermediate hosts of human infections. DISEASES
CAUSEED BY CRUSTACEANS Some of the Crustacea serve as
intermediate hosts of human parasites, their involvement being known from
Biblical times (Matheson 1950). The
larval stage of the Guinea worm (Dracunculus
medinensis) occurs in copepods of the genus Cyclops. Animals and humans are infected from the drinking water. When the infected Cyclops are swallowed the Guinea
worm larvae exit and penetrate the mammal's intestinal tract. They move through the tissues ending up in
cutaneous connective tissue. After
10-14 months adult female worms may reach over 100 cm. in length. They produce blistered lesions mainly on
the lower parts of the body. The
offspring of the females are emitted from lesions that break when the mammal
enters fresh water. There are few
symptoms in the mammal until the worm females are mature, making it difficult
to control an infestation. The
late-appearing symptoms in animals include itching, vomiting, diarrhea and
unrest. Distribution of the Guinea
worm is now widespread in both hemispheres (Matheson 1950). Lung fluke of
the Far East and the Neotropics, Paragonimus
westgermani, larvae infest freshwater crabs and crayfish. The lungs of mammals including humans
become infected. In the life cycle a
snail (Melania) and a
freshwater crayfish or crab are involved.
Humans become infected when consuming raw crabs or crayfish raw. Usually the parasite passes from the lungs
to other body organs and even the brain.
Symptoms vary depending where the parasite ends up in the body. An important
parasite of humans is the broad fish tapeworm, Diphyllobothrium latum, which is common in North
America. The larval stages develop in
freshwater crustaceans Diaptomus
and Cyclops. Infected crustaceans are consumed by fish
after which the larvae penetrate the stomach wall. After a few days the larvae attach to the tissues where they
remain in an encysted state called "plerocercoids." Consuming uncooked fish results in human
infestation. Tapeworms mature in
about six weeks after ingestion. They are widely distributed throughout the
world. Infections can result in
severe anemia and weakness (Matheson 1950).
Key References: <medvet.ref.htm> Brusca, R. C., and Brusca G. J. 2003.
Invertebrates. Second Edition. Sunderland, Massachusetts: Sinauer Pub. Brusca, R. C. & G. J. Brusca. 2003. The Crustacea. In: Brusca R.C. & G. J.
Brusca, Invertebrates. Sunderland, MD: Sinauer Associates
pp. 511–587. Ferrari, F., Fornshell, J., Vagelli,
A., Ivanenko, V., and Dahms, H. 2011. Early
Post-Embryonic Development of Marine Chelicerates and Crustaceans with a Nauplius. Crustaceana 84 (7): 869–893. Matheson, R. 1950. Medical Entomology. Comstock Publ. Co, Inc. 610 p. Service, M. 2008.
Medical Entomology For Students.
Cambridge Univ. Press. 289 p Legner, E.
F.
1995. Biological
control of Diptera of medical and veterinary importance. J. Vector Ecology 20(1): 59-120. Legner, E. F.. 2000.
Biological control of aquatic Diptera. p. 847-870. Contributions
to a Manual of Palaearctic Diptera, Vol. 1, Science Herald, Budapest. 978 p. Turner, J. 2004. The
Importance of Small Planktonic Copepods and Their Roles in Pelagic Marine
Food Webs. Zoological
Studies 43(2): 255–266 |
