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Marine Biology Phylum Arthropoda
Arthropods include an incredibly diverse group of taxa such as insects, crustaceans, spiders, scorpions, and
centipedes. There are far more species of arthropods than species in all other phyla combined, and the number
of undescribed species in the largest assemblage of arthropods, the insects, probably numbers in the tens of
millions. Members of the phylum have been responsible for the most devastating plagues and famines mankind
has known. Yet other species of arthropods are essential for our existence, directly or indirectly providing us
with food, clothing, medicines, and protection from harmful organisms.
A. General characteristics
1. Segmented bodies - most have 2 body segments (crabs have only one)
a. Segmentation affects both external and internal structure.
b. Tagma (plural Tagmata) - Some segments are fused to form specialized body regions. These include
the head, thorax and abdomen, and the process and condition of fusion is called Tagmosis.
2. Exoskeleton - the body is covered with an exoskeleton, secreted by the epidermis, made up primarily of
the protein chitin (pronounced Kaitin); lipids, other proteins, and calcium carbonate also play a role.
a. A nitrogenous polysaccharide insoluble in water, alcohol, dilute acids, and digestive juices of most
animals
b. Serves as a protective armor and a place of muscle attachment
c. Jointed appendages -The exoskeleton is jointed to allow movement of limbs, or.
d. Movement of appendages is controlled primarily by a complex muscular system, divided into
smooth and striated components as in chordates.
3. Arthropods generally grow by molting their exoskeletons in a process called Ecdysis (ek -da-sis Gr.
Ecdyem - to strip off). The periodic shedding of the old cuticle and the formation of a larger, new one.
a. Occurs most frequently during larval stages and less often as the animal reaches adulthood.
b. The molting process and preparation for it, is a continuous process going on during most of the
animals life
4. Bilateral symmetry
5. Open circulatory system, one way digestive system, and a ventral nerve cord.
The phylum Arthropoda is made up of many classes. The following are some of the more important marine
classes:
Phylum Arthropoda
Class Merostomata – (meaning thigh mouth) - Horseshoe Crabs
Class Pycnogonida (meaning “all leg”)– Sea Spiders
Class Crustacea
(1) Subclass: Branchiopoda (branchi meaning gill) --fairy shrimp, brine shrimp
(2) Subclass: Copepoda (cope meaning oar)--copepods
(3) Subclass: Cirripedia (meaning "curl-footed“)--barnacles
(4) Subclass: Malacostraca: (Greek: "soft shell")
Order: Stomatopoda-- (stomato, meaning
“mouth”) mantis shrimp
Order: Euphausiacea—(meaning “young fry of fish”) krill
Order: Decapoda--shrimps, lobsters, crabs, crayfish
Class: Merostomata
A. Merostomata includes two rather different groups of marine organisms, the horseshoe crab and the
eurypterids. Eurypterids (sea scorpions) are now extinct; they lived 200 to 500 million years ago. Some
were huge, reaching a length of 3 m.
1. Their morphology suggests that they fed on a variety of kinds of foods.
2. Some may have been amphibious, emerging onto land for at least part of their life cycle.
B. The horseshoe crabs are and ancient group of only 5 species that exist today
1. They feed on small invertebrates.
2. Horseshoe crabs are often used as laboratory animals by physiologists.
C. The most common type of horseshoe crab has the scientific classification of Limulus polyphemus.
1. The head and thorax of the horseshoe crab are fused together to make the Cephalothorax.
2. The body is covered with a hard, thick shell or shield.
3. A narrow shell that is hinged from the main shell at the front and ends in a long, sharp spine protects the
abdomen.
4. The animal has a large compound eye on each side of the Cephalothorax, two pairs of smaller, simple
eyes between the compound eyes, and five light-receptive organs beneath the shell.
5. Its mouth is located in the middle of the Ventral (underside) of the Cephalothorax.
a. The 1st pair of pincers, the chelicerae, for seizing food is found on each side of the mouth.
b. The second pair of appendages, the pedipalps, resemble walking legs. These, along with the
additional five pairs of appendages, are used as walking legs.
6. They have a long, spike-like appendage called a Telson that projects from the rear of their bodies,
used for steering and to turn itself over (righting itself).
7. Respiration is via book gills, flat leaf-like gills found on the abdominal appendages.
8. It feeds on small invertebrates that are buried beneath sand or mud.
9. In spring the female lays eggs in the intertidal zone of bays and estuaries.
Class: Pycnogonida (sea spiders also called Pantopoda (pan: meaning all) or pycnogonids)
A. Sea spiders appear to be a sort of marine "spider," but in fact their relationships are enigmatic. They may
represent a very early branching of the chelicerate lineage. There are approximately 1000 described species
of Pycnogonida, all of which are marine.
1. Pycnogonids can be found from the intertidal regions to depths of around 7000m. Most are small, but a
few deep-sea forms reach up to 70 cm diameter across the legs.
2. They feed by sucking juices from soft-bodied invertebrates through a long proboscis.
3. Pycnogonids vaguely resemble spiders, with small bodies and relatively long, hinged legs.
B. Unique characteristics include an unusual proboscis, which varies in size and shape among species, but
amounts to a chamber with an opening at the distal end (the true mouth lies between the proboscis
chamber and the esophagus).
1. The body itself is not divisible into neatly- organized Tagmata or regions as it is in most arthropods.
2. an anterior region bears, besides the proboscis, three or four pairs of appendages, including the first pair
of walking legs.
3. Special appendages called ovigers when present make up the four pair; these play a role in the brooding
of young and are used in cleaning.
4. Following the first segment is a series of segments making up a "trunk," each segment bearing a pair of
walking legs.
5. A terminal segment includes a tubercle that projects dorsally and an anus. Some species have more than
four pairs of walking legs.
C. Pycnogonids are also unique in bearing multiple gonopores, found on the second segment of some or all of
the walking legs.
Class: Crustacea
General Characteristics of Crustaceans:
A. Approximately 30,000 species make up this Class or Subphylum. Most are aquatic; of these, the majority are
marine but some are found in fresh water. Members of the Class (Subphylum) include lobsters, crabs,
crayfish, shrimp, copepods, barnacles, and several other groups of organisms.
B. All have two pairs of antennae, a pair of mandibles, a pair of compound eyes (usually on stalks), and two
pair of maxillae on their heads, followed by a pair of appendages on each body segment (crustacean bodies
usually are made up of head, thorax, and abdomen, although the segments composing these Tagmata differ
among different Classes). The appendages are primitively branched (biramous), and although this condition
is modified in many species, adults always have at least some biramous appendages.
C. Crustaceans respire via gills. Like other arthropods, all have a hard but flexible exoskeleton called a
carapace.
D. Most crustaceans are free-living, some are sessile and a few are even parasitic, and there are four main
methods or feeding techniques used by Crustaceans:
1. Predation - Most use their maxillae and mandibles to take in food. The walking legs, including
specialized Chelipeds, which may be used to help capture prey and tear it into edible pieces with their
appendages.
2. Scavenging - scavenge organic material by scraping it from the substrate, from living fish in the case of
cleaner shrimp, or even from fallen leaves and other plant material (detritus) in the case of terrestrial
isopods.
3. Parasitism - A few species live on other crustaceans or on fishes.
4. Filter feeding - Some crustaceans filter tiny plankton or even bacteria from the water
E. Most crustaceans are dioecious. The actual mechanisms by which fertilization is achieved vary greatly. Some
crustaceans hatch young that are like miniature adults; others go through a larval stage called a Nauplius.
F. Many species, including lobsters, crayfish, barnacles, and crabs are important to human economies, some
very much so. Others, such as krill, are at the base of extremely important marine food chains. Still others
are crucial in recycling nutrients trapped in the bodies of dead organisms.
Subclass Malacostraca (malac, meaning soft & ostrakon, meaning a shell)
With over 20,000 species, Malacostraca is by far the largest Subclass of Crustaceans. Its members, who are
primarily marine but also occupy some freshwater and terrestrial habitats, are extremely diverse. They include
isopods (sow bugs), amphipods, euphausiids (krill), beach hoppers, mantis shrimp, and a very large order, the
Decapoda, which contains many kinds of shrimp, crabs, crayfish, and Lobster.
A. Malacostracans are characterized by a trunk with eight thoracic and six abdominal somites, each bearing a
pair of biramous appendages. As in branchiopods and cephalocaridans, primitive malacostracans feed by
filtering food from water; currents that move water for feeding are generated by thoracic appendages.
B. The Subclass Malacostraca contains a number of species of considerable economic significance. These
include edible lobsters, shrimp, crayfish and crabs. Equally or more significant are the many malacostracans
which, as larvae or adults, contribute to plankton and as such are at the base of an immensely important
marine food chain.
Order Decapoda (Gr. Deka, ten + pous, podos, foot)
The best known Order - includes lobster, shrimp crayfish and crabs, prawns. Important food source for humans
and economically important to human society.
A. External Anatomy - Most have two body segments, crabs have only one.
1. Cephalothorax - head and thorax fused to form the first body segment.
2. Carapace - The hard outer cuticle which covers and protects the body or Cephalothorax. This outer
shell contains 3 layers.
a. Outer Epidermis - contains the pigment cells (chromatophores) which are controlled by
neurosecretory cells in the eyestalks.
b. Middle - contains the polysaccharide for strength
c. Inner - Membraneous layer
3. In some groups, the carapace may cover most if not all of the body. Not found on underside of abdomen
on those decapods with two body segments (shrimp, lobster, crayfish etc.)
a. Walking legs are attached to the thoracic (thorax) body region.
4. Chelipeds - the first set of appendages that are modified into claws for grasping and holding prey.
a. Also used for defense and during sexual display.
5. Abdomen - second body segment (portion you eat) which can contain swimmerets on underside.
a. The swimmerets are used for swimming, and in some are modified to hold eggs.
b. The blue crab uses its last set of oar shaped appendages for swimming.
6. Uropods - The last pair of appendages which are wide and are used as rudders for swift backward
movements.
7. Telson - found in the middle of the uropods and along with the uropods form a protective device for
eggs on the swimmerets
8. Mouth parts -Decapods contain 3 pair of mouth parts:
a. Maxillipeds - they are jawed feet that situate and hold the food in front of the mouth.
b. Maxillae - these shave the food and bring towards the mouth.
c. Mandibles - crush and grind the food. Along with the maxillae, the mandibles are involved in actual
ingestion.
9. Exoskeleton - Advantages:
a. Provides maximum protection
b. Strength do to the angle of muscle attachment.
10. Disadvantages:
a. Cumbersome
b. They are vulnerable to predation, especially during and right after molting.
11. Molting: (inhibits maneuverability), and they must molt to grow (Their body is very soft right after
molting, and they are vulnerable to predation). Molting is partially controlled by the water temperature,
and the photoperiod (length of daylight).
12. Eyes - Stalked and compound eyes contain visual units: Lens, light sensitive cell, and nerve connection
to the brain.
B. Crayfish (Crawfish, Crawdad, etc.)
1. Crayfish have a two-part stomach:
a. Gastric Mill (food-crusher) - first part of stomach - where food, which has, been torn up by the
mouthparts, can be ground into fine particles by three calcareous teeth.
b. Filter of setae (strainer) - second stomach, where the nutritive juices are separated from the
innutritious hard parts of the food and passed on into the intestine. Food particles pass through and
then enter the intestine for chemical digestion.
2. Respiration - the gills of crustaceans vary in shape (treelike, leaf like, or filamentous)
a. All are provided with blood vessels or sinuses
b. They are usually attached to the appendages, and kept ventilated by the movement of the
appendages.
c. Protected by the carapace
d. Some smaller crustaceans breathe through the general body surface
3. Green Glands - Excretory and osmoregulatory organs in crustaceans are these paired glands located on
the head, at the base of the antennules.
a. Waste products are mostly ammonia, with some urea and uric acid
b. Wastes also are diffused through the gills as well as through the excretory glands.
C. Circulation - Most of the body cavity is an open "hemocoel," or space filled loosely with tissue, sinuses, and
blood. The circulatory system is open and consists of a heart, arteries, and the open spaces of the hemocoel. The
blood flows through these sinuses.
1. Neuro-genic Heart - nerve impulse cause the heart to beat
2. The movement of organs and appendages as circulates the blood more effectively in open sinuses
than in the capillaries.
a. A Neurotoxin can wipe out these nerve impulses (Cephalopods like the octopus contain
neurotoxin glands)
b. Ours is a Myogenic impulse, which originates from within the muscle itself
c. Some smaller crustaceans lack a heart
3. The blood may contain as respiratory pigments either hemocyanin or hemoglobin and it has a
property of clotting to prevent loss of blood in minor injuries.
4. Most crustaceans contain hemocyanin, which is copper base fluid
D. Nervous System - nerve cords are located on the ventral surface (better protected) and are ladder shaped 2 brains and 2 spinal cords, send nerve to viscera, appendages, and muscles
1. Statocysts - chambers filled with fluids that deal with balance and equilibrium, are located at the base
of each antennule, adjacent to the eyes.
a. These chambers are lined with sensitive hairs that project into the fluid
2. Chemosensitive Setae - Used to detect chemicals in the water and are most numerous on the antenna,
antennules, and mouthparts.
3. Tactile Receptors (setae) – Sensitive to touch, and are found on the cuticle of most of the body, legs
and antenna.
4. Movement, and touch and located within tissues and muscles,
5. Compound eyes - stalked eyes similar to insects, so they are precise and can detect or perform the
following:
a. Wide field of vision - 200 degrees or more
b. analyze polarized light
c. detect motion
d. Most crustacean eyes are adapted either to bright or dim light, depending on either nocturnal or
diurnal habits
6. The median eye is found in the Nauplius larva and in some adult copepod forms may be the only adult
eye.
E. Reproduction - Most crustaceans have separate sexes (dioecious)
1. Fertilization - Sexual external fertilization is the most common method
a. Sperm are generally released into the water close to the female's swimmerets
b. The eggs are released through the gonopore and held on the abdomen (swimmerets) by a
sticky substance.
c. Most crustacean’s brood their eggs in some manner.
d. Malacostracans usually carry eggs and young attached to their appendages(swimmerets)
2. Most larvae produced by crustaceans must go through a series of physical changes (abrupt or gradual)
over a series of molts to assume the adult form.
F. Decapods have a variety of mating responses:
1. Fiddler crab - male has one large (oversized claw) used for ritualistic sexual display, and fighting with
other males
2. Blue crabs - mass up off-shore
a. Abdomen strongly flexed under the carapace for protection.
b. Gills are attached to the base of the walking legs, and are protected by the carapace
c. They walk or swim sideways, and stalked eyes allow for good vision
d. In certain species of crab and crayfish, the male will defend the female during molting. Mating
occurs after loss of the shell, and the male will protect until the new shell hardens.
3. Shrimp, crayfish and lobster - 2 body regions are the Cephalothorax and abdomen.
a. Lobster - form line and walk to deep water in single file after first winter storm (helps cuts
resistance)
b. The Florida spiny lobster has no Chelipeds, but the male does have one long small claw used for
reproduction
c. Protected by law during certain months of the year, from April 1st through the beginning of
August.
d. Lobster farming - they capture egg bearing females, and then the larvae are given a diet of
minced clams to inhibit cannibalism. Over 30% will survive to 4th molt, then released.
G. Autotomy - Crustaceans have the ability to release an appendage (legs and antenna), whether injured or
to get away from a predator, and then later regenerate that lost appendage.
Subclass Cirripedia (l. Cirrus, curl of hair, + pes, pedis, foot)
A. Barnacles (2 types) which are usually enclosed in a shell of calcareous plates, as well as three smaller Orders
of burrowing or parasitic forms.
1. Sessile as adults and may be attached to the substrate by a stalk (goose barnacles) or directly (acorn
barnacles)
a. Stalked - fleshy flexible muscular stalks capable of lengthening and contracting to feed. Found
on floating organisms, debris, or attached to hard substrate.
b. Acorn - shell attached on substrate
2. Nauplius - larval form, which drifts with plankton for 1-12 weeks, molts several times, and then
attaches to substrate.
a. The carapace (mantle) surrounds the body and secretes a calcareous shell
b. The head is reduced, the abdomen absent, and thoracic legs are long, many-jointed cirri with
hair like setae.
3. The cirri (used for feeding)are extended through an opening between the calcareous plates to filter from
the water the small particles like plankton and detritus on which the animal feeds
4. Head is attached to substrate by a cement gland.
a. They secrete a calcareous shell around themselves.
b. Some attach to motile organisms like whales or other shelled animals
c. Some attach to boats, piers, and docks etc.
Subclass Copepoda (ko-pepo-da) (gr. Kope, oar, + pous podos, foot)
A. Second only to the Malacostraca in number of species
1. Copepods are small (usually a few millimeters or Less in length), rather elongated, tapering toward the
Posterior end, lacking a carapace, and retaining the simple, median, Nauplius eye in the adult.
2. They have four pairs of rather flattened, thoracic Swimming appendages, and a fifth, reduced pair
3. Ecologically, the free-living copepods are of Extreme importance, often dominating the primary
consumer level in aquatic communities (bases of the food Chain).