Marine Arthropods..(crustaceans,
Insects, arachnids) .
Lecture
Notes -
Arthropods make up the
largest phylum with over a million species. They are jointed legged found in every
habitat. Crustaceans are the dominant class of marine Arthropods such as
lobster, shrimp and barnacles. The bodies are divided into a cephalothorax and
abdomen. The success of crustaceans in the sea is bought about by the gills,
walking legs, swimming legs, and feeding appendages. Water striders and tide
pool insects are the representatives of insecta and the arachnids are the sea
spider (Class Pycnogonida) and horseshoe crabs (Limulus).
The success of
Arthropods is due to their segmentation, and jointed external covering, which
allows for great mobility and strength. Growth is accompanied by shedding or
molting of the exoskeleton because otherwise growth would be impossible and
therefore imposes limitations in terms of size and growth.
Now in addition to
crabs and lobsters, crustaceans make up a major component of plankton.
Crustaceans bear 5 pairs of appendages, two pairs of antennae, mouth parts. One
major line of crustaceans have evolved with large walking animals. Extensions
of the body wall at the base of the legs are used for gills. The blood contains
hemocyanin, a copper based respiratory pigment like hemoglobin. Sexes are
separate and they develop through a series of larval stages increasing size and
numbers of segments with their associated limbs. The simplest larva stage is
the nauplius larva, with three pairs of appendages.. 1st and 2nd pair of
antennae and mandibles and often swims and suspension feeds (different
functions of appendages in larva than adult.)..larger larva develop from the
nauplius according to the type of crustaceans. Some do bypass the nauplius by
developing within the egg.
Small crustaceans are
everywhere, plankton, bottom, among sediments, in and on other animals and
plants. These are the bugs, flies and mosquitoes of the sea.
Copepods The class
Copepoda includes minute sea inhabitants which provide a major source of food
for fish, mollusks, crustaceans and other animals as major components of the
plankton. Some are parasitic, infesting invertebrates or vertebrates (fish
lice) and the free living are small and capable of rapid population turnover.
The large first antennae may be used for quick escape but more commonly are act
as parachutes against sinking. They can filter feed on phytoplankton but can't
survive on this alone all year round because of low phytoplankton levels, and
sometimes seize large prey items like other members of the zooplankton. During
the rich spring bloom, filtering becomes worthwhile and the Copepods can pass
through several generations quickly before returning the normal rate , often
using the fat reserves built up in the bloom for the rest of the year (# of
generations per year. related to time period of phytoplankton bloom.)
Barnacles (video
of barnacles)
Barnacles are
sedimentary marine crustaceans permanently attached to the substratum. For
protection barnacles have carried calcification of the cuticle to an extreme
and have a shell resembling that of a mollusk. The shell is derived from the
cuticle of the barnacle head and enclosed the rest of the body. Class Cirripedia,
feed with six pairs of thoracic legs (cirri) which can protrude though the
shell plates to filter food suspended in the seawater.. They can filter fine
material, including phytoplankton and even bacteria. They are hermaphrodites
and carry out cross fertilization between neighbors. There are 6 nauplius
stages increasing size which swim and filter phytoplankton over a period of a
month or so before giving rise to a non-feeding larva--the cypris larva. This
is the sediment stage of the life cycle , able to drift and swim in the
plankton before choosing a settlement site in response to environmental factor
which the larva detects by an array of sense organs. Barnacles colonize a
variety of substrata including living animals such as crabs, whales turtles, sea
snakes and with this moving host, the barnacle does not need to use energy in
beating its cirri.
Beach fleas and other
Amphipods the most familiar being the beach fleas or sand hoppers found on
sandy shores. The Amphipods are mostly marine and scavengers of detritus able
to creep using the thoracic periopods (legs) and swim with the abdominal ones.
They can feed by scraping sand grains or filtering phytoplankton. The head and
tail curve downwards and they commonly are found among debris washed ashore. Whale
lice belong to this group of 5,000 species.
Isopods are found in
many of the same environments as Amphipods and can be recognized by their
flattened top to bottom form. Fish lice belong to this group.
The eucarids...Krill
and decapods
The planktonic krill
and decapods (shrimps lobsters, crabs) are classified in the suborder Eucarida.
they have a well developed carapace, fertilized eggs are carried beneath the
abdomen.
Krill have primitive
features . All are marine, eggs hatch as nauplii, most have luminescent organs,
usually on the eyes, at the base of the 7th thoracic limbs and underside of the
abdomen. They are probably used for communication in swarming and reproduction.
Krill are pelagic and filter feed when phytoplankton conditions are suitable
but otherwise prey on larger planktonic organisms. Phytoplankton rich seawater
enters the tips of the legs and is strained as it passes between the leg bases.
Whale krill reach 2 in. long and dominate the zooplankton of the Antarctic Ocean and is the chief food of many baleen
whales.
In decopod, the first
3 pairs of thoracic appendages are adapted as auxiliary mouthparts leaving 5
pairs of legs (decopods) (and the 1st pair of these can be claws). Decopods
have been divided into swimmers (natantains) and crawlers (reptantains)
essentially, shrimps and prawns ///and lobsters, crayfish and crabs. Now they
are divided morphologically into two sub-orders..shrimplike with many branched
gills and planktonic eggs hatching as nauplius larva (Dendrobranchiata) and
Pleocyemata which have gills lacking secondary branches and eggs carried on
pleopods before hatching as zoeae.
Prawns and shrimp have
no exact zoological definition and are interchangeable. The most important
shrimp families are the penaeids and sergestids. The first contain commercial
shrimp. Most pelagic shrimps are active predators feeding on crustaceans of the
zooplankton, such as krill and Copepods. Bottom dwelling species are scavengers
but range from carnivores to herbivores. Sexes are distinct although some
females pass through an earlier male stage.
Lobsters and
freshwater crayfish ...belong to a group known historically as macrurans (large
tails) but now make up 3 infra-orders , Astacidea (lobsters, freshwater crays,
scampi) the Palinura (spiny and Spanish lobsters and Thalassinidae (mud
lobsters and mud shrimps).
Lobsters and
freshwater crays walk on the substratum on the four pairs of back legs. The 1st
pair is modified as a pincer. They are carnivorous scavengers living in holes
on rocky bottoms. The American lobster can reach 2'long (48 lbs) and live for
100 years. Fresh water crays are omnivorous about 4 inch long.
Squat lobsters and
hermit crabs are intermediate between lobsters and crabs. The abdomen is
variable in structure and hermit crabs probably evolved from ancestors that
lived in crevices and eventually specialized into using discarded gastropod
shells. Hermit crabs live as carnivorous scavengers on sea bottoms ranging from
bottom to sea shore and have a terrestrial existence in the tropics.
True crabs.
These crabs all
possess a reduced abdomen held permanently flexed beneath the cephalothorax The
reduction of the abdomen has brought the center of gravity of the body directly
over the walking legs making locomotion very efficient and rapid. The sideways
gait assists this. The crab shape is therefore the ultimate shape in efficient
crustacean walking. They live beyond the top of the shore to the deep sea to
the hydrothermal vents 1.6 miles deep and on the other hand the ghost and
fiddler crabs live at the top of sandy and muddy shores up rivers and into
fresh water. Most crabs burrow to escape their predators descending back-first
into the sediment . Some can swim using the last pair of legs as paddles, run
rapidly (ghost crabs) , pea crabs live in the mantle cavity of bivalves and the
female coral gall crabs become imprisoned by surrounding coral growth with a
small opening left for plankton and the small male for reproduction. Most crabs
are scavengers although terrestrial ones can eat plan material.
Economically the
crustaceans are important as a food source for man and as plankton for the
organisms of the sea. Crayfish--over a million pounds a year are caught and
another 2 million reared artificially.
Horseshoe crabs and
Sea Spiders..Chelicerated Arthropods...no jaws, no antennae
CLASS MEROSTOMATA
Chelicerated
Arthropods with abdominal gills and a long spikelike telson. The class contains
Limulus the horseshoe crab and fossil sea scorpions called eurypterids.
This genus appeared
175 million years ago and not undergone any evolutionary change since. It also
occupies a unique place in Arthropods. Like spiders it possesses chelicerae,
claws instead of chewing jaws, lacks antennae and has 6 pair of appendages.
Horseshoe crabs or
king crabs have a protective hinged carapace which covers the crab with a long
caudal spine protruding behind. There are compound eyes on the carapace and
median simple eyes. Beneath the carapace lie the chelicerae and 5 pairs of
walking legs (comparable in evolution to the chelicerae, pedipalps and 4pr of
walking legs in spiders. They live on sandy or muddy bottoms in the sea,
plowing their way through the upper surface of the sediment. During burrowing,
the spine levers the body down while the 5th pair of walking legs acts as
shovels. The carapace form helps move through sand and the spine is used to
right itself if it gets turned over. They are scavenging carnivores and have
jawlike extensions on the bases of their walking legs used to trap and macerate
prey such as clams and worms before it passes to the mouth. The last pair of
legs use the bases to crack open bivalves. The appendages at the rear are
modified and each is expanded into 150 gill lamella resembling leaves of a book
and appendage movement maintains a current over the gills. Small horse shoe
crabs can swim upside down using their gills as paddles. Reproduction occurs at
night when they congregate in intertidal zones, the female laying 2-300 eggs
which get fertilized by the clasping male. Eggs hatch into larva which mature
in 3 years.
Class Pycnogonida
500 species about an
inch long...except giant sea spider. The abdomen is reduced to a knob. The
intestines extend into the legs as well as gonads located in legs. Between
palps and 1st pair of legs are an extra set of legs with which the male carries
the eggs. It has 4 eyes near the anterior end of the animal and has no
respiratory or excretory system.
Sea Spiders are
exclusively marine found in the intertidal zone and deep sea. They are able to
grip the substratum with their claws and sway from one individual to another
They feed by either sucking up the preys body tissues or cutting of pieces with
their chelicerae and eating them. Colors are usually white/transparent but red
in the deep sea species. Economic importance seems to be nothing.
Chapter Questions
Amphipod
Reading and Questions
Sand-Burrowing Amphipods
1. Where are amphipods found?
2 What is “sand lickng”?
3 How do the pleopods function?
4. Why are amphipod inhabitants of sandy
beaches?
5. How is the food supply replinished?
Readings