marine invertebrates biol 505

MARINE Invertebrates

BIOL 505Understanding Marine

Invertebrates, Their Environments

and Processes

Phylum Arthropoda (Jointed Animals)Defining Characteristics:

1. Epidermis produces segmented, jointed, hardened (sclerotized) chitinous exoskeleton, with internal musculature between individual joints of appendages.

2. Complete loss of motile cilia in adult and larval stages.

Phylum Arthropoda (Jointed Animals)

>75 % of all animals described to date belong to this phylum.

Thus arthropod body plan is most represented in all animal kingdom.

Like annelids, arthropods basically metameric, with new segments arising from special budding zones at posterior of animal as larva develops.

However, in most extant spp, the metamarization is masked when segments fuse and modify for specialized function (tagmatization), also seen in polychaetes, but reaches greatest extent in arthropods.

General Characteristics


Two of the major arthropod groups (Insecta and Crustacea) have 3 distinct tagmata: head, thorax, and abdomen.

Arthropods lack cilia, even in larval stages.



Exoskeleton.

A hard, external, protective covering. However, unlike molluscan hard outer covering. Covering in the 2 phyla produced by very different processes, have different chemical composition, different physical properties, and perform different functions.

Molluscan covering serves mainly as protection for soft body within.

Arthropod covering does this, but also functions as locomotory skeleton.



Exoskeleton secreted by epidermal cells.

Outermost layer (epicuticle) usually waxy, made of lipoprotein layer, underlain with layers of lipids.

Advantage: epicuticle is impermeable to water – so water loss minimized through body surface.

Disadvantage: body surface can’t be used for gas exchange.

Most of exoskeleton made up of the endocuticle, or procuticle, mainly made of the polysaccharide chitin, associated with several other proteins.

General CharacteristicsExoskeleton

Phylum Arthropoda (Jointed Animals)General Characteristics


Chitin is used in protection, support, and movement, and provides rigid skeletal system.

The procuticle is strengthened by several hardening elements. In the crustaceans, hardening is partly achieved by depositing CaCO3 in some procuticle layers.

“Tanning” the procuticle’s protein component also helps hardening. This process (sclerotization) involves forming cross-links between protein chains.

The procuticle varies in thickness, and does not harden uniformly over the entire body. This is where the main functional significance lies.



In many regions of body, procuticle is thin and flexible in different directions, forming the joint.

Appropriate musculature joined to exoskeleton provides for jointed appendages that function in much the same way as vertebrate skeleton, with pairs of muscles that antagonize through system of rigid levers.

The jointed, flexible exoskeleton is the secret of arthropod success.



For animals encased in suit of rigid plates, coelom can play no role in locomotion.

Thus, for arthropods, coelom is greatly reduced.

Instead, main body cavity is hemocoel, part of the blood circulatory system, as in molluscs.

General CharacteristicsThe Hemocoel


Unlike the shell of molluscs, arthropod skeletons do not grow gradually. Instead, secreted over all regions of body simultaneously.

Once hardened, arthropod is encased in it’s shell, except where sensory hairs protrude and gland openings occur.

Major regions of hind and foregut also lined with cuticle.

To grow, arthropods must shed cuticle (including that lining gut), grow, the harden new cuticle around larger, sometimes morphologically different, body.

Old cuticle is degraded by enzymes, and split before molting.

General CharacteristicsMolting


Old cuticle splits by uptake of water and increased blood pressure that cause body to swell.

Ecdysis (Gk. “escape”, or “slipping out”) is process of removing old cuticle.

New cuticle is secreted before old on removed, so animal can stay partially active.

Temporarily “soft” crabs rely on high blood pressure in hemocoel to maintain locomotory function, so hemocoel acts as internal ???

hydroskeleton, until new exoskeleton hardens.



Proecdysis (premolt): the period before molting when the new exoskeleton is deposited below the old one. Lost appendages will begin to regenerate as limb buds that will unfold at the time of molting. Crab actively storing salts which will be necessary in the shedding process as well as storing water and food reserves.

Ecdysis (molt): active shedding of exoskeleton. During this stage which is the shortest in duration of the four, salts and water used by crab’s hemolymph to build up hydrostatic pressure and crack the exoskeleton to be able to withdraw from it. Land hermit crabs molt while in their shell which acts as a mold for the soft crab.



Metecdysis (postmolt): freshly molted crab begins to harden and recover movement ability. The crab will consume his exoskeleton to recycle necessary minerals and salts to aid in the calcification process.

Anecdysis (intermolt): the longest period during which the exoskeleton will begin to bulk up as calcium and minerals are consumed and deposited. As the crab grows, this phase gradually increases in length.



Although increases in size discontinuous, tissue growth (biomass) continuous process.

If number of epidermal cells increases continuously (as in some arthropods), additional tissue becomes folded into pleats until molt, and increase in size takes place.

Ecdysis and formation of new exoskeleton are under neural and hormonal control.

The Y-organ (gland located in head of crustaceans) produces ecdysteroid hormones that stimulate molting.

Production of EH inhibited during intermolt by 2nd hormone.



Second hormone produced by neurosecretory complex (X-organ) in the eyestalks.

When X-organ ceases, Y-organ activity no longer inhibited and ecdysone produced.

Or, X-organ secretions may not turn Y-organ off, but may inhibit action of ecdysone directly.

Eyestalk oblation results in premature production of ecdysone ecdysis.



Several other functions under neurohormonal control:

1. Regulation of reproductive cycle

2. Regulation of body fluid osmolarity

3. Migration of light-screening pigments in eye

4. Movements of pigment granules within chromatophore cells gradual color changes of body.



Arthropod nervous system different than both vertebrates and other invertebrates.

In contrast to vertebrate muscle contraction, in arthropods, strength of contraction depends on rate at which impulses delivered to muscle fibers.

A single fiber may be innervated by up to 5 different types of neurons.

Contraction type (slow, long vs fast, short), partly depend on source of stimulation.

Also, some neurons are inhibitory.

General CharacteristicsNerves and Muscles


Additionally, arthropods have muscle fiber types that differ physiologically and functionally, so rate of contraction is partly function of individual muscle fiber.

Thus, fine control of movement depends on both on types of muscle fibers stimulated and interaction of several neuron types on single muscle fiber.

Also, single neuron may innervate many muscle fibers, so a given muscle may be innervated by only a few neurons.



Arthropod musculature differs from other invertebrates in that arthropod muscle is ALL striated (most other inverts have mainly (or all) smooth muscle.

Striated muscle can contract much more quickly.



Circulation begins when blood is collected directly from the hemocoel into the heart through ostia (holes in the heart).

Blood leaves heart through closed vessels (anterior and posterior aorta), that dump the oxygenated blood into the hemocoel.

Thus, the circulatory system is “open” with blood moving through a series of sinuses.

General CharacteristicsCirculation


Circulation


Arthropod vision uses one of two forms:

1. Ocelli

2. Compound eyes

An ocellus is a cup with a light-sensitive surface backed by light-absorbing cells.

Cup is often covered with lens (corneal lens). Photosensitive pigment underlying cup is derivative of vitamin A combined with a protein. Stimulation by light causes chemical change in pigments (rhabdom), sending signal down neurons. Not usually image forming.

General CharacteristicsVisual System


Visual System


Compound eyes can form images. This eye system well represented among crustaceans and may occur along with ocelli.

Composed of many individual units (ommatidia) that are each oriented in slightly different direction from the other, due to eyes convex shape.

Each ommatidia has 1) a fixed-focus lens (cornea) that has depth of field from 1 mm – several m all in focus at receptor; 2) underlying gelatinous crystalline cone that acts as lens in most crustaceans; 3) series of up to 8 photoreceptors (retinular cells) each with light-sensitive pigment;



4) Cylindrical cells (collars) containing shielding pigment, that isolates every ommatidium from it’s neighbor; and

5) a neural cartridge at the basal end that is a cluster of neurons receiving information carried by retinular cells, and sending action potentials to optic ganglia for processing.

Crustacean eyes sensitive to polarized light. Polarized light known to be used by some arthropods as navigational cue.



Light-sensitive pigments of retinular cells contained in thousands of rhabdomeres, fine microvilli that fold out from retinular cell walls.

Rhabdomeres in each omatidium form distinct, ordered association (a central shaft) called the rhabdom.

Thus, the rhabdom is not really a structure, but a central area formed by the microvilli of the retinular cells.

Rhabdom records light intensity at center of image that falls on its tip; not the entire image. So, tip of single rhabdom analogous to single rod in mammal eyes.



Visual System


Remainder of cylindrical rhabdom acts to guide light for this segment of the image down to the neural cartridge at the base of the rhabdom.

The brain then constructs the complete image from the thousands of omatidia. However, image is not put together as single image, but as many images from slightly different angles.

Each omatidium functionally isolated from neighbor by shielding pigments, or reflective trachioles.



Visual System


Basic compound eye called apposition eye, because each lens positioned directly apposed to receiving rhabdom.

Since each lens is very small, each rhabdom receives only small amount of light. Thus apposition eyes work best with high light intensity.

To work well in low light, each neural cartridge must receive light from more than one ommatidium.

If screening pigment absent between ommatidia, many facets can combine light received into single image on retina.

This type eye is superposition eye.



Visual System

Superposition eye Apposition eye


In superposition eye, ommatidium has large space between distal end of crystalline cone and rhabdom.

Pigments that typically inhibit light passage, migrate out of the way, and allow light to pass from several lenses into one rhabdom.

This produces signal of greater intensity than received through single lens.

Pigment migration under hormonal control, so superposition eye takes time to light- or dark-adapt.



Sharpness of image formed depends on:

1. How much the light hitting rhabdomere enters at angle parallel to long axis of that ommatidium.

2. How much light from other ommatidium impinge on receptor pigments of an ommatidium (reduced resolution).

3. Amount of difference adjacent ommatidia are oriented (decreased angle = increased resolution).

4. Number of ommatidia/eye (more = better resolving power).



Sharpness of image formed depends on:

5. Complexity of brain receiving impulses from ommatidia.

Even most complex and largest compound eye forms somewhat coarse-grained image, since image is formed from collaborative effort of from 6 – few thousand neural cartridges.



Defining Characteristics:

1. No antennae

2. Body divided into two distinct sections (prosoma and opisthosoma)

3. First pair of appendages (chelicerae) on prosoma adapted for feeding.

Subphylum Chelicerata


The only arthropods without antennae.

First anterior segment has no appendages at all.

Second anterior segment has pair of clawed appendages (chelicerae), next to mouth, for grabbing and tearing food material.

Chelicera also have no mandibles (appendages found next to mouth in other arthropod groups, used for chewing and grinding food during digestion).




1. Body NOT divided into tagmata (distinct regions).

2. Unique proboscis with opening at tip, located at anterior of animal.

3. Number of walking legs varies among spp.

Class Pycnogonida



Small group of ~ 1000 spp.

Known as “sea spiders”, all spp are marine and have obviously long legs; from 3 – 16 times the length of the body.

Body length ranges from few mm (shallow spp) to >10 cm (deep water spp).

Majority of the body is prosoma. Abdomen (opisthosoma) reduced to short stump.

Found in all world oceans and have lengthy fossil record (Ordovician).

Class Pycnogonida

Phylum Arthropoda (Jointed Animals)Class Pycnogonida


Unlike true spiders, pycnogonids lack special respiratory or excretory systems.

Do have complete digestive system with sucking mouth at anterior tip of proboscis.

Both digestive system and gonads extend well into the legs.

Most spp have 4 pairs of walking legs, posterior to pair of chelicerae and pair of palps.

Some spp have 5 or 6 pairs of walking legs.

The head also holds pair of ovigers used by both sexes (gonochoristic) to groom other legs and trunk.

Class Pycnogonida


Ovigers also used by males to carry eggs after fertilization.

Unlike most other arthropods, juvenile pycnogonids grow during both the molt and intermolt stages. The thin flexible joint membranes stretch as tissue mass increases.

Adults mostly free-living, but extremely slow moving.

Larvae, if leave egg sac before completing development, grow as parasites on cnidarians.

Many adults and juveniles are parasitic in, or on, or are commensal with other marine invertebrates, including gastropods, bivalves, echinoderms and scyphozoans.

Class Pycnogonida


Most adults carnivores, feeding on bryozoans, colonial hydozoans, poriferans, and anthozoans.

One well-studies sp, Pycnogonum litorale, can be starved for several months.

Class Pycnogonida


Anoplodactylus evansi


Unidentified fluorescent pycnogonid with fluorescent cyanobacteria

Phylum Arthropoda (Jointed Animals)Subphylum Mandibulata


1. Appendages on third head segment modified as mandibles for chewing/grinding food.

2. Retinula of compound eyes contains 8 cells.

3. All members have mandibles on the third head segment for feeding.

4. Group contains animals with both uniramous and biramous appendages.

5. Mandibulata have 3 classes: Myrapoda (centipedes); Insecta; Crustacea.



1. Head has 5 pairs of appendages, including 2 pairs of antennae.

2. Development contains a triangular larval form (nauplius) with 3 appendages and a single middle eye. Even spp that hatch at later stage of development pass through nauplier stage.

~ 45,000 crustacean spp

Most crustaceans divided into 6 subclasses:

Class Crustacea


Crustacean Subclasses:

1. Malacostraca

2. Branchiopoda

3. Ostracoda

4. Copepoda

5. Pentastomida (all parasitic)

6. Cirripedia

Class Crustacea


Subclass Malacostraca


1. Thorax with 8 segments, abdomen with 6 – 7 segments plus telson.

2. Appendages on sixth abdominal segment, flattened to form uropods.

Class Crustacea


Class CrustaceaSubclass Malacostraca


This subclass contains ~ 60% of all described crustacean spp.

Includes the decapods, euphausiids, stomatopods, isopods, and amphipods.

Basic body plan is tripartite, with head, thorax, and abdomen.




A. Euphuasiida

B. Isopoda

C. Amphipoda

D. Stomatopoda

E. Decapoda


Unlike insect head that is separated from thorax by flexible joint, crustacean head is almost always fused with thorax, and may be covered by a carapace extending posteriorly from head.

May function as a cephalothorax.

In some spp carapace may have anterior projection (rostrum).

Have large stalked compound eyes, as well as 2 pair of head appendages; 1st (antennules) and 2nd (antennae).


marine invertebrates biol 505

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