MARINE INVERTEBRATES


  Marine invertebrates make up most of the zooplankton and bottom fauna, and include a wide variety of animals such as sponges, jellyfish, coral, worms, starfish, and sea urchins.  Plankton are free-floating or very weakly swimming marine organisms.  The phytoplankton are the producers, like photosynthetic algae and seaweeds, and chemosynthetic bacteria.  Zooplankton are the animal plankton mainly fish and other animal eggs, larvae, microscopic animals and jellyfish.  Most zooplankton eat phytoplankton or other zooplankton and are eaten by fish and other predators.   
Crustaceans like shrimp and lobster, and mollusks like clams, scallops, and squid are favorite marine invertebrate seafoods for many fish, marine mammals and people.  The objective of this lab is to examine the major types of marine invertebrates.  


Safety Cautions:

1.      Safety goggles must be worn during the dissection of the squid.

2.        Wear gloves when handling preserved specimens not on prepared slides..

A.   Sponges

  Sponges are primitive multicellular animals that lack specialization of groups of cells into tissues with different functions.  The interior of a sponge contains a complicated system of canals and cavities connected with the exterior by many incurrent pores and one main exhalant opening.  Flagellated cells beat constantly in the cavities, pulling water into the sponge through the pores.  Food particles and tiny plankton are trapped inside the sponge, and filtered water is expelled through the exhalant opening.  Proteinaceous spongin fibers or mineral spicules are embedded in the soft tissues of sponges.  Commercial sponges are the spongin skeletons of a variety of marine sponges from the Caribbean, the Mediterranean, and Australia.  Fire sponges are quite painful to touch and are a hazard for the recreational diver who may brush against them.  These species produce poisonous substances, which are introduced into a diver’s skin when spicules penetrate the skin.

  Procedure: (Optional)

  1.     Examine some specimens of marine sponges.  Spicules are also available if you are interested in putting them
        on a microscope slide and seeing them.


  1.     Some marine animals hide inside sponges, swimming in through the exhalant opening. 
        Some decorator crabs pick up sponges and place them on the crabs’ backs or legs.  Why do crabs 
        do this?    

  B.  Cnidarians - Hydroids, Sea Anemones, Jellyfish and Coral  

  Cnidarians have specialized tissues, with an outer ectoderm and inner endoderm constituting the body wall.  Between them is a thin, noncellular layer.  Coordination is achieved by means of an intercellular nervous system.  Cnidarians are tentacled predators which kill their prey with special stinging cells called nematocytes.  Fire coral gives swimmers a painful sting, while the most dangerous jellyfish, the Pacific Sea Wasp of the tropical Pacific, has venom so lethal people who are stung can die within minutes. 
Cnidarians reproduce sexually, and asexually by budding.  Most cnidarians alternate sexual and asexual generations.  The attached polyp is the asexual generation; the floating medusa is the sexual generation.

  Procedure: (Optional)

   1. Examine a live hydra, and/or a preserved hydra on a prepared slide.

         For a high power look at a live hydra, use a toothpick to transfer a live Hydra to a slide,
        place a cover slip over it and examine it first under low power.  Locate the gastrovascular
        cavity, which is its complete digestive system.  Note any buds being formed. 
        Locate the nematocytes in the ectoderm; they appear as small beads.

 2.     Place a drop of acetic acid at the edge of the coverslip.  Record what happens to the
        nematocytes. On high power, look for the tiny bristlelike trigger, the cnidocil, on the
exterior of the undischarged nematocytes.  Look for the barbs and long thread of the
discharged nematocytes.

  3.     Examine the prepared slides of Obelia polyps and medusaeObelia is a hydroid
        whose polyps are often abundant on seaweeds, especially kelp.  On the slide of the
        polyp colony, note two types of individuals: tentacled hydranths which each have a
        mouth at the base of the tentacle ring, and nontentacled gonangia which produce
.  On the medusa slide, observe the resemblance of medusas to umbrellas,
        with tentacles at the rim and a central mouth.  Radial canals make up much of the
        digestive tract, and in mature specimens you will see gonads as dark-staining bodies
        below the radial canals. 


1.     Gonangia produce the sexually reproducing medusa stage; what is the function of the

2.  What advantage is alternating generations to cnidarians?

  C.    Annelid Worms - Sandworms, Feather Dusters and Leeches

  Annelid worms, like the terrestrial earthworm, are segmented worms with digestive, circulatory, excretory, nervous and reproductive systems.  Most marine worms are predatory crawlers on the sea floor or deposit-feeding burrowers in the sediment.  Projecting from each segment are many bristles which can be long and poisonous.  Fireworms of Caribbean reefs have such bristles which easily penetrate and break off in skin if they are picked up.  “Feather-duster” worms of reefs live in calcareous tubes and filter-feed with a network of gills and cilia that looks like a feather duster sticking out of the tube.  While some marine leeches are bloodsuckers, and these are fairly common on sharks and rays, many marine leeches are predators feeding on snails and other marine invertebrates.

  Procedure: (Optional)

  1.     Examine the live sandworms and leeches.


  1.     In the past, leeches were used to treat many medical problems believed to be caused
        by “bad blood” because of their bloodsucking ability and their anticoagulant
        secretions.  Leeches are still used today in the most modern hospitals, on rare
        occasions when appropriate, because of their anticoagulant abilities.  What
        circumstances or injuries might merit leech treatment today in Boston hospitals?
        (see Leeches at

   D.   Mollusks - Clams, Oysters, Scallops, Snails, Slugs, Nudibranchs, Octopuses and

  Mollusks are characterized by having a mantle cavity between the main body and an enclosing envelope, the mantle.  Many possess shells of calcium carbonate secreted by the mantle.  Mollusks also have a muscular “foot” usually used for propulsion; the foot has developed into muscular tentacles in octopuses and squids.  Oysters, clams and scallops are bivalves with two shells.  They are filter-feeders and important seafoods, and pearl oysters are cultured to produce pearls because they secrete layers of mother-of-pearl (also called nacre) around foreign objects like sand particles between the shell and the mantle.  Typical marine snails have shells; sea slugs and nudibranchs are shelless marine snails.  Nudibranchs (nude gills”) actually do not have true gills; respiration takes place through the skin on gill-like structures. Marine snails browse algae or prey on other invertebrates.  Some predatory Pacific cone snails have venom that is deadly to man.  Octopuses and squids have a beak that delivers a poisonous bite to their prey.  In this part of the lab, you will dissect a squid.


Obtain a squid and examine its external characteristics.  Note the ten arms and the funnel.  Jets of water forced through the funnel propel the squid in the opposite direction to which the funnel is pointed.  The mantle extends posteriorly into triangular fins.  
Begin a careful dissection to reveal internal structures.  Cut across one of the tentacles to see the hollow tubelike blood vessels inside.  Examine the mantle skin.  Look for dark dots in the skin.  These are chromatophores, pigment cells.  These cells contain blue, purple, red and yellow pigments.  By increasing or decreasing chromatophore size, squid can change color rapidly.  
The pen, or shell, of the squid is a toughened feather (or rod)-shaped plate concealed beneath the skin of the back  The true head is the short region between the arms and the mantle collar.  It contains two eyes.  The digestive system includes a muscular pharynx (throat), esophagus, salivary glands, stomach, cecum (like our appendix), intestine, rectum and liver. Cut between the two bottom arms to reveal the pharynx.  There is a powerful beak in the pharynx, and a rasping tongue organ, the radula, with rows of chitinous teeth.  Digestion starts in the stomach and is finished in the cecum.  Wastes move from the intestine to the anus, which opens in the funnel. Above the rectum is the dark ink sac, with a duct which opens near the anus.  An inky cloud can be squirted out through the funnel to confuse a predator.  There is a circulatory system and two gills with a heart at the base of each which pumps blood through them.  From the gills, the blood goes to the primary heart, which pumps it to the body.  Determine the sex of your squid - females have an ovary;  males have a penis and testis.  Males produce sperm packaged in sacs called spermatophores, which they transfer to a small pouch, the sperm receptacle, near the female’s mouth.     


  1.  Why does the mantle expand posteriorly (near the rear end) into triangular fins?  

2.     When a squid grabs prey in its tentacles, it flushes the tentacles full of blood.  Why?  

  E.  Echinoderms - Starfish and Sea Urchins 

  Echinoderms, such as starfish, brittle stars, sea urchins and sea cucumbers, have internal skeletons of calcareous plates, and a water-vascular system connected to tube feet.  Brittle stars and sea cucumbers are scavengers feeding on organic debris;  sea urchins are grazers feeding on plant and animal matter, while starfish are predators which can be very destructive on oyster beds and coral reefs.  Sea cucumbers are a delicacy in Oriental restaurants.  

Procedure: (Optional)

1.   Obtain a starfish and examine its external characteristics.  The upper surface has many short blunt spines, with modified spines called pedicellariae at their base.  The pedicellariae resemble little stalked pincers.  Their function is to cleanse the surface of debris and small organisms.  This is critical because respiration occurs through minute openings on the upper surface of the rays.  These areas look soft and furry, but actually are covered with cilia.  A large madreporite is the entrance to the water vascular system, and there is also an anal opening on the upper surface.  On the lower, oral side is the mouth.  The madreporite connects to a ring canal which encircles the mouth and from this a radial canal passes outward in each ray just above the groove.  The radial canals branch into the tube feet, which the starfish uses to move.  The foot can be extended by forcing water into it, or contracted by muscular contractions that force water out.  Starfish can move about 6 inches per minute.  The tube feet are also the main sensory organs, with connections to the nervous system.


  1.  What is the advantage of having sucker disks at the end of the tube feet?


  Brine shrimp are crustaceans, like lobsters, crabs and shrimp people eat.  Crustaceans are very abundant members of the zooplankton community.  Even lobsters and crabs are members of the microscopic zooplankton community in their larval stage.  Brine shrimp can respond to some harmful microclimatic changes, like drying out, by going into diapause, a hibernationlike state.  In doing so, a brine shrimp forms a capsulelike cyst, as a protective coating.   There are many varieties of marine shrimp.  For example, pistol shrimp stun prey with high intensity sound by snapping their claws.  Mantis shrimp smash or spear their prey.  Cleaner shrimp act like barbers and dentists, cleaning up other species by removing food debris from their teeth and ectoparasites from their skin.  Krill are large shrimplike crustaceans abundant near Antarctica.  Gammarus, a scavenging crustacean also known as a beach flea,, is common under seaweed washed up on the beach and can often be seen hopping away when the seaweed is disturbed.  Barnacles and parasitic fish lice are also crustaceans.

Procedure: (Optional)

1.      Examine a few dry brine shrimp (Artemia franciscana) diapause cysts.

2.  Additional crustaceans are available for viewing under the microscope if you are interested and there is time
near the end of lab.  These include prepared slides of zooplankton crustaceans, as well as live brine shrimp.  Use a depression slide with the brine shrimp.  The prepared slides include crab larvae, barnacles, parasitic fish lice, and Gammarus the beach flea.


  1.   What is the advantage to a brine shrimp which lives in the ocean of being able to
survive drying out by morphing into a diapause cyst?  What circumstances could cause
it to dry out?  Explain.  

  2.   Crustaceans are important food sources for many animals.  Krill are the main food for
what marine mammals?  What is chitin, and why do people take chitosan pills made
      from chitin? (see Chitin (Chitosan pills) at

  3.  Which of the invertebrates groups examined in this lab is most important for the ocean, and why?

While some procedures in this lab are optional, your lab report should contain answers to ALL the analysis and final conclusions questions.