Chapter 11: Marine Invertebrates

Phylum Porifera

A stovepipe sponge, image from Wikipedia

Sponges are common in the benthic habitats of all the world’s oceans. These incredibly simple and almost entirely sessile creatures are believe by scientists to be the oldest animals on Earth. Sponges consist of two layers of cells, perforated by lots of tiny pores and a few larger openings that make up a complex system of channels that the sponge uses to filter feed. This filter feeding is powered by a group of specialized flagellated cells that can beat in a coordinated motion removing bacteria and other food particles from the water column. Encrusting sponges have a internal skeleton made of calcium carbonate. Other sponges contain glass like structures called spicules that can serve both for support or defense. Sponges are on the menu for lots of organisms including sea turtles and nudibranchs. Despite their lack of organized tissue, they are animals, and they are capable of sexual reproduction when conditions call for it.

Phylum Cnidaria

Sea nettles, image from Wikipedia

Sea anemones, hydra’s, corals, and jellyfish are all members of the phylum Cnidaria. Nearly all cnidaria spend some part of their life in one of two physical forms; the medusa and the hydra. The medusa is found as jelly filled “bell” with tentacles hanging down, that to some degree, can swim. Click on the link to watch a Vimeo video of a jellyfish swimming. The hydra form is typically sessile with the tentacles facing up. Both forms have a one opening is both mouth and anus and most cnidarians have a simple nerve net to help coordinate their movement. Most cnidarians are carnivorous and equipped with specialized stinging cells called cnidocytes or nematocysts. The basic body plan of cnidarians is organized around a circle (radial symmetry) with two tissue layers. The inner layer (endoderm) that functions in both digestion and absorption and an outer layer (ectoderm.)

Phylum Ctenophora

A sea gooseberry, image from Wikipedia

Comb jellies are truly bizarre and wonderful marine animals. Named for the eight rows of cilia (combs) that move them through the water. Ctenophores are gelatinous organisms with two cell layers much like the cnidarians. Ctenophores range in size from a few millimeters to over a meter and are nearly all carnivorous. Most surface dwelling ctenophores are transparent, and some deep sea species carry a red pigment, but nearly all are also bioluminescent, capable of producing their own light.

Phylum Platyhelminthes

A bedford flatworm, image from Wikipedia

Flatworms have perfected the role of the parasite. Of the four groups of flatworms, three are entirely parasitic. Nonetheless, there are some remarkably beautiful flatworms with body shapes and behaviors that are significantly more complicated that the sponges and jellies. Flatworms have a bilateral symmetry, meaning they have a front, a back, a left, and a right side. This allows flatworms significantly improved locomotion and the ability to hunt using sensitive eye spots and a very simple brain. Flatworms have two layers of tissue rather than the three that other worms have. This means they lack a true digestive, respiratory, and circulatory systems and are limited to flat bodies through which oxygen and carbon dioxide can easily diffuse through. Flatworms are mostly hermaphroditic and some flatworms participate in an unusual battle for sexual gender rolls called penis fencing. Click on the link for a purely G-rated YouTube video.

Phylum Annelid

A christmas tree worm, image from Wikipedia

The annelids are segmented worms. When you think of worms you probably think of the common earthworm. While the earthworm is in fact an annelid, there are many more interesting species in this phylum, such as the christmas tree worm to the right. Annelids have three true tissue layers, giving them a complete digestive system that includes a mouth and anus. Annelids also have a simple circulatory and respiratory system that draws oxygen and releases carbon dioxide through their skin. Each segment of an annelid contains muscles and in most species appendages. This makes annelids powerful diggers and squirmers. Many marine annelids have elaborate appendages that make them very talented filter feeders.

Phylum Mollusk

A striking marine chiton, image from Wikipedia

Mollusks have been hugely successful in the marine environment. They represent the most diverse phylum in the ocean, making up 23% of marine species. The mollusks include class cephalopod (octopus, cuttlefish, and squid) where the shell have been reduced to near nothing or eliminated. Clams and other members of class bivalvia have two shells sealed together by a strong ligament. Snails and slugs (many of them marine) make up class gastropod. Chiton (seen above) have eight hinging plats making up a dorsal shell. They are part of the class Polyplacophora. Lots of them move on a slime layer secreted below the undulations of their foot. All mollusks breath through gills, have a muscular foot used for movement, and a scraping tongue called a radula.

Phylum Arthropoda

A krill, image from Wikipedia

Out there in the terrestrial world phylum Arthropoda contains more species than all other animal phylum combined. Marine arthropods, most of which make up the subphylum Crustacea are less diverse, but make up a large number of benthic and planktonic species.  Arthropods all have an exoskeleton made of chiton, jointed appendages, and a segmented body. This basic design is incredibly versatile and arthropods were certainly the first animals to leave the oceans and venture out on land. Common marine species associated with this phylum are crabs, lobsters, shrimp, copepods, barnacles, and krill.

Phylum Echinoderm

A purple sea urchin, image from WIkipedia

Echinoderms are somewhat unique in the animal kingdom. They are the only phylum that is exclusively marine. They can be recognized by their five part radial symmetry and their circulatory system that uses seawater rather than blood. This is interesting because in the invertebrate world they are a mix of the primitive (they have no head or tail) and the modern (a complex nervous system that controls the movement of thousands of tube feet.) The phylum includes sea stars, sea urchins, brittles stars, sand dollars, and sea cucumbers. Echinoderms are ubiquitous in the oceans, found at every depth and in every habitat. Here is a beautiful sea cucumber from the Exocet Seamount.

Questions to Research

  1. Sponges are considered to be the most primitive of animals. Take advantage of a flash animation of a sponge filter feeding or watch a Vimeo video of a sponge feeding. Describe how they do it.
  2. Watch an YouTube video from MBARI called “There is no such thing as a Jelly Fish.” Describe two things the video taught you about Cnidarians or Ctenophores.
  3. Explain how sea anemones battle one another for control of intertidal communities.
  4. Giant clams are the largest mollusk (by weight). How did they get so big? Click on the video link, watch the video then click “biology” to the left. Describe the symbiotic relationship of the algae that live within the clams. How does the relationship help both species?
  5. Ever heard of “kleptopredation?” Probably not, check out the sea slug Cratena peregrina is a kleptopredator and then define what that term means.
  6. Bivalves have been gathered along coastlines by people for thousands of years. Periodically shellfish become infected with a condition call PSP making them toxic to humans. Click on the attached Paralytic Shellfish Poisoning Fact Sheet, and describe what PSP is, which species your most likely to find it in, and which months of the year it’s most common.
  7. Horseshoe crabs are not really crabs at all (that is not malacostracoda.) They are members of an ancient sub-phylum Chelicerata. Click on the link, watch the video, and then click on biology. Describe one thing you learned about their ecological importance. Horseshoe crabs may also have saved your life, read the post and find out how.
  8. Watch this incredible video of a deep sea vent off the coast of Mexico. Identify each of the marine invertebrate phylums that you see in the video.
  9. From 2013-2017 sea stars were reported dying in large numbers up and down the Pacific North West, including in Alaska.  Explain how they were dying and the best hypothesis for the cause.
  10. Recopy and complete the following chart.
    Phylum common names distinguishing features symmetry respiratory exchange circulatory system
    asymmetrical cell to water none
    nematocysts none
    Annelid bilateral
    snails, clams, octopi foot, radula open or closed
    jointed legs gills