Phylum Echinodermata

Sea stars, brittle stars, sea urchins, sea cucumbers, and crinoids

Useful articles on echinoderms by Ron Shimek:

General Overview of Echinoderms
(this general article is very useful)

Seastars
(this article has a very nice discussion of the various ways seastars feed)

Sea Urchins

Sea Cucumbers

 

The above articles have more detail than you need to do well on the exam, but you will find them to be useful supplements to the text.

 

1. Echinodermata means "spiny skinned"

2. Exclusively marine, mostly bottom-dwelling

3. VERY important in marine habitats

  • Some of the most abundant of the larger animals
  • If you just consider larger animals (e.g. larger than insects), the most abundant animal on earth is a particular species of sea cucumber that lives in continental shelf areas
  • Very influeantial in the ecological communities in which they live. For example:
  • Sea stars are often MAJOR predators
  • often shape ecologial communities by reducing abundance of their prey species.
  • Sea urchins are often MAJOR herbivores
  • often shape communities by severely reducing algae and "seaweed" abundance
  • coral reef health depends on long-spined urchins (genus Diadema) clearing algae off rocks, which allows coral larvae to settle and establish new colonies.

    • 4. Echinoderm anatomy:

  • Radial symmetry in adults (in many echinoderms) - often pentandrial (5 sections)
  • this is secondarily developed from bilaterally symmetrical larvae
  • many other echinoderms are bilaterally symmetrical as adults
  • e.g. some urchins (especially sand dollars)
  • e.g. sea cucumbers
  • e.g. some seastars

    • Some echinoderm larvae - planktonic (and bilaterally symmetrical!):

  • Internal skeleton (mostly lacking in sea cucumbers)
  • calcareous plates embedded in body wall
  • sometimes fused (e.g. in sea urchins)
  • portions of skeleton often protrude from body (spines)
  • Well-developed coelom (body cavity)
  • in many species, extensions of coelom protrude from body and function as gills
  • Water vascular system
  • connected to outside world via madreporite
  • tube from madreporite runs to ring canal
  • radially arranged canals branch from ring canal
  • radial canals lead to hollow tube feet
  • ampulla pumps water into tube feed to extend feet
  • muscles retract tube feet
  • collective grip of many tube feet can be tremendous
  • some seastars can pull open bivalves!
  • Nervous system - radially organized nerve network - no brain
  • Complete gut (except for brittle stars, which lack an anus!)
  • Pedicillaria - small jawlike structures on surfaces of many echinoderms (lacking in sea cucumbers)
  • prevent other organisms from colonizing surface
  • defend against predators
  • venomous in some urchins

    •  

     

    Surface of a sea urchin, showing spines, pedicillaria, tube feet

    5. Sea stars

  • Anatomy:
  • Sea star internal anatomy: http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=ASTE004B&File_type=cdr AND http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=ASTE007B&File_type=CDR
  • Sea star arm cross section: http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=ASTE002B&File_type=cdr
  • Sea star water vascular system: http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=ASTE001B&File_type=cdr
  • Seastars walk on tube feet:

  • Seastar lifestyles and feeding habits (see: Seastars)
  • most are formidable predators of anything that doesn't walk or swim away
  • many evert stomach onto food, do intitial digestion externally
  • some pull open live bivalves, insert stomach, digest bivalve inside its own shell!
  • many evert stomach onto surfaces and digest biological films (bacteria, algae, etc.) and/or detritus
  • many eat live corals or other sessile invertebrates
  • some ingest prey whole (some of these can catch moving prey!)

    • 6. Brittle stars

  • Central oral disk with long slender arms
  • Jaw-like mouth structures on bottom of oral disk (see photo on right above)
  • Ingest food
  • No anus
  • Much more active and faster moving than most sea stars
  • Move by rowing arms
  • Some can catch fish!
  • Feeding habits:
  • most are scavengers
  • many are filter feeders
  • stay in one place with arms extended into water
  • some are predators
  • How food is brought to mouth:
  • small particles brought to mouth in "bucket brigade" fashion, handed off from tube foot to tube foot
  • larger particles- wrap arms around food and bring directly to mouth
  • Brittle stars can be EXTREMELY abundant
  • in some places, filter-feeding brittle stars totally cover seafloor!
  • Basket stars
  • basically modified brittle stars with branching arms
  • filter feeders

    • Basket star:

     

    7. Sea urchins:

    Lower surfaces of sea surchins, showing mouth and 5 teeth

  • Sea urchin anatomy:
  • Urchin cross sections: http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=ECHN001B&File_type=cdr AND http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=ECHN002B&File_type=cdr
  • Tube feet, pedicillaria, etc: http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=ECHI013B&File_type=CDR
  • Typically have tube feet all over surface
  • Mouth on bottom, anus on top
  • Have 5 teeth - Aristotle's lantern
  • Feeding habits
  • Most scrape algae off rocks
  • A few are predators
  • Some flat, short-spined species (sand dollars) live in sand and feed on detritus (Sand dollar diagram)

    • 8. Sea cucumbers

  • Sea cucumber diagram with labels: http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=HOLO001B&File_type=cdr
  • Basically shaped like a tall skinny urchin laying on its side
  • Bilaterally symmetrical in most cases
  • Internal gills (branched respiratory trees) in cloaca (share opening with outside world with anus)
  • Have feeding arms (modified tube feed) surrounding mouth
  • Trap food, and one by one arms move food to mouth
  • Arms "licked clean", and coated with more sticky glue
  • Feeding habits
  • many species mop up sand/sediments and detritus
  • dirty sand goes in, clean sand comes out!
  • many other species are filter feeders

    • Photo of feeding arms of a filter feeding sea cucumber:

    9. Crinoids - the sea lilies

  • Filter-feeders
  • Were VERY abundant in prehistoric times
  • less abundant today, but not extinct
  • Many were sessile (and some like this still live today):

    • Diagram of sessile crinoid, and photo of part of a fossil crinoid:

  • Others not quite so completely sessile- can walk about, though usually sit and filter feed:

    •