Chapter 7: Marine Invertebrates

Bilateral Symmetry and the Advancements of the Worms

Oh, to be a Worm!

Adaptive trends exhibited by worm phyla:

Bilateral symmetry

Cephalization –development of a head region

Coelom development

Increasing development of nervous sensory

systems.

Bilateral Symmetry

“Bilateral symmetry refers to a basic animal body plan in which one plane of symmetry exists to create two mirror-image halves.”Sumich (1999) An Introduction to the Biology of Marine Life

gecko.gc.maricopa.edu/.../platyhelminthes/ platyhel.htm

Planaria

Bilateral Symmetry

Organisms with bilateral symmetry have developed an anterior “head” region and a posterior “tail” region.

In addition they also display a top or back side (dorsal) and a belly or underside (ventral).

Worms with Direction

“Animals with a front end [anterior] region generally move in a forward direction.”

Villee, et. Al. (1989) Biology

Thus the tendency would naturally be to concentrate sensory organs in this anterior region to detect changes in the environment.– Leads to more active predation– More sophisticated behaviors

This process is termed “cephalization” – from the Greek for “getting a head”

A Bit About Germ Layers

Early in embryonic development, the structures of most animals develop from three tissue layers call germ layers.

Ectoderm – outer layer Mesoderm – middle layer Endoderm – inner layer

Digestive cavity

Germ Layers

Ectoderm – outer layer– outer covering of the body and the

nervous system Mesoderm – middle layer

– gives rise to most of the body structures

Endoderm – inner layer– lines the digestive tract

As organisms become more sophisticated anatomically, the development of a body cavity or coelom [see-luhm] is observed.

The coelom is lined by mesoderm tissue and is essentially an open tube within the organism’s body in which digestive, reproductive and other organs arise.

A Tube-Within-A-Tube

PHYLUM: PlatyhelminthesFlatworms – A Tiny “Inch” Forward

Exhibit bilateral symmetry and cephalization Acoelomate Mouth and anus are still shared Simplest organisms with well-developed organs Have a simple brain called a ganglia in the head

with two nerve cords that extend the length of the body.

Flatworms

Turbellarians– Planarians– Marine, free-living

Trematodes – Flukes– Mostly parasitic

Cestodes– Tapeworms– Parasites that live in the

intestines of vertebrates (including humans!)

Anatomy of a Flatworm

Flatworms – Another Look

Anatomical diagram of a planarian – a typical flatworm found in both fresh and marine waters as well as terrestrial habitats

Flatworm Media

Planaria

Swimming Turbellarians

Trematode infection of salamanders

Warning: Colonoscopy showing tapeworm !

PHYLUM: NemerteaProboscis Worms/ Ribbon Worms

Simplest animals to possess definite organ systems.

Almost exclusively marine Possess a proboscis – a

long, hollow, muscular tube which can be everted from the head to capture food or for defense.

Proboscis Worms/ Ribbon Worms

Are truly a “tube-within-a-tube.” The digestive tract is a complete tube with mouth at one end and anus at the other.

First example of separate circulatory and digestive systems

Acoelomates Non-parasitic, mostly benthic Claim to fame – one species has been observed up to

30 m long (the longest invertebrate!)

PHYLUM: NematodaRoundworms

Most common worms in the world – inhabit almost every species of plant and animal.

Mostly parasitic, some benthic Have a tough, outer covering called a cuticle

which keeps them from drying out. Sexes separate and dimorphic – separate male

and females that look different (male smaller)

Roundworms

Pseudocoelomates Have a cavity filled with incompressible fluid

which acts as a hydrostatic skeleton.– Cavity is not completely lined by mesoderm.– When muscles in the body wall contract they flex

and squeeze against this fluid causing the shape of the worm to deform and therefore move.

– Excellent technique for sediment burrowing.

Roundworm in cat gut

Marine roundworm

Good slide show of various roundworm images

20,000 species including marine and terrestrial species (e.g. earthworms)

Defining characteristics– Body divided into

segmented units called metameres.

– Chaetae (or setae) – hair-like structures on each segment

PHYLUM: AnnelidaSegmented Worms

Other Innovations of Annelids

Digestive tract (or gut) extends through all segments. Coelomates

– Acts as a hydrostatic skeleton– Organism can move each segment individually. This permits

localized and more efficient movement. Have a closed circulatory system In aquatic species, respiratory exchange is through gills

Annelid Classes

Polychaeta– All marine, may be free-swimming or live in benthic

aggregations– Include bloodworms, sandworms, lugworms, bristle worms,

fan worms, feather duster worms, beard worms, etc. Oligochaeta

– Aquatic or terrestrial, live in mud or sand bottoms’– Include earthworms

Hirudinea– Mostly freshwater, but some marine species– Leeches

Polychaete Biology

Anatomy:– Chaetae emerge from flat parapodia which are stiff extensions on each body

segment

Life History: – Have a planktonic larval stage called a trochophore– As adults, some crawl on bottom, others burrow, others build tubes and live in

aggregations, while still others remain planktonic

Feeding: – Some are carnivorous, some are suspension feeders, and others are deposit

feeders.– Crawling worms have well developed parapodia, a proboscis, and jaws.– Suspension feeding worms often have tentacles, cilia, or mucus to capture

prey

Serpula vermicularis – reef building tube worm

Common lug worm (Arenicola marina) Plymouth, Devon, England

Lug worm casts on the coast of North Ireland

King Ragworm (Nereis virens)

Tubeworm (Spirorbis tridentatus) Batten Bay, Mount Batten, Plymouth, Devon.)

Myrianida pachycera, a polychaete (worm) (60x)

Christmas tree worms on coral head

Trochophore larvae of a bristle worm

Note the bristles anchored in the body for swimming and the reddish eye spots.

Polychaete sandworms - Notice the tubes sticking up from the mud.

Some sandy beaches can contain up to 32,000 polychaete worms/m2 that consume 3 tons of sand/ year.

Feather duster worms, Bimini, Bahamas

Polychaete epitokes swarming . Glover’s Reef, Belize

Pogonophora beard worms

Deep water species – live near hydrothermal vents

No mouth or gut Tuft of tentacles absorbs

dissolved nutrients from the water Symbiotic bacteria inside the

worm use these nutrients to make food.

Formerly classified in their own phylum

Oligochaeta

Found in mud/sand bottoms Usually deposit feeders Lack parapodia Includes the common

earthworm

Hirudinea leeches

Usually parasitic and blood-sucking

Inject a chemical into prey that is both an anticoagulant and an anesthetic.

Have a sucker on anterior and posterior.

Lack parapodia

Sipuncula peanut worms

Strictly marine Unsegmented Burrow in shallow water soft

bottom sediments Possess a long anterior

portion that can be retracted into the body.

Deposit feeder 1-35 cm long Approximately 320 species

Echiura innkeepers/ spoon worms

proboscis

Strictly marine Unsegmented, though now

classified with annelids Have a non-retractable, spoon-

like proboscis for gathering organic material.

One species creates a U-shaped burrow that is often shared with other organisms.

Deposit feeder Approximately 135 species

Unifying Characteristics of Worms

Ubiquitous in marine environment (benthic, parasitic, free swimming)

Usually small Responsible for mixing marine sediments. Recycle bacteria and detritus into the food chain. Have highly developed feeding appendages and

digestive systems. Important food for higher invertebrates and some fish. May have important health effects on marine

vertebrates

Image Citations

Brown, Hugh. “Serpulid polychaete worm” Digital Image. Serpulid reefs. The Scottish Association for Marine Science (SAMS). 5 January 2009. <http://www.sams.ac.uk/research/departments/ecology/ecology-projects/reef-ecology/researchproject.2007-04-18.1807501867>

Fiege, Dieter. “Glyceridae” Digital Image. Senchenbergische Naturforschende Gesellschaft. 2008. 5 January 2009. <http://www.senckenberg.de/root/index.php?page_id=2301>

“Leech.” Digital Image. Annelids Live Invertebrates – Niles Biological, Inc. 2006. Niles Biological, Inc. 5 Jaunary 2009 <http://www.nilesbio.com/subcat288.html>

Rouse, Greg. “Chaetae of an Annelid” Digital Image. Annelida 2004. Tree of Life Web Project. 5 January 2009 <http://www.tolweb.org/Annelida>

Rouse, Greg. “Myrianida pachycera, a polychaete.” Digital Image. Nikon Small World – Gallery. 2008. Nikon Small World – Photomicrography Competition. 5 January 2009. <http://www.nikonsmallworld.com/gallery.php?grouping=year&year=2003&imagepos=2>

Siddal, Mark. “Medicinal leech” Digital Image. Leech on Me. 2007. Science Friday Newsbriefs. 5 January 2009. <http://www.sciencefriday.com/newsbriefs/read/120>

“Social feather duster worm close-up” Digital Image. ReefNews. 2001. 5 January 2009. http://www.reefnews.com/reefnews/photos/bimini/sfdust2.html

“Swarming polychaetes” Digital Image. Rpolychaete epitokes Ryan Photographic. 5 January 2009. <http://www.ryanphotographic.com/epitoke.htm>

“Trocophore larvae” Digital Image. Bristleworms and their larva. 1995. Mic-UK: Bristle worms. 5 January 2009. <http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artmar99/poly2.html>

Veitch, Nick. “Lug worm casts” Digital Image. Wikimedia Commons. 2008. 5 January 2009. <http://commons.wikimedia.org/wiki/File:Lugworm_cast.jpg>