"Life in, on, and around the Mud"

A lesson plan for exploring the marine invertebrate diversity at Garrison Bay


The microhabitats of Garrison Bay provide an excellent system in which to explore the roles of biotic and abiotic variables, such as substrate type and desiccation, in structuring the distributions of species in the intertidal. Interesting comparisons can be made between the composition and abundance of marine invertebrates found in association with different microhabitats (e.g., rocks, wood, shells, and mud) at the site.

Soft sediment is the primary substrate in terms of aerial extent at Garrison Bay. Much of the diversity that you will be exploring is found in, on, or around the mud. Precautions should be taken to minimize your impact on the soft sediment habitats found at the bay. As you will discover, moving across the tidal flat can be challenging and results in a swath of churned up mud. Try to restrict the movement of your group and all activities to small subsets of the intertidal zone, leaving much of it in the state in which you found it.

Click here for a copy of the handout provided during the 2004 MIZ class field lesson, which includes major themes and questions, temperature graphs, and a species checklist.

Lesson Goals

Supplies Needed

Surficial Transect Exercise

  • 2 ropes
  • 4 stakes and flagging tape
  • Shovels
  • Trowels
  • Clipboards
  • Worksheets
  • Small containers for collecting live specimens for identification in lab
  • 3 different types of candies to symbolize species from the three primary microhabitats (e.g., foil-wrapped chocolates for the diatomaceous layer; hard candies for the rocks; and foil-wrapped chocolate bars for the mud)

Dead Shell Microhabitat Exercise

  • Sieves
  • Buckets
  • Clipboards
  • Worksheets
  • Small containers for collecting live specimens for identification in lab

Optional: A bag of candy for the first person who makes a "mud angel" on the tidal flat
(just like a snow angel, only messier!).


Surficial Transect Exercise

Lay out transect across different microhabitats using two ropes, stakes, and flagging tape.

The two ropes should be spaced approximately one meter apart and run from the upper intertidal diatomaceous layer to the lower intertidal mud, and intersect exposed rocky patches along the way.

Dead Shell Microhabitat Exercise

Buckets of water should be placed along the edge of the dock along with sieves to facilitate sieving of samples in the case where low tide precludes sieving directly in the bay.





Introduction (10 minutes)


Sketch from the log, looking towards the northeast, a portion of the intertidal noting microhabitats or changes in surficial characteristics (5 minutes)


Sediment depth demo (10 minutes)


Split into two groups (approximately 5 students each) with each group doing one of the two activities (45 minutes)

Group A: Survey along Transect

Group B: Shell microhabitat activity on the dock
V. Groups rotate - Group A to shell microhabitat activity and Group B to Transect (45 minutes)


Discussion & synthesis


Diversity along the transect

Faunal list of the site is read by one of the instructors

Groups are given a bucket for candy collection

For every species found, the group receives a candy coded for the microhabitats it occurred in (e.g., hard candy for species found in association with rocks)


Discussion of the microhabitats

How was diversity (taxonomic richness and abundance) partitioned among the microhabitats examined?
c. Discussion of the microhabitats created by the shells of dead bivalve molluscs at the site

Did the composition or abundance of the invertebrate fauna vary between those living in association with dead shells and those living only in the surficial sediments?

What roles might dead shells play at Garrison Bay? How might these roles vary with tidal height?
VII. Opportunity to make mud angels and compete for a bag of candy (5 minutes)

Sediment depth demonstration

Using a shovel, excavate a hole in the mud to show the students the layering of sediments with depth (diatomaceous upper layer, anoxic mud, and clay layer).

Discuss the influence of each of these layers on the distribution of organisms in the sediment, as well as the physical factors creating these depth gradients. How might the inclusion of coarse shell material in the sediment affect the infaunal environment?

Discuss the role of the diatomaceous layer in contributing to high oxygen content at the sediment-water interface, its role in inhibiting sediment mixing and water infiltration into the sediments below the diatomaceous layer, and its contribution to the preservation of traces and organisms in the fossil record.

Also, point out different types of surficial features on the mud for students to look for throughout the exercises (e.g. sizes and types of burrows, tracks).
Provide a few examples of organisms and their associated burrow and/or trail (e.g. Paranemertes and its mucous trail, Abarenicola and the coiled sediment excrement near its burrow).

Transect Exercise

Starting in the upper intertidal, groups should traverse the transect, making note on their faunal list of different taxa they observe. Groups should indicate what microhabitat each species was found in.

Encourage groups to excavate a few small holes using either a shovel or trowel to look at infaunal diversity.

Encourage groups to explore some of the burrows they observe - can they determine what animals are responsible for those burrows?

Dead Shell Microhabitat Exercise

Have students pick up approximately 5 dead shells located on the sediment surface which contain sediment. Using the sieves, students should screen wash the sediments within the shells and keep a list of the diversity and abundance they observe.

Students should then use the shells to scoop up surficial sediment, sieve it, and compare their results with what was observed in the dead shell microhabitats.

Are there any differences you observe between the animals found living in association with dead shells and those living directly in the surficial sediment?