Indicator of Benthic Health   
Hydra species

Station Location: Lake Knapp Pavilion, GPS coordinates: 38°57’07.91”N, 76°14’00.83”W
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  • Domain: Eukarya
  • Kingdom: Animalia
  • Phylum: Cnidaria
  • Class: Hydrozoa
  • Order: Anthoathecata
  • Family: Hydridae
  • Genus/Species: Hydra species

“Benthos” is a term used to describe the community of organisms that live in close association with the bottom of bodies of water (e.g. seabeds).  These organisms can be further classified according to their trophic status as “phytobenthos” (primary producers) and “zoobenthos” (consumers), or by their size, where macrobenthos includes organisms 1 mm or larger, meiobenthos includes organisms less than 1 mm and greater than 0.1 mm, and microbenthos, organisms 0.1 mm or less.   Finally, benthic organisms may be described by where they live, in relation to the aquatic floor, i.e endobenthos (organisms that live in the sediment), epibenthos (organisms that live on top of the sediment), and hyperbenthos (organisms that live just above the sediment).  The example organism in this tour, Hydra species, are members of the macrobenthos (growing up to 10 mm in size).

Scientists use surveys of benthic populations to determine the health of the associated body of water. The types and numbers of organisms that form the benthos are reliable indicators of the effects of pollution and other environmental stresses.  A program named “Chesapeake Bay Long-Term Benthic Monitoring and Assessment Program in Maryland” was established in 1984.  It’s run by a private contractor, in coordination with the Maryland Department of Natural Resources and the U.S. EPA Chesapeake Bay Program. This program has two components, including “a fixed-site monitoring sampling effort directed at identifying trends in benthic condition, and a probability-based sampling effort intended to estimate the area of the Maryland Chesapeake Bay with benthic communities meeting and failing to meet the Chesapeake Bay Program’s Benthic Community Restoration Goals (i.e. the “state of the bay”).”¹ The program monitors a large number and variety of benthic species¹.

Hydra species are among the organisms monitored in the study described above.  Hydra species can usually be seen with the naked eye, but a magnifying loupe is usually useful for visualizing them. Hydra, like jellyfish, are members of the phylum Cnidaria, a group of primitive invertebrates that are characterized by a radially symmetrical body plan and stinging cells called nematocysts.  Hydra are freshwater invertebrates that are common in temperate and tropical waters.  They live attached to aquatic vegetation or other submerged material.  They eat organisms that pass within range of their stinging tentacles.  Hydra are of particular scientific interest because they apparently do not age and have what is described as an unlimited lifespan.


Learn More:

Society for Freshwater Science: What Is The Benthos?

PBS NatureWorks: Benthos

Chesapeake Bay Benthic Monitoring Program

Chesapeake Bay Program: Life at the Bottom

Northern State University: The Natural Source – Hydra

Introduction to Cnidaria

Notes & References:

Photo attribution: (distributable under Creative Commons license)

¹Chesapeake Bay Benthic Monitoring Program: Reports and Other Documents