Cryptic Habitats and Hard-Grounds

Hard Ground and Cryptic Habitats

Many organisms bore into hard substrates like mud banks, trees, pilings, or other organisms. These animals are often hard to see and hard to study unless one searches for them. Other animals attach to hard surfaces either by cementation, attachment by secreted threads called byssae, or by tightly cling to them. Hard surfaces are uncommon on the Georgia seashore, but exist in the form of other animals, skeletal trees, and, more recently, as pilings and docks.


Mud banks exposed on the beach are colonized by boring bivalves (Modified after Morris and Rollins, 1977).

Skeletal trees are covered with attached mussels, oysters, barnacles, and numerous crevice-living invertebrates.

Shells of organisms provide a surface upon which cemented forms, like the American Oyster, can grow.

Relict Mud Habitat

The relict marsh mud being exposed by erosion of St. Catherines beaches acts as a hard-ground in some ways. The semi-solid mud acts as a hard substrate for attached epifauna including the bivalves Brachidontes recurvus and Lyonsia sp., the barnacle Balanus sp., the green alga Ulva sp., polychaete worms, hermit crabs and the gastropod Terebra dislocata (Morris and Rollins, 1977).

Skeletal Trees, docks, and Pilings

Skeletal trees and pilings act as hardground hosts to the zebra periwinkle (Littorina ziczac), the pulmonate snailSiphonaria alternata, tightly packed barnacles, and small-ribbed mussels. The sea isopod Sphaeroma quadridentata is a common surface inhabitant of the wood. Skeletal tress low on the beach have been observed heavily encrusted by mason worms (Sabellaria vulgaris).

Organisms as Habitat

Many instances of organisms as hard-ground substrate have been observed on St. Catherines Island. These include the American Oyster growing one upon another as bioherms, Knobbed and Channeled Whelks with fringes and crowns of oysters, barnacles, bryozoans, and hydroids. Other mutalistic relationships include Knobbed Whelks with layer upon layer of encrusting bryozoans covering their shell, Sand Dollars with symbiotic crabs, and Loggerhead Sea Turtles carrying their exotic carpet of carapace epizoans (Frick & Slay, 2000).


The carapace of Loggerhead sea turtles is often nearly covered with an exotic fauna of cemented barnacles, attached hydroids, and clinging invertebrates such as skeleton shrimp.

Cryptic (Bored) Habitats

1. Relict Mud patches on the beach provide a boring habitat for many infaunal organisms. Bivalves, Petricola pholadiformis, Barnea truncata, and Cyrtopleura costata, and polychaete worms bore into the relict mud and peat to establish their domiciles (Morris and Rollins, 1977).

Bored Wood Habitat

Stumps, logs, and driftwood provide shelter and attachment for numerous invertebrates, some of which, like the southern gribble isopodLimnoroa tripuntata, bore into the wood, producing extensive borings within the wood. This habitat, when fossilized has been called the Teredolites ichnofacies (Bromley, Pemberton, and Rahmani, 1984).

The Golden Isles and St. Catherines Island provide an excellent opportunity for the collection of shells of common marine invertebrates. The teacher-interns in the St. Catherines Sea Turtle Conservation Program have found a local shell collection to be of immense importance in their classroom teaching. In order to expedite this activity, we are presenting two web resources for persons interested in this activity, a K-12 Atlantic Shell Collection Activity and a series of pages documenting specific organisms of interest.

Shell Collection

A student’s shell collection made from the beaches of St. Catherines Island illustrating the types of materials that are taken by teacher-interns for use in teaching in their classrooms.

Please remember to take only shells of dead organisms, we would not be good stewards of the land if we took live organisms. Be especially careful to check shells for secondary inhabitants, such as hermit crabs! If your study needs to involve collecting or manipulating live animals, be sure to apply for proper permits from the Federal and State Governments, and gain approval from your local Institutional Animal Care and Use Committee (IACUC).

Last updated: 7/27/2016

Sea Turtle Program at St. Catherines Island • (912) 478-1744 •