Attachment strategies of organisms on hard substrates: A palaeontological view

Attached organisms on hard substrates have been termed sessile, fixed, encrusting, cemented. These terms reflect the palaeontological aspect of these communities, the securely fixed organisms having the best taphonomic chances of surviving fossilization in life position. Neontologists commonly refer to organisms attached to hard substrates as “fouling organisms”. However, there are numerous other styles of attachment that are characteristic of hard substrate communities. Many of these involve soft-part anatomy (e.g., echinoderm podia, octopus suckers, gastropod foot) and chemical adhesives (e.g., gastropod pedal glue, macroalgal hapteron glue, bivalve byssus), which may leave no direct palaeontological evidence. Different organisms present widely differing degrees of permanence of attachment; some indeed are attached for only part of their life span. In fact, attachment styles range from the permanently affixed, temporarily affixed, periodically mobile to permanently mobile attachment. Each of these categories includes some organisms that etch or abrade their subjacent substrate, creating incipient trace fossils, but they do so for different reasons and by different means. Thus, the styles of attachment do not lend themselves to a compact, formal terminology, owing to the wide scope of attachment strategies and the overlap of their individual features. The categories are therefore retained here in ‘open terminology’. The following styles of attachment have been distinguished. Muscular sucker attachment includes single sucker attachment (e.g., limpets) and multiple sucker attachment (e.g., Octopus). Holdfasts include the releasable bunches of byssus secreted by bivalves and the attachment of crinoids using cirri, and of crabs using legs. Organic glue secretions from basal membranes attach foraminifera, hydroids, bryozoans, tunicates, and some barnacles. Some groups of basal membrane gluers etch the substrate and produce incipient trace fossils. These include verrucid and some balanid barnacles, a few bryozoans and the anomiid bivalves. Hapterons include the permanent attachment of brown macroalgae onto rocky shores. Etchings include pits and dishes (e.g., foraminifera) and roots (etchings by brachiopod pedicles). Some vermetid gastropods show combined etching and cementation. Extensive to subtle cementation without etching includes oysters and other bivalves, and some vermetid gastropods. The many organism groups that etch carbonate substrates are unable to pursue this strategy on noncarbonate substrates. There is a much larger variety of strategies available to organisms attaching themselves to carbonate substrates than to noncarbonate. The great variety of attachment styles provided by organisms colonizing hard substrates provides a corresponding variety in taphonomic starting points for the fossilization process, including bioimmuration.