First evidence of biogenic habitat from tubeworms providing a near-absolute habitat requirement for high-intertidal Ulva macroalgae

Abstract

Disturbances in ecological systems can cause new resources to become available and can free the resources held by strongly competitive species. In intertidal boulder fields, wave-action causes disturbance by overturning boulders and freeing space for re-colonisation. In this study, mensurative experiments showed that boulder disturbance may also cause new biogenic-habitat resources to become available, if pre-disturbance boulders originally had tubeworm encrustations on their undersides. On the high-shore of a South Australian rocky coast, a small proportion of boulders had extensive encrustations of serpulid and spirorbid worm-tubes on their uppersides, and were likely to have recently been overturned, as spirorbid tubeworms are almost always only underneath boulders while living. Ulva macroalgae was absent from all boulders, except those with worm-tubes, where up to 61% Ulva cover was observed. Many boulders with tubes did not, however, have much algae, and this was likely caused by grazing. While limpets were seldom observed attached to tube encrustations, snails such as Nerita atramentosa and Bembicium nanum were equally abundant on and off tubes. N. atramentosa was likely the main grazer, as its densities were negatively correlated with Ulva cover. The mechanism causing association of Ulva and worm-tubes is unknown, but may be related to retention of moisture or algal spores within the complex topography of the tubes. Alternatively, some tubes may still have been living and providing nutrients for Ulva from excretory products. This study takes the first step towards understanding a very distinct habitat requirement which allows an important alga to persist in the hostile environment of the rocky-intertidal high shore.

Facilitation cascades across space: Monitoring estuarine foundation species from satellites to the microscope

“…Furthermore, Austrovenus has short siphons that restrict its burial depth, causing parts of its shells to typically protrude a few millimeters above the sediment surface, thereby allowing Ulva spores to settle (by contrast, most bivalves are buried deeper and therefore do not have attached Ulva ). The combination of protruding shells and radial shell structures thereby facilitates settlement, survival, and growth of Ulva spores, as well as provides a substrate to stay in place against the tidal currents, to ensure that Ulva can complete its life history in an otherwise sediment‐dominated system (Callow et al, 2002; Liversage, 2017; Zhang et al, 2011). In addition to providing an essential hard substratum (Keough, 1984; Thomsen et al, 2016; Zhang & Silliman, 2019), Austrovenus may also facilitate Ulva by enhancing water clarity through filter‐feeding and deposit nutrient‐rich organic matter for assimilation (Hargrave et al, 2021).…”