Characteristic fauna of South African cobble and pebble shores

Van Rensburg, Chelsea

Characteristic fauna of South African cobble and pebble shores

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

Abstract

Pebble and cobble shores (those comprising grain sizes of 4 -< 64 mm and 64 -< 256 mm respectively) are far less common on the coasts of South Africa than either rocky or sandy shores and thus are highly neglected in terms of publications. This study samples three cobble and pebble shores to investigate the faunal composition and migratory patterns of species and individuals on these shore types. Chapter 1 is a review of the existing literature on cobble and pebble shores in South Africa, and globally, highlighting the lack of research on these shore types. Chapter 2 focuses on two cobble and pebble shores on the west coast of South Africa where samples of mean grain size and faunal composition were taken across the shore from high to low shore, as well as vertically downwards into the substratum. Mean grain sizes transcended the dimensions of either solely cobble (64 - <256 mm) or solely pebble (4 - <64 mm) shores, with grains near the surface being classified as cobbles and those vertically deeper below being classified as pebbles, while even deeper the substratum became pebbles buried in gravel or sand. The Ganzekraal site was chosen as a pebble shore and had a mean grain size of 30.46 mm (± 6.045 mm), however, the Grotto Bay site was chosen as a cobble shore but had a mean grain size of 52.44 mm (± 12.527 mm) even though some grains where in fact >64 mm, thus classifying it as a pebble shore as well. The mean abundance of organisms found on each shore supported this as there was no significant difference found between Ganzekraal (5301.33 ± 1376.775 #/m²) and Grotto Bay (9093.33 ± 1376.775 #/m²), (p = 0.7). Species collected were a mixture of those previously reported on rocky shore and those associated with sandy beaches, with two specialized species characteristic of these intermediate shores, those being the semiterrestrial isopod Marioniscus spatulifrons and the pulmonated gastropod Myosotella myosotis. Chapter 3 examined the diurnal migratory patterns of three dominant, terrestrial, air-breathing isopods, Deto echinata, Ligia dilatata and Marioniscus spatulifrons, found on a single cobble shore in South Africa over two 24-hour periods. All three species exhibited peak densities within the drift kelp and were observed favoring different states of the kelp, suggesting that they co-inhabit a single shore with little to no competition. Ligia dilatata appeared to prefer the fresh kelp; Marioniscus spatulifrons appeared to prefer the hard, dried, broken pieces; Deto echinata appeared to show less specific preference suggesting that they may scavenge kelp between the cobbles. All three species appeared to have different migratory patterns. Marioniscus spatulifrons was most active during the diurnal tide and D. echinata during the nocturnal tide. Ligia dilatata was the fastest moving of the three species, migrating the furthest down the shore, and occupying most of the intertidal. Marioniscus spatulifrons was the most abundant species (1703.82 ± 3128.83 #/m² during spring tide and 2058.68 ± 3300.34 #/m² during neap tide), even though it was only recently reported on South African shores, as compared to Ligia dilatata (1071.53 ± 1784.81 #/m² during spring tide and 1002.94 ± 2334.09 #/m² during neap tide) and Deto echinata (386.11 ± 832.95 #/m² during spring tide and 293.06 ± 570.91 #/m² during neap tide).

Keywords

pebble shore

South Africa

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Masters

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