The November 2011 irruption of buoy barnacles <i>Dosima fascicularis</i> in the Western Cape, South Africa

Abstract

November 2011 saw an unprecedented irruption of buoy barnacles Dosima fascicularis in coastal waters off the Western Cape, South Africa. Buoy barnacles not uncommonly strand in the region attached to feathers, plastic litter and other small objects, but the 2011 irruption saw exceptional numbers of unusually large colonies (average 23.5 individuals; SD 18.5), most of which relied on their own floats. The largest individuals (49 mm capitulum length) were larger than previous records for the species. Buoy barnacles were first observed at sea off the Cape Peninsula on 2 November 2011. During the following three weeks they washed ashore along at least 500 km of coast from Paternoster on the West Coast to De Hoop, east of Cape Agulhas. Mass strandings attained densities of up to 42 (SD 23) colonies per metre of beach (n = 10), with a wet mass of around 10 kg m^–2. They were accompanied by large numbers of bluebottles Physalia physalis and other neustonic organisms. Of 100 buoy barnacle colonies examined, only three were attached to obvious floating items (two Janthina shells and one piece of plastic). Dissection failed to reveal foreign attachment sites in 40 floats, but digesting 70 floats in potassium hydroxide revealed small plastic fragments in eight floats, one tar ball and one Velella skeleton. The 100 study colonies were comprised solely of buoy barnacles, but a Janthina shell had one buoy barnacle and 18 Lepas pectinata goose barnacles. Float size was poorly related to the number of barnacles, but was strongly correlated with the size of the largest animal. Most colonies had multiple large individuals, raising the possibility that several cyprid larvae settle together, jointly forming the communal float. Unusually persistent westerly winds prior to the strandings may have carried colonies into coastal waters from the South Atlantic gyre, but it is not evident why such events are so rare.

Keywords: attachment substrata, colony size, float volume, Janthina mass stranding, Physalia, size distribution, Velella

African Journal of Marine Science 2012, 34(1): 157–162