The “suitcase hypothesis” – can eddies provide a pathway for gene flow between Madagascar and KwaZulu-Natal?

Abstract

Similarities in marine fauna found off the coasts of southern Madagascar and KwaZulu-Natal (KZN, South Africa) led to the development of the “Suitcase Project”, with the aim of establishing whether eddies that form off southern Madagascar may package and transport biological material, as if in a suitcase, across the Mozambique Channel. In pursuit of this question, sampling was conducted on the southern Madagascan shelf and along a transect across a cyclonic eddy which originated off the southern tip of Madagascar, between the 15th and 23rd of July 2013. Bongo nets (300 and 500 μm-mesh) and a neuston net (900 μm-mesh) were used to collect zooplankton within the upper 200 m and at the surface, respectively. Samples were sorted for meroplankton (larval stages of fish and benthic invertebrates) under a stereo microscope, particularly seeking species known to be common to both the east coasts of Madagascar and South Africa and, thus potential indicators of connectivity between these regions. Larvae of crabs, rock lobster, and fish were used for DNA barcoding. Zooplankton biovolume and abundance were compared between the eddy core, eddy periphery and outer regions of the eddy, as well as stations from the Madagascan shelf. Mean neuston biovolume on the Madagascan (0.08 mL m-3) was not significantly higher than that in the eddy (0.06 mL m-3). Mean bongo biovolume in the upper 200 m was much higher on the Madagascan shelf (0.62 mL m-3) than in the eddy (0.16 mL m-3) although only 2 stations were sampled on the shelf. Highest biovolume in the eddy was recorded in the west eddy zone (0.25 mL m-3) and west outer zone (0.23 mL m-3), which was not statistically significantly higher than the eddy core (0.12 mL m-3) and east eddy zone (0.17 mL m-3). Meroplankton was comprised of coastal origin taxa and was most abundant on the shelf and in the eddy perimeters. Larval goat-fish, Parupeneus fraserorum was identified, a newly described mullid, and has been recorded on both the coasts of Madagascar and KZN, SA. Larvae of coastal invertebrate species identified, include the squat lobster Allogalathea elegans and camel shrimp Rhynchocinetes durbanensis. Other larval fish identified, but not found in high abundance include the families of reef associated fishes, for example: Apogonidae, Labridae, Pomacentridae, Priacanthidae, Serranidae and Sparidae. Higher zooplankton biovolumes, larval abundances and reef-associated larval assemblages found on the Madagascan shelf and in the periphery of the cyclonic eddy compared to the core in this study provide support for the suitcase hypothesis that planktonic organisms are entrained within eddies as they propagate south-westwards of the Madagascan shelf. However, further studies are required to determine whether planktonic larvae are able to cross the Mozambique Channel and reach the KZN coast in time to settle.