Abstract
The sea-surface microlayer (SML) at the air-sea interface is a distinct, under-studied habitat compared to the subsurface and copepods, important components of ocean food webs, have developed key adaptations to exploit this niche. By using automated SML sampling, high-throughput sequencing and unmanned aerial vehicles, we report on the distribution and abundance of pontellid copepods in relation to the unique biophysicochemical signature of the SML. We found copepods in the SML even during high exposure to sun-derived ultraviolet radiation and their abundance was significantly correlated to increased algal biomass. We additionally investigated the significance of the pontellids’ blue pigmentation and found that the reflectance peak of the blue pigment matched the water-leaving spectral radiance of the ocean surface. This feature could reduce high visibility at the air-sea boundary and potentially provide camouflage of copepods from their predators.
Highlights
Among zooplankton taxa living within the surface microlayer (SML), neustonic copepods of the family Pontellidae have been frequently recorded in tropical regions of all oceans[6,7,8]
Various theories have been developed to explain the significance of the blue colouring in copepods, including protection from strong solar and/or UV radiation16,17, 1Institute for Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University Oldenburg, Schleusenstraße 1, 26382, Wilhelmshaven, Germany. 2Lamont-Doherty Earth Observatory, Columbia University, Palisades, NewYork, 10964, USA. 3German Centre for Marine Biodiversity Research, Senckenberg am Meer, MartinLuther-King Platz 3, 20146, Hamburg, Germany. 4Center for Earth System Research and Sustainability (CEN), University of Hamburg, Olbersweg 24, 22767, Hamburg, Germany. 5Marine Biological Association of the United Kingdom, Plymouth, PL1 2PB, UK. 6Present address: Group for Aquatic Microbial Ecology (GAME), University of Duisburg-Essen, Campus Essen-Biofilm Centre, Essen, Germany
High-throughput sequencing of the total eukaryote 18S ribosomal RNA encoding genes on additional manually-collected SML and ULW samples confirmed predominant SML enrichment for the order Calanoida including the families Pontellidae and Paracalanidae, whereas different copepods of the order Cyclopoida were most prevalent in the ULW (Fig. 1B)
Summary
Blue copepods (prosoma length of ~2 mm) collected with the S3 were counted and identified to the calanoid copepod family Pontellidae in subsamples from stations 11, 13, 14 and 17 (Supplementary Fig. S1). While pigmented copepods generally experience a higher predation risk compared to transparent individuals[27], some blue copepods such as Pontella mimocerami can exhibit green fluorescence serving a potential role in counter-shading, a mechanism being analogous to bioluminescence[28]. This might in turn aid the crustaceans to escape predation during the night, e.g. during diel vertical migration. While being inhospitable to most other meso-zooplankton, pontellids most likely benefit from their pigmentation and adaptive behavior[25,35] to cope with the disadvantages of high visibility and increased solar and UV radiation at the air-sea interface
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