3D spatial point patterns of bioluminescent plankton: a map of the minefield

As the open ocean environment lacks points of refuge from visual predators, it has favored the evolution of extraordinary adaptations for optical concealment, such as vertical migration, trans- parency and counterillumination. Bioluminescent plankton, which respond to a mechanical disturb- ance with a flash of light, are ubiquitous in the ocean and potentially a threat to any organism seeking darkness as a means to evade detection. Estimating encounter probabilities for organisms maneu- vering through luminescent 'minefields' requires characterization of the three-dimensional distri- bution patterns of the potential light emitters. In order to measure nearest neighbor distances (NNDs) of bioluminescent biota, we have developed a spatial plankton analysis technique (SPLAT) for 3D reconstruction and statistical analysis of the spatial point patterns of identified bioluminescent displays. Analysis of aggregations of bioluminescent biota in the Gulf of Maine (Wilkinson Basin) revealed that encounter probabilities were highest in the temperature minimum zone (temperature <5°C) where dinoflagellates (Protoperidineum depressum) exhibited NNDs of 3.5-3.8 cm in a regular distribution pattern. This technique was also used to examine the internal organization of thin layers of the bioluminescent copepod, Metridia lucens. Comparison of night-time surface (migrating) and deep (non-migrating) layers indicated complete spatial randomness in both populations and no significant difference in spacing (NNDs: 9.5-13 cm).