# A study of the bioluminescence of larger zooplankton and the effects of low-level light changes on their behavior

#### Abstract

A bio-optical study was undertaken to quantify the relationships which exist between counter-illuminating organisms and the downwelling spectral light field in which they exist. The basic hypothesis behind counter-illumination is that the animal emits light using ventrally-oriented photophores to disrupt or eliminate the shadowed area on ventral surfaces. An organism lacking photophores sharply silhouettes against the highly directional downwelling irradiance, whereas by distributing photophores over the ventral surface of the body and closely matching the spectral and intensity characteristics of the downwelling light, this silhouette is obscured.^ Analysis carried out on changes in vertical distribution patterns in response to low-level intensity changes in ambient surface light suggested that diel migrating organisms begin to shift vertically in the water column when surface scalar irradiance decreased below or increased above $1.0\times10\sp{-2}\ \mu$Ein m$\sp{-2}$ sec$\sp{-1}$. Maximum aggregations of organisms, as defined by MOCNESS net sampling or single-frequency acoustic backscatter, appeared to remain within definable in situ blue-green isolume ranges varying less than a factor of ten throughout each night. Comparisons made between organism counter-illumination capacity and modeled in situ downwelling irradiance levels suggested that euphausiids, decapods and myctophids use between 1-10 percent of their maximum counter-illumination capacity to match the ambient downwelling light conditions. Modeling also suggested that up to 40 percent of the maximum measured bioluminescence output is required to match ambient irradiance in the shallower surface zones where aggregations of copepods, potential food sources, were commonly found at night.^ An optical study to quantify the radiative transfer of bioluminescence from a point source revealed that non-isotropic point sources produce radiance patterns that cannot be simply explained by inverse square losses. Therefore simple inverse-square estimates of bioluminescent propagation loss rates from organisms in the ocean are an oversimplification of the radiative transfer processes that occur when these emissions occur. Additionally, in evaluating counter-illumination, the distance of the receptor, such as the eyes of a potential predator, is critical in determining the effectiveness of the organisms in matching the uniform light field of their surrounding environment and ultimately avoiding detection and predation. ^

#### Subject Area

Physical Oceanography|Biology, Oceanography|Physics, Optics

#### Recommended Citation

Jeffrey Robert Van Keuren, "A study of the bioluminescence of larger zooplankton and the effects of low-level light changes on their behavior" (1994). Dissertations and Master's Theses (Campus Access). Paper AAI9513255.
http://digitalcommons.uri.edu/dissertations/AAI9513255

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