Midwater animals get a little help from their friends

Another area of patchiness we’ve investigated is how squid, fish, and crustaceans like krill and shrimp organize within layers. Throughout the world’s oceans, animals in the relatively featureless midwater will group together into layers that can cover tens or even hundreds of kilometers. This area is collectively referred to as the “deep scattering layer” because of the reflective characteristics of the collective group. In fact, the deep scattering layer’s strong response to sonar led the layer to be incorrectly interpreted as the ocean floor by early sonar.

Acoustic analysis of the internal organization of different layers of animals off southern California revealed that animals of the same kind and size grouped together to form small schools inside the layers, something that was a surprise. Traditionally, schooling was believed to be a behavior mostly found closer to the ocean surface.

Our analysis also revealed that these individual aggregations are responsive to changes in neighboring aggregations, showing that they are organized at multiple levels. As Risso’s dolphins dove to hunt within an aggregation of deep-sea squid, the squid would form a closer group. The open space left by the consolidating squid was quickly absorbed by neighboring groups of organisms, whether crustaceans or fish. Rather than a response from just the aggregation of targeted prey, this response from non-prey animals that seemed to fill in the gaps indicates a larger, community-wide benefit to keeping an unbroken horizontal layer as well as an important anti-predator benefit from schooling.