The biology of glass shrimps
There is currently much focus on mesopelagic ecosystem (depth range ~200-1000 m), particularly since mesopelagic fish appear to be very abundant. They form acoustic scattering layers occurring worldwide. Also pelagic shrimps are common and important, yet they are weaker acoustic targets than fish and their acoustic signature may be swamped by that of the fishes. To what degree they contribute to the mesopelagic scattering layers are uncertain. Relatively little is known about the biology of many species.
Pelagic shrimps may not be strong targets at 38 kHz, which is the usually applied acoustic frequency in mesopelagic studies, but they are clearly detected at higher frequencies, particularly if not occurring together with fish. The inner Oslofjord is too shallow for mesopelagic fish. However, in the deepest part of the inner fjord (160-170 m) we record acoustic scattering layers that can be ascribed to glass shrimps, Pasipahea sp, a genera with species occurring in the mesopelagic realm throughout the worlds’ oceans. Their relatively shallow occurrence in the Oslofjord can likely be ascribed to the fact that waters in the Oslofjord has higher light absoption than in the open ocean, so that sufficiently dark waters to avoid visually searching predators occurs at relatively shallow depth. Also, the shrimps in the fjord appear to be young stages, which in general occur shallower than the adults.
The presence of easily detected acoustic scattering layers of Pasipahea in the Oslofjord provides the opportunity of using the fjord as an experimental basin for study of this shrimps.
We plan to:
Map their presence and vertical distribution over time
Assess vertical distribution relative to light
Establish diel vertical migration pattern for the population as well as individual behavior.
Individuals can be studied by submerging an echosounder into deeper waters, thereby getting much better resolution. Depending on the students’ interest, it is possible to additionally address more acoustically related questions.
The thesis work may also include a component addressing feeding biology, and to test a hypothesis that the shrimps will change their pigmentation in course of the diel cycle as an antipredator mechanism. In both cases, this may be done using digital photography. The glass shrimps are basically transparent so that gut filling can be assessed visually. Level of pigmentation can be quantified from photographs.