Fish or jellies—a question of visibility?

Abstract

Light is an important limiting factor for the visual foraging process in fishes, and the light regime may potentially affect the competition between visual and tactile predators. We investigated two equal‐sized fjords of quite different pelagic food web structure. Earlier studies have revealed that the jellyfish Periphylla periphylla dominates Lurefjorden, while fish predators dominate in the other fjord, Masfjorden. Furthermore, the mesozooplankton stock of Lurefjorden is larger in both total biomass and size of the individuals. Hence, earlier hypotheses linking the competitive advantage of tactile gelatinous plankton predators to smaller sized mesozooplankton communities are unable to explain the present phenomenon. To see if the difference in the pelagic biota of the two fjords could be associated with characteristics of the light regime, we measured the light absorbance in the basin water of the two fjords. We found that, due to a slightly stronger influence of coastal water in the basin water of Lurefjorden, the exponential light absorbance coefficient below 100 m is two to three times higher there than in other fjords. This results in a reduction in light flux of several orders of magnitude, effectively reducing the possibility of visual foraging. The tactile mode of predation in jellyfish, however, is not influenced, and we hypothesize that the visibility regime has a decisive role in structuring the pelagic food webs of the two fjords.

The rate of metabolism in marine animals: environmental constraints, ecological demands and energetic opportunities

“…transparency: Johnsen 2001) or more (e.g. bioluminescence : Widder 2002) conspicuous and can ultimately drive species composition and the structure of pelagic ecosystems (Eiane et al 1999). Furthermore, de Queiroz (1999) found, looking only at shallow-living taxa, a strong association between imageforming eyes and activity levels.…”

“…Body form is constrained, in clear, well-lit oceanic waters, by the demand for streamlined bodies that consist largely of locomotory muscles used for high-speed locomotion during predator-prey interactions. Vision-limited predators rely more on tactile or sit-and-wait predation strategies and, in fact, turbid water masses favour tactile predators by diminishing the efficiency of visual predation (Eiane et al 1999;Aksnes et al 2004).…”

Predation efficiency in visual and tactile zooplanktivores

“…A literature review of predation efficiency in visual and tactile zooplanktivores indicated that these two animals can be considered as representative models of the two predation modes. Eiane et al (1999) hypothesized that tactile predators, which are adapted to feed continuously, should increase their com-1 Corresponding author (tom.sornes@ifm.uib.no).…”

Concurrent temporal patterns in light absorbance and fish abundance

“…The fjord populations are therefore considered semi-enclosed entities, suggesting that population regulations take place on a local scale. Eiane et al (1999) found a negative relation between light absorbance and salinity and attributed the elevated levels of absorbance in Lurefjorden to the high proportion of NCW. NCW originates from the Baltic and North Sea and flows northwards along the Norwegian coast (Fig.…”