Seeing in the Sea

A For animals living in water, seeing clearly is far more complicated than it is for creatures on land. A single coastal region may contain bright surface water, shadowy depths and estuaries so muddy that very little light penetrates them. The amount of available light can change dramatically with depth, weather, tides and the concentration of suspended material in the water. Its colour also varies: blue wavelengths travel furthest through clear ocean water, whereas coastal and estuarine environments tend to transmit greener, yellowish or reddish light.

No visual system can perform perfectly in all these conditions. An eye designed to gather as much light as possible may function well in darkness but usually processes detail more slowly. Conversely, an eye that detects rapid movement accurately often requires stronger illumination. Aquatic animals must therefore make compromises between sensitivity, speed and sharpness. Natural selection tends to favour the combination most appropriate for the habitat and behaviour of each species.

B To investigate such adaptations, marine biologist Andrij Horodysky and an international team studied five members of the fish family Sciaenidae found in and around Chesapeake Bay in the eastern United States. These fish, commonly known as drums and croakers, are closely related, yet they occupy noticeably different ecological niches.

The species selected were weakfish, spotted seatrout, red drum, Atlantic croaker and spot. Some pursue prey in open or relatively shallow water, while others search for food on or close to the seabed. Their habitats differ greatly in brightness and water clarity, making them particularly useful for studying how visual systems become specialised. The researchers expected that the eyes of each species would reflect the light conditions in which it normally fed and avoided predators.

C The scientists measured the fishes’ vision using electroretinography. This technique records small changes in electrical activity in the retina when the eye is exposed to light. Six individuals from most of the species were examined. Before testing began, each fish was kept in darkness for at least 30 minutes so that its visual system could adjust to low-light conditions.

The team then conducted three main experiments. First, they varied the intensity of white light to establish how little illumination each fish could detect and how wide a range of brightness its eyes could process. Second, they presented rapidly flickering light. By gradually increasing the flicker rate, they could identify the point at which the fish’s visual system was no longer able to distinguish separate flashes. This measurement provided an indication of how quickly visual information could be processed. Finally, the researchers exposed the fish to different wavelengths to determine which colours produced the strongest retinal responses.

D The results broadly supported the idea that lifestyle shapes visual performance. Atlantic croaker and spot, which commonly feed near the bottom, displayed particularly high sensitivity to faint light. Their visual systems also operated effectively across a relatively broad range of light intensities. This would be useful in habitats where clouds of sediment, changing tides and uneven depths cause illumination to fluctuate.

However, these bottom-feeding species did not dominate every aspect of vision. Their eyes appeared to be general-purpose systems, balancing sensitivity, speed and resolution rather than being exceptional at one particular task. This represents a practical evolutionary solution: an animal feeding in unpredictable conditions may benefit more from reasonable performance across several functions than from outstanding performance in only one.

E The most striking contrast occurred between weakfish and spotted seatrout. Although the two species belong to the same genus, their visual systems process changing images at very different speeds. Under strong illumination, spotted seatrout produced the fastest response to flickering light of all five species. This suits a predator that pursues moving prey in comparatively bright, shallow environments.

Weakfish, by contrast, had the slowest response. At first sight, slow visual processing might seem disadvantageous. Yet by collecting light over a slightly longer period, the weakfish eye can capture more photons before transmitting a signal. This improves sensitivity in dim or turbid water, although it reduces the animal’s ability to distinguish extremely rapid movement. Its visual system has consequently exchanged speed and fine detail for better performance when light is scarce.

F The colour experiments revealed another unusual feature of weakfish vision. Most of the species responded primarily to wavelengths within the visible range from approximately violet to orange-red. Weakfish, however, also reacted to ultraviolet light. Examination of the tissues inside their eyes showed that ultraviolet wavelengths could pass through the cornea, internal fluid and lens, making some form of ultraviolet vision possible.

This ability may help weakfish detect objects or prey under suitable conditions, although its usefulness is probably reduced in highly turbid water because ultraviolet light is absorbed and scattered quickly. The researchers also observed that the spectral sensitivity of weakfish and Atlantic croaker shifted towards shorter wavelengths at night. Red drum and spot showed no comparable day-to-night change. These differences demonstrate that even related species do not necessarily solve the problem of underwater vision in the same way.

G The findings suggest that the five fishes are well matched to the visual environments in which they evolved. Nevertheless, the researchers warned that those environments are being altered rapidly. Industrial development, population growth and nutrient pollution have increased the amount of organic material and suspended particles entering many estuaries. As a result, water may become darker and more turbid, changing both the quantity and the colour of the available light.

Evolutionary adjustment occurs over many generations, whereas human-driven changes in water quality can happen within decades. A species whose eyes were once suitable for its habitat may therefore find it increasingly difficult to locate prey, recognise potential mates or avoid predators. Horodysky argues that studying the relationship between sensory physiology and animal behaviour is consequently important for fisheries management. Protecting a population requires more than counting the number of fish; it also requires an understanding of whether those animals can still perceive and function effectively in their changing environment.