ANALYSING A SPECIALIST
ADAPTED FOR LIFE ABOVE AND BELOW WAVES
Seals have large eyes with powerful iris muscles that allow for a huge increase in the size of the pupils in response to low light conditions. In daylight, seal pupils appear like vertical slits; as light reduces, pupil size expands dramatically. Grey seals also have a reflective layer behind their retina, the tapetum cellulosum, which allows light to reflect back into the eye. A flat region of the cornea enables the seal to focus clearly below and above water. There is debate over the ability of grey seals to see in colour, but a 2016 study concludes that common, or harbour, seals can see coloured triangles and it’s likely the same is true of greys.
Seals use their sense of smell to locate food from some distance. They are also able to detect certain chemicals at very low concentrations. Of particular note, seals seem acutely sensitive to dimethyl sulphide (DMS), a chemical produced by phytoplankton in response to zooplankton grazing; the distribution of DMS at sea is linked to the abundance of zooplankton and thus to that of shoaling fish, on which seals feed.
Above water, grey seals have a similar hearing range to humans, but underwater it appears that their hearing range is greater, extending from less than 75Hz to over 75kHz. Their calls have been grouped into guttural ‘rups’ and ‘rupes’ (100–3000Hz) and lowfrequency growls (100–500Hz). Other less common vocalisations recorded to date include low-frequency clicks (3000Hz), as well as loud knocks. The seals are more vocal as the breeding season progresses. Studies have also shown more seals haulout when responding to boat sonar.
Seals have a highly tuned ability to differentiate between seawater samples of varying salinity. Salinity gradients can be used as a valuable clue when seals are orienting themselves or foraging; for example, studies have shown that the seasonal development of phytoplankton is linked to surface salinity conditions.
Seal vibrissae, or whiskers, are incredibly sensitive. They have an undulating shape that reduces vortices generated behind them as the animals move through the water. The base of each whisker is packed with nerves. Research in Germany concluded that seals could detect hydrodynamic trails left by fish over 100m away.