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Marine Mammal Physiology: Requisites for Ocean Living
at the water surface to which they have to return for breathing. When diving, low light
levels are caused by sunlight getting more and more absorbed and scattered with depth.
Absorption and scattering is increased if particles are dissolved in the water which especially characterizes coastal waters or rivers. Under these dim light conditions, are marine
mammals able to use vision? Which role could vision nevertheless play in their visual
environment?
Marine mammals possess well-developed eyes with one exception which will be highlighted later in this chapter (for a review, see for example, Mass and Supin 2007; Kröger
and Katzir 2008; Reuter and Peichl 2008; Hanke et al. 2009; Mass and Supin 2009). In most
marine mammals, many adaptations have evolved to increase sensitivity comparable to
adaptations found, for example, in crepuscular or nocturnal animals. Most marine mammals have large eyes and large ranges of pupillary opening, the retina is dominated by lights ensitive rods, visual pigments possess maximum sensitivity in blue light, giant ganglion
cells pool visual information, and tapeta line the fundus (see Section 12.2.1). Moreover, diving straight down from the potentially very bright water surface to deep waters, the eyes of
marine mammals also quickly and efficiently adapt to the light intensities that dominate
the light conditions at the depth at which they are foraging (Levenson and Schusterman
1999). Some marine mammals such as elephant seals or beaked (Ziphiidae) and sperm
(Physeter macrocephalus) whales even dive down to the deep sea and hunt deep-sea squid
and fish at more than 1500 m depth. Although sunlight is never reaching these depths, the
deep sea is not necessarily completely dark. Bioluminescent point-like flashes contribute
light for vision in the deep sea (Warrant and Locker 2004). This visual environment has
shaped the eyes of deep-diving marine mammals. In addition to a very fast dark adaptation
(Levenson and Schusterman 1999) mediated by large pupillary openings (Levenson and
Schusterman 1997), the rod pigment, the rhodopsin, of deep divers is highly blue-shifted
and thus maximally sensitive to light of 480–486 nm wavelength (Lythgoe and Dartnall
1970; McFarland 1971; Fasick and Robinson 2000; Southall et al. 2002; Levenson et al. 2006).
These blue-shifted pigments might be a response to bluish light that penetrates best into
deep waters and to bluish bioluminescence. Moreover, the blue pigments have a reduced
noise levels which helps to detect a signal and thus is required to gain a reasonably good
image when photons are scarce and consequently noise is high (Reuter and Peichl 2008).
Furthermore, the number of axons within the optic nerves of deep-diving animals is highest among the marine mammals with, for example, 750,000 axons in southern elephant
seals (Mirounga leonina; Pütter 1903) and 1,000,000 in the pygmy sperm whale (Kogia breviceps; Dawson 1980). The putatively resulting high resolution might enable these animals to
detect small bioluminescent sources even at distance (Kröger and Katzir 2008).
Marine mammals encounter dim light conditions when diving deep or at night,
however, also when, even under perfect light conditions, dissolved particles additionally
cause absorption and scattering (Jerlov 1976; Mobley 1994). With increasing turbidity,
the underwater visual acuity drops drastically as shown for harbor seals (Phoca vitulina)
(Weiffen et al. 2006). Thus, the detection of small prey items is limited. However, visual
orientation underwater requires more than just object detection in a foraging context.
Recently, researchers came up with the idea that the particles that seem to render vision
in murky waters difficult, if not impossible, create a perfect optic flow environment when
passed by an animal (Gläser et al. 2014). Optic flow is defined as the pattern of visual
motion elicited on the retina of a moving observer (Gibson 1950). When moving forward, an optic flow pattern is generated in which all motion vectors emanate from the
point of heading, the focus of expansion (FOE) (Figure 12.1a). The interpretation of optic
flow patterns may allow an animal to avoid colliding with obstacles in its surround
Marine Mammal Physiology: Requisites for Ocean Living
at the water surface to which they have to return for breathing. When diving, low light
levels are caused by sunlight getting more and more absorbed and scattered with depth.
Absorption and scattering is increased if particles are dissolved in the water which especially characterizes coastal waters or rivers. Under these dim light conditions, are marine
mammals able to use vision? Which role could vision nevertheless play in their visual
environment?
Marine mammals possess well-developed eyes with one exception which will be highlighted later in this chapter (for a review, see for example, Mass and Supin 2007; Kröger
and Katzir 2008; Reuter and Peichl 2008; Hanke et al. 2009; Mass and Supin 2009). In most
marine mammals, many adaptations have evolved to increase sensitivity comparable to
adaptations found, for example, in crepuscular or nocturnal animals. Most marine mammals have large eyes and large ranges of pupillary opening, the retina is dominated by lights ensitive rods, visual pigments possess maximum sensitivity in blue light, giant ganglion
cells pool visual information, and tapeta line the fundus (see Section 12.2.1). Moreover, diving straight down from the potentially very bright water surface to deep waters, the eyes of
marine mammals also quickly and efficiently adapt to the light intensities that dominate
the light conditions at the depth at which they are foraging (Levenson and Schusterman
1999). Some marine mammals such as elephant seals or beaked (Ziphiidae) and sperm
(Physeter macrocephalus) whales even dive down to the deep sea and hunt deep-sea squid
and fish at more than 1500 m depth. Although sunlight is never reaching these depths, the
deep sea is not necessarily completely dark. Bioluminescent point-like flashes contribute
light for vision in the deep sea (Warrant and Locker 2004). This visual environment has
shaped the eyes of deep-diving marine mammals. In addition to a very fast dark adaptation
(Levenson and Schusterman 1999) mediated by large pupillary openings (Levenson and
Schusterman 1997), the rod pigment, the rhodopsin, of deep divers is highly blue-shifted
and thus maximally sensitive to light of 480–486 nm wavelength (Lythgoe and Dartnall
1970; McFarland 1971; Fasick and Robinson 2000; Southall et al. 2002; Levenson et al. 2006).
These blue-shifted pigments might be a response to bluish light that penetrates best into
deep waters and to bluish bioluminescence. Moreover, the blue pigments have a reduced
noise levels which helps to detect a signal and thus is required to gain a reasonably good
image when photons are scarce and consequently noise is high (Reuter and Peichl 2008).
Furthermore, the number of axons within the optic nerves of deep-diving animals is highest among the marine mammals with, for example, 750,000 axons in southern elephant
seals (Mirounga leonina; Pütter 1903) and 1,000,000 in the pygmy sperm whale (Kogia breviceps; Dawson 1980). The putatively resulting high resolution might enable these animals to
detect small bioluminescent sources even at distance (Kröger and Katzir 2008).
Marine mammals encounter dim light conditions when diving deep or at night,
however, also when, even under perfect light conditions, dissolved particles additionally
cause absorption and scattering (Jerlov 1976; Mobley 1994). With increasing turbidity,
the underwater visual acuity drops drastically as shown for harbor seals (Phoca vitulina)
(Weiffen et al. 2006). Thus, the detection of small prey items is limited. However, visual
orientation underwater requires more than just object detection in a foraging context.
Recently, researchers came up with the idea that the particles that seem to render vision
in murky waters difficult, if not impossible, create a perfect optic flow environment when
passed by an animal (Gläser et al. 2014). Optic flow is defined as the pattern of visual
motion elicited on the retina of a moving observer (Gibson 1950). When moving forward, an optic flow pattern is generated in which all motion vectors emanate from the
point of heading, the focus of expansion (FOE) (Figure 12.1a). The interpretation of optic
flow patterns may allow an animal to avoid colliding with obstacles in its surround