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Marine Mammal Physiology: Requisites for Ocean Living
With this knowledge in mind and returning to benthic prey, is there any hydrodynamic
information that a marine mammal could use to detect even benthic prey? Every animal
irrespective of lifestyle has to breathe. The breathing current expelled from the gills is causing water movements that are within the perception range of the vibrissal system of seals
and sea lions (Bublitz 2010). Moreover, harbor seals seem to be able to localize artificially
generated breathing currents over a large area (Niesterok et al. 2014). Thus the hydrodynamic detection of breathing currents of benthic fish might answer the question raised by
Bowen et al. (2002), namely, which cues seals and marine mammals in general can rely on
for a successful detection of benthic prey.
12.2 Knowledge by order
12.2.1 The visual system: Adaptation to low light levels
The marine mammal eye is of general vertebrate eye bauplan (Walls 1942), however,
specific adaptations to the visual ecology and lifestyle of a particular species are apparent. A common trait of the eyes of marine mammals is that they have evolved many
adaptations to increase sensitivity as a consequence of low light levels dominating
their visual environment—important to mention, the eyes of manatees, sea otters, and
polar bears (Ursus maritimus) share only some of these adaptations probably as manatees and polar bears inhabit light-rich environments and sea otters are phylogenetically young semi-aquatic animals with eyes still largely resembling eyes of terrestrial
mammals.
First, an effective means of increasing sensitivity is to increase pupil diameter.
And as the pupil diameter cannot exceed eye size, a further increase in sensitivity can
only be achieved by increasing eye size (Land and Nilsson 2002). And indeed, most
marine mammals possess large eyes; the largest extant eyes in vertebrates can actually
be found in cetaceans (Walls 1942). Their eyes are also larger in comparison to their
terrestrial relatives (Debey and Pyenson 2013). And moreover, among the pinnipeds,
the pupil can dilate up to an area of 422 mm 2 as measured in northern elephant seals
(Mirounga angustirostris) in dim light (Levenson and Schusterman 1997) which is more
than five times larger than the fully dilated pupil of a young human (Rogers 2011).
Second, the retina is dominated by rods. Generally, rods are more sensitive to light
than cones as they contain a very light-sensitive pigment, the rhodopsin, they collect
photons over longer time periods (temporal summation), and they pool photons over a
larger area (spatial summation) due to a higher convergence. The ringed seal (Phoca hispida) retina only contains approximately 1.5% cones with a mean cone/rod ratio of 1:64
(Peichl and Moutairou 1998) in comparison to approximately 5% cones and a cone/rod
ratio of 1:19 in humans (Jonas et al. 1992). Pronouncedly in whales, the visual information is spatially pooled in large receptive fields of giant ganglion cells that also have a
very low density across the retina (Mass and Supin 2007; Reuter and Peichl 2008). And
third, a thick tapetum (Pütter 1903; Walls 1942; Nagy and Ronald 1970; Jamieson and
Fisher 1971; Young et al. 1988) can be found behind the inversely organized retina. This
thick cell layer is reflecting photons that were not absorbed during the first passage of
the retina and thus enables photon absorption during the second passage of the retina.
According to Walls (1942), the tapeta of seals are the thickest tapeta found in the animal kingdom. In some marine mammals (see for example, Johnson 1901; Dawson et al.
1987), the tapetum backs the whole ocular fundus and is not restricted to the ventral
fundus as in terrestrial carnivores. This way, the tapeta can increase the probability of
Marine Mammal Physiology: Requisites for Ocean Living
With this knowledge in mind and returning to benthic prey, is there any hydrodynamic
information that a marine mammal could use to detect even benthic prey? Every animal
irrespective of lifestyle has to breathe. The breathing current expelled from the gills is causing water movements that are within the perception range of the vibrissal system of seals
and sea lions (Bublitz 2010). Moreover, harbor seals seem to be able to localize artificially
generated breathing currents over a large area (Niesterok et al. 2014). Thus the hydrodynamic detection of breathing currents of benthic fish might answer the question raised by
Bowen et al. (2002), namely, which cues seals and marine mammals in general can rely on
for a successful detection of benthic prey.
12.2 Knowledge by order
12.2.1 The visual system: Adaptation to low light levels
The marine mammal eye is of general vertebrate eye bauplan (Walls 1942), however,
specific adaptations to the visual ecology and lifestyle of a particular species are apparent. A common trait of the eyes of marine mammals is that they have evolved many
adaptations to increase sensitivity as a consequence of low light levels dominating
their visual environment—important to mention, the eyes of manatees, sea otters, and
polar bears (Ursus maritimus) share only some of these adaptations probably as manatees and polar bears inhabit light-rich environments and sea otters are phylogenetically young semi-aquatic animals with eyes still largely resembling eyes of terrestrial
mammals.
First, an effective means of increasing sensitivity is to increase pupil diameter.
And as the pupil diameter cannot exceed eye size, a further increase in sensitivity can
only be achieved by increasing eye size (Land and Nilsson 2002). And indeed, most
marine mammals possess large eyes; the largest extant eyes in vertebrates can actually
be found in cetaceans (Walls 1942). Their eyes are also larger in comparison to their
terrestrial relatives (Debey and Pyenson 2013). And moreover, among the pinnipeds,
the pupil can dilate up to an area of 422 mm 2 as measured in northern elephant seals
(Mirounga angustirostris) in dim light (Levenson and Schusterman 1997) which is more
than five times larger than the fully dilated pupil of a young human (Rogers 2011).
Second, the retina is dominated by rods. Generally, rods are more sensitive to light
than cones as they contain a very light-sensitive pigment, the rhodopsin, they collect
photons over longer time periods (temporal summation), and they pool photons over a
larger area (spatial summation) due to a higher convergence. The ringed seal (Phoca hispida) retina only contains approximately 1.5% cones with a mean cone/rod ratio of 1:64
(Peichl and Moutairou 1998) in comparison to approximately 5% cones and a cone/rod
ratio of 1:19 in humans (Jonas et al. 1992). Pronouncedly in whales, the visual information is spatially pooled in large receptive fields of giant ganglion cells that also have a
very low density across the retina (Mass and Supin 2007; Reuter and Peichl 2008). And
third, a thick tapetum (Pütter 1903; Walls 1942; Nagy and Ronald 1970; Jamieson and
Fisher 1971; Young et al. 1988) can be found behind the inversely organized retina. This
thick cell layer is reflecting photons that were not absorbed during the first passage of
the retina and thus enables photon absorption during the second passage of the retina.
According to Walls (1942), the tapeta of seals are the thickest tapeta found in the animal kingdom. In some marine mammals (see for example, Johnson 1901; Dawson et al.
1987), the tapetum backs the whole ocular fundus and is not restricted to the ventral
fundus as in terrestrial carnivores. This way, the tapeta can increase the probability of