99
Chapter five: Feeding mechanisms
5.2.1.2 Cetacea: Mysticeti
Baleen whales (Mysticeti) evolved from toothed whales presumably to exploit aggregations of prey, rather than single prey items. Baleen whales are edentulous and exhibit calcified alpha-keratin baleen racks that act as the primary feeding structure (Szewciw et al.
2010). All baleen whales are obligate bulk filter feeders that engulf a volume of prey-laden
water that is subsequently filtered out of the mouth using two racks of vertically oriented
baleen plates that hang down from the top of the rostrum. Each baleen rack consists of an
array of plates that are obliquely angled relative to the whale’s long body axis. Each plate
consists of a series of tubules that are imbedded in a calcified, keratin matrix (Fudge et al.
2009; Szewciw et al. 2010). The plates are frayed on the lingual side, thereby exposing the
core tubules that derive from the palatal epithelium. Because these plates are embedded
in a cream white Zwischensubstanz less than 1 cm apart (Pinto and Shadwick 2013), these
exposed tubules, or fringes, collectively form a fibrous mat that acts as the filter that separates prey that are suspended in engulfed water. There is a tremendous amount of diversity among mysticetes with respect to plate and fringe morphology, and these variants
correlate generally with prey preference and ecological niche. For example, finer fringes
are associated with smaller prey, as exemplified by bowhead and right whales that feed
primarily on copepods (Werth 2012).
There are different modes of filter feeding represented among different baleen whale
families. These modes include suction and ram hydraulic phenomena, the latter of which
can be either intermittent or continuous, but these elements manifest in baleen whales
in very different ways compared to toothed whales. Mysticetes have very large skulls
and most lack soft tissues that completely occlude the lateral gape, so enhanced suction
performance may not be possible in most species. However, one monotypic family of
gray whales (Eschrichtidae) is known to use suction to feed along the seafloor on benthic invertebrates (Nerini 1984). Gray whales have well developed hyolingual musculature and ventral grooves that facilitate the depression of the hyoid and tongue to generate
suction (Werth 2007). However, it is unclear how adequate suction is produced without
lateral occlusion of the gape. Gray whales have been observed both in captivity and virtually using digital movement tags to roll onto their lateral sides during feeding (Ray and
Schevill 1974; Woodward and Winn 2006). Therefore, the flow of water and prey must enter
and exit through the side of the mouth, and relatively short baleen plates may enable this
unique filtration mechanism (Werth 2001). In addition, gray whales could use the substrate
and the geomorphology of the seafloor to passively increase suction distance. The ability
to use the substrate to passively increase the suction distance on benthic prey is known
for both ray-finned and chondrichthyan fish (Carroll et al. 2004; Nauwelaerts et al. 2007).
Interestingly, other observations suggest that gray whales are generalist filter feeders
and may also use ram in combination with suction to feed throughout the water column
(Pyenson and Lindberg 2011). Despite several observational and anatomical studies, the
filter feeding mechanisms in gray whales remain poorly understood. Even more enigmatic
are pygmy right whales (Cetotheriidae/Neobalaeninae; Caperea marginata), which share
many morphological characters with some mysticetes that exhibit continuous ram feeding (Balaenidae; Bowhead and right whales) and others that are obligate intermittent ram
feeders (Balaenopteridae; rorqual whales) (Fordyce and Marx 2012).
The two mysticete families Balaenidae and Balaenopteridae are extremely divergent in both the anatomy of the feeding apparatus and the hydrodynamic mechanisms
employed to capture and filter prey from seawater. Broadly, balaenids have large and stiff
tongues that direct continuous flow of prey-laden water past long baleen plates, whereas
Chapter five: Feeding mechanisms
5.2.1.2 Cetacea: Mysticeti
Baleen whales (Mysticeti) evolved from toothed whales presumably to exploit aggregations of prey, rather than single prey items. Baleen whales are edentulous and exhibit calcified alpha-keratin baleen racks that act as the primary feeding structure (Szewciw et al.
2010). All baleen whales are obligate bulk filter feeders that engulf a volume of prey-laden
water that is subsequently filtered out of the mouth using two racks of vertically oriented
baleen plates that hang down from the top of the rostrum. Each baleen rack consists of an
array of plates that are obliquely angled relative to the whale’s long body axis. Each plate
consists of a series of tubules that are imbedded in a calcified, keratin matrix (Fudge et al.
2009; Szewciw et al. 2010). The plates are frayed on the lingual side, thereby exposing the
core tubules that derive from the palatal epithelium. Because these plates are embedded
in a cream white Zwischensubstanz less than 1 cm apart (Pinto and Shadwick 2013), these
exposed tubules, or fringes, collectively form a fibrous mat that acts as the filter that separates prey that are suspended in engulfed water. There is a tremendous amount of diversity among mysticetes with respect to plate and fringe morphology, and these variants
correlate generally with prey preference and ecological niche. For example, finer fringes
are associated with smaller prey, as exemplified by bowhead and right whales that feed
primarily on copepods (Werth 2012).
There are different modes of filter feeding represented among different baleen whale
families. These modes include suction and ram hydraulic phenomena, the latter of which
can be either intermittent or continuous, but these elements manifest in baleen whales
in very different ways compared to toothed whales. Mysticetes have very large skulls
and most lack soft tissues that completely occlude the lateral gape, so enhanced suction
performance may not be possible in most species. However, one monotypic family of
gray whales (Eschrichtidae) is known to use suction to feed along the seafloor on benthic invertebrates (Nerini 1984). Gray whales have well developed hyolingual musculature and ventral grooves that facilitate the depression of the hyoid and tongue to generate
suction (Werth 2007). However, it is unclear how adequate suction is produced without
lateral occlusion of the gape. Gray whales have been observed both in captivity and virtually using digital movement tags to roll onto their lateral sides during feeding (Ray and
Schevill 1974; Woodward and Winn 2006). Therefore, the flow of water and prey must enter
and exit through the side of the mouth, and relatively short baleen plates may enable this
unique filtration mechanism (Werth 2001). In addition, gray whales could use the substrate
and the geomorphology of the seafloor to passively increase suction distance. The ability
to use the substrate to passively increase the suction distance on benthic prey is known
for both ray-finned and chondrichthyan fish (Carroll et al. 2004; Nauwelaerts et al. 2007).
Interestingly, other observations suggest that gray whales are generalist filter feeders
and may also use ram in combination with suction to feed throughout the water column
(Pyenson and Lindberg 2011). Despite several observational and anatomical studies, the
filter feeding mechanisms in gray whales remain poorly understood. Even more enigmatic
are pygmy right whales (Cetotheriidae/Neobalaeninae; Caperea marginata), which share
many morphological characters with some mysticetes that exhibit continuous ram feeding (Balaenidae; Bowhead and right whales) and others that are obligate intermittent ram
feeders (Balaenopteridae; rorqual whales) (Fordyce and Marx 2012).
The two mysticete families Balaenidae and Balaenopteridae are extremely divergent in both the anatomy of the feeding apparatus and the hydrodynamic mechanisms
employed to capture and filter prey from seawater. Broadly, balaenids have large and stiff
tongues that direct continuous flow of prey-laden water past long baleen plates, whereas