102
Marine Mammal Physiology: Requisites for Ocean Living
as molar progression (Lanyon and Sanson 2006a,b; Unger 2010). Unlike manatees, however, dugongs have a finite number of molars (six). However, some evidence suggests that
dugongid cheek teeth may not be functional at all. Instead the enlarged, heavily cornified
and rugose palatal pads may function to masticate, process, and transport sea grasses into
the buccal cavity. Although trichechids also possess such cornified palatal pads, those of
dugongs are much more robust and cover a larger surface area (Marsh et al. 1999; Lanyon
and Sanson 2006a,b).
The facial muscles of all sirenians form a muscular hydrostat (Kier and Smith
1985), or short muscular snout, that is capable of highly complex and varied movements
(Marshall et al. 1998a, 2003). The complex facial muscles surround a series of six perioral
bristle fields, or modified vibrissae, which are located on both the broad and expanded
upper and lower lip margins (Reep et al. 1998, 2001; Marshall et al. 2003). Vibrissae are
specialized hairs that transmit tactile information from the environment to the central
nervous system. Although these bristles are homologous with mystacial vibrissae of
other mammals, manatee vibrissae differ in that they are short, thick, and robust. These
are also unusual in they function in both motor and sensory roles. When contracted,
semicircular facial muscles protrude the largest pair of perioral bristles on the upper
(U2 bristle fields) and lower (L1) lips, which are then used to handle and manipulate
vegetation (Marshall et al. 2000). How this is accomplished differs between manatees
and dugongs (Marshall et al. 1998b, 2003). In manatees, the paired upper bristle fields
are protruded anteriorly and then medially in a grasping motion, pushing vegetation in
the mouth. This is alternated with a sweeping motion of the lower bristle fields to further push vegetation into the mouth. For dugongs, the upper bristle fields have a slightly
more horizontal distribution across the lip margin (oral disk) but are also protruded (as
much as 6 cm). However, rather than moving toward the midline, the upper bristles are
moved laterally, in a breast-stroke-like motion (Marshall et al. 2003). As dugongs graze
along the seafloor, this functions to part the sea grass in front of the animal and introduce plant material into the side of the mouth. As in manatees, the bristle fields on the
lower jaw alternate and sweep vegetation further into the mouth. When feeding upon
small species of sea grasses, this mechanism can be used to literally excavate the root
system (belowground biomass), with rhizomes, from the benthic substrate. This action
can be seen as a signature feeding trail on the seafloor, which creates substantial bioturbation. Feeding trails span the width of the dugong rostrum, can be as deep as 5 cm, and
as long as 10 m (Anderson and Birtles 1978). Up to 90% of the vegetation can be removed
from these feeding trails (Preen 1995).
5.2.3 Order Carnivora: Pinnipedia
Pinnipeds are a monophyletic lineage of carnivores that include sea lions (Otariidae), seals
(Phocidae), and walruses (Odobenidae). Each family has successfully transitioned back
to the aquatic environment, albeit at different time scales. Extant pinnipeds consume a
diversity of prey that include fish, cephalopods, bivalves, crustaceans, invertebrates, and
large amniote prey (penguins, seabirds, other marine mammals) (King 1983; Riedman
1990; Pauly et al. 1998). Although pinnipeds are a major carnivoran lineage, we know less
regarding their aquatic feeding mechanisms than other marine mammal groups (i.e., cetaceans and sirenians). Our knowledge of how pinnipeds feed is largely d escriptive, and
lacks the detailed functional and phylogenetic analyses found for basal aquatic vertebrates (e.g., actinopterygian fish; but see Jones and Goswami 2010; Jones et al. 2013). Three
themes in the feeding function of pinnipeds are (1) loss of m astication, (2) a reduced and
Marine Mammal Physiology: Requisites for Ocean Living
as molar progression (Lanyon and Sanson 2006a,b; Unger 2010). Unlike manatees, however, dugongs have a finite number of molars (six). However, some evidence suggests that
dugongid cheek teeth may not be functional at all. Instead the enlarged, heavily cornified
and rugose palatal pads may function to masticate, process, and transport sea grasses into
the buccal cavity. Although trichechids also possess such cornified palatal pads, those of
dugongs are much more robust and cover a larger surface area (Marsh et al. 1999; Lanyon
and Sanson 2006a,b).
The facial muscles of all sirenians form a muscular hydrostat (Kier and Smith
1985), or short muscular snout, that is capable of highly complex and varied movements
(Marshall et al. 1998a, 2003). The complex facial muscles surround a series of six perioral
bristle fields, or modified vibrissae, which are located on both the broad and expanded
upper and lower lip margins (Reep et al. 1998, 2001; Marshall et al. 2003). Vibrissae are
specialized hairs that transmit tactile information from the environment to the central
nervous system. Although these bristles are homologous with mystacial vibrissae of
other mammals, manatee vibrissae differ in that they are short, thick, and robust. These
are also unusual in they function in both motor and sensory roles. When contracted,
semicircular facial muscles protrude the largest pair of perioral bristles on the upper
(U2 bristle fields) and lower (L1) lips, which are then used to handle and manipulate
vegetation (Marshall et al. 2000). How this is accomplished differs between manatees
and dugongs (Marshall et al. 1998b, 2003). In manatees, the paired upper bristle fields
are protruded anteriorly and then medially in a grasping motion, pushing vegetation in
the mouth. This is alternated with a sweeping motion of the lower bristle fields to further push vegetation into the mouth. For dugongs, the upper bristle fields have a slightly
more horizontal distribution across the lip margin (oral disk) but are also protruded (as
much as 6 cm). However, rather than moving toward the midline, the upper bristles are
moved laterally, in a breast-stroke-like motion (Marshall et al. 2003). As dugongs graze
along the seafloor, this functions to part the sea grass in front of the animal and introduce plant material into the side of the mouth. As in manatees, the bristle fields on the
lower jaw alternate and sweep vegetation further into the mouth. When feeding upon
small species of sea grasses, this mechanism can be used to literally excavate the root
system (belowground biomass), with rhizomes, from the benthic substrate. This action
can be seen as a signature feeding trail on the seafloor, which creates substantial bioturbation. Feeding trails span the width of the dugong rostrum, can be as deep as 5 cm, and
as long as 10 m (Anderson and Birtles 1978). Up to 90% of the vegetation can be removed
from these feeding trails (Preen 1995).
5.2.3 Order Carnivora: Pinnipedia
Pinnipeds are a monophyletic lineage of carnivores that include sea lions (Otariidae), seals
(Phocidae), and walruses (Odobenidae). Each family has successfully transitioned back
to the aquatic environment, albeit at different time scales. Extant pinnipeds consume a
diversity of prey that include fish, cephalopods, bivalves, crustaceans, invertebrates, and
large amniote prey (penguins, seabirds, other marine mammals) (King 1983; Riedman
1990; Pauly et al. 1998). Although pinnipeds are a major carnivoran lineage, we know less
regarding their aquatic feeding mechanisms than other marine mammal groups (i.e., cetaceans and sirenians). Our knowledge of how pinnipeds feed is largely d escriptive, and
lacks the detailed functional and phylogenetic analyses found for basal aquatic vertebrates (e.g., actinopterygian fish; but see Jones and Goswami 2010; Jones et al. 2013). Three
themes in the feeding function of pinnipeds are (1) loss of m astication, (2) a reduced and