Manatees are non-ruminant herbivores that specialize in hind-gut fermentation. They man consume approximately 8% of body weight daily and feed for 6 - 8 hours per day (Best 1981; Hartman 1979). Manatees and are apparently unparticular in their choice of feeding sites when food is abundant, and will often continue to exploit specific areas until food resources are depleted (Best 1981; Hartman 1979).The stomach is small relative to body size. A fingerlike projection, called the cardiac gland, protrudes from the cardiac portion of the stomach and secretes most enzymes used in digestion (Best 1981). Digestive efficiencies in manatees are similar to those of terrestrial herbivores such as the horse (Best 1981). Intestines may measure up to 40 m (130 ft.) in length (Reynolds 1979).Manatees feed exclusively on submerged, emergent and floating vegetation in freshwater, brackish and marine waters, with seagrasses an important staple in the diet (USFWS 2001). They are indiscriminate feeders that ingest whatever vegetation is available, and move freely between habitats in search of food (Hartman 1979). Feeding generally occurs in shallow water 1- 4 m deep. Plants are consumed in situ or taken below the surface to be manipulated into the mouth using the foreflippers. The upper lips of a manatee are bilobed, prehensile, and covered with stiff bristles. To feed, the lobes are everted so they project forward to the food source. Upon contact with food, Lips then close laterally, the bristles grasping the food and tucking it into the mouth (Hartman 1979).In fresh water, manatees tend to favor submerged, vascular plants such as Hydrilla verticilata, Myriophyllum spicatum, Ceratophyllum demersum, Vallisneria sp. and Callisneria neotropicalis (Snipes 1984; Best 1981). In marine and estuarine habitats, algae such as Anabena, Cladophora, Enteromorpha, Gracilaria, Oscillatoria, and Spirogyra are often consumed, as well as a variety of seagrasses, especially Thalassia testudinum (turtle grass) (Best 1981, Hartman 1979). Provancha and Hall (1991) found spring aggregations of manatees feeding in seagrasses dominated by Syringodium filiforme (manatee grass) and reported an apparent preference for both Syringodium and Halodule wrightii (shoalgrass) over other seagrasses and macroalgae such as Caulerpa spp.Manatees continuously replace worn teeth. As anterior-most teeth wear and are lost, replacement teeth grow at the posterior end of the tooth row (Best 1981).Competitors: Manatees do not strongly compete with other herbivores, likely due to their flexibility in making food choices based on availability (Best 1981). Predators: Manatees have few known predators, but all members of the Sirenia have been hunted for food (Rathbun 1984; Best 1981; Domning 1978), with some populations, especially in the southern hemisphere, still noted to be at risk from poaching and subsistence hunting (IUCN 2006, Jiménez 2002; Domning 1982). A related species, Steller's sea cow (Hydrodamalis gigas), was a 25-foot kelp-feeder that inhabited the Bering Sea. It was hunted to extinction by fur seal hunters within 25 years of its discovery in 1741 (Hartman1979; Marmontel et al. 1997; Domning 1978). In Florida, evidence shows manatees were hunted by pre-Columbian societies. After Spanish occupation of Florida, the increase in human population increased hunting pressures on the manatee, heavily impacting population levels. Commercial and subsistence hunting during the 1800s also significantly reduced the population in Florida. In 1893, the State of Florida passed legislation banning the killing of manatees and they have been protected since that time (USFWS 2001). Parasites: Manatee skin supports a variety of parasitic or commensal organisms. Ectoparasites and commensals include the copepod Harpacticus pulex; Lyngbya, a blue-green algae; diatoms, balanid barnacles, protozoans, nematodes, isopods, small gastropods, and leeches (Hartman 1979). Endoparasites include digenetic trematodes and a variety of nematodes (Hartman 1979). Habitats: Manatees are habitat generalists that utilize canal systems, mangrove creeks, saltmarshes, estuaries, bays and nearshore coastal waters. They move freely between habitat types and regularly seek out fresh water sources for osmoregulation (Haubold et al. 2006; Lefebvre et al. 2001; Hartman 1979). Manatees typically inhabit waters less than 3.7 m (12 feet) in depth (Haubold et al. 2006). Hartman (1979) reported manatees in Western Florida generally inhabited depths of 1.5 - 2 meters (4.9 - 6.6 feet), and observed that manatees tended to avoid waters less than 1.5 m deep unless these areas had access to deeper waters nearby. Feeding is sometimes observed on grassflats less than 50 cm (19.7 inches.) deep. Estrous females often escape the attentions of following males by moving into waters as shallow as 60 cm (23.6 inches), with some even stranding themselves for brief periods. Hartman (1979) reported that fast moving currents, generally over 5 km/hr. (3 miles/hr.), discourage manatees from occupying certain areas. In Sebastian Inlet, currents can exceed 11 km/hr. (6.8 miles/hr.) at certain points during the day. These strong currents likely deter manatees from entering or exiting the inlet during periods of peak velocity. Rather, manatees may swim adjacent to the shoreline, or slip in or out of the Inlet during slack tides.Activity Time: Manatees are generally arrhythmic (Hartman 1979), with feeding, resting, traveling, socializing, and other activities showing no consistent differences between day and evening hours. Feeding occurs over 1 - 2 hour periods and totals 6 - 8 hours daily. Socialization among manatees is highly variable, but is apparently higher in winter when animals aggregate in warm-water refuges. Though manatees spend much of their days seeking out food; sleeping, or traveling (Best 1981), they also spend approximately 6 - 10 hours at rest with no apparent pattern their activities.
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