The Biogeography of the Hawaiian Monk Seal

The Biogeography of the Hawaiian Monk Seal
by Ingrid Overgard, student in Geography 316 Fall 2003

The Hawaiian monk seal’s specific name, Monachus schauinslandi, honors German scientist H.H. Schauinslandi who described the species for the first time in 1899 (Reeves et al. 2002). Monachus schauinslandi is taxonomically classified as a member of the order Carnivora and the family Phocidae. Along with the families Otariidae and Odobenidae, Phocidae are part of a group called the pinnipeds. Sometimes this group is listed as the suborder Pinnipedia. Derived from Latin, the term pinniped means “fin-footed” (Reeves, Stewart and Leatherwood 1992; Reeves et al. 2002). Debate continues over the suborder classification, however, there is general agreement as Reeves et al. (1992) suggests that the term pinniped refers to “all the amphibious aquatic Carnivora whose front and hind limbs are flippers” (3).

The Hawaiian monk seal is a member of the Phocidae family, called the “earless” or true seals because they lack external ear flaps or pinnae. A second phocid characteristic is the inability to rotate the hind flippers under the body in the way that other pinnipeds do. As a result, all phocids use their front flippers on land to pull themselves forward in an undulating motion, like an immense caterpillar with their hind flippers dragging. On the other hand, in the water they are extremely agile and appear almost to fly, using the rear flippers for steering (Reeves, Stewart and Leatherwood 1992; Reeves et al. 2002). The body of M. schauinslandi is a slender torpedo shape, streamlined for swimming. Adults weigh roughly 170-240 kilograms and measure 2.1-2.4 meters, with the females slightly larger than the males (Gilmartin and Forcada 2002.) Females have two pairs of mammae (Reeves et al. 2002). Both genital organs and mammary glands are retracted under the skin (Berta 2002a).

The Hawaiian monk seal has large black eyes, which aid in underwater vision. Its head is broad, flat, and relatively small (Reeves et al. 2002). The snout is short and has smooth vibrissae on each side with nostrils on top of the snout rather than facing forward as in other phocids (fig. 1)(Berta and Sumich 1999). The nostrils are vertical slits when closed during diving. Monchus schauinslandi has eight pairs of teeth in both the upper and lower jaw (Reeves et al. 2002).

The two short fore flippers are positioned on each side of the body ventrally and the hind flippers are posterially orientated and are of intermediate length. All flippers have webbing between the digits. Each flipper has five digits with nails and is covered in hair (Reeves, Stewart and Leatherwood 1992). Pinnipeds have a short tail located between the two rear flippers (Berta 2002a).

Fig. 2 A Hawaiian monk seal rests on a Laysan beach amidst the garbage that typically litters the beaches of the Northwestern Hawaiian Islands. Note the lighter ventral and darker dorsal coloration of the species.

The short pelage of the adult is uniformly grayish after the yearly catastrophic molt and darkens to a dull dark brown on the back and yellowish color on the underside (fig. 2), with age these colors can darken. Some Hawaiian monk seals return from sea with a green or red tinge from algal growth (Reeves, Stewart and Leatherwood 1992; IMMA 1999). Pups are born with a woolly black coat (fig. 3), weighing 16-18kgs and are about 1 meter long (Reeves et al. 2002; IMMA 1999). They nurse for four to six weeks at which time they lose the black coat.

Pinnipeds exhibit a number of evolutionary adaptations that are advantageous to life in the marine environment and benthic foraging. A thick blubber layer provides insulation and the ability to shunt blood away from the extremities and control respiration and heart rate aids in deep diving (Hammill 2002). Phocids, including the Hawaiian monk seal, are more adapted to swimming than the otariids. This improved swimming ability and hydrodynamic body shape comes at the expense of mobility on land and increases vulnerability to terrestrial predators (Hammill 2002).

Fig. 3 A three to four week old Hawaiian monk seal pup on Sand Island, Midway Atoll. Note the distinctive black coat of the young M. schauinslandi. It will have this coat until it is weaned at about six weeks of age (Reeves, Stewart and Leatherwood 1992).

The Hawaiian monk seal is named for its solitary nature. Generally preferring to be alone, the closest social bond existing between mother and pup (Reeves, Stewart and Leatherwood 1992). Most of what is know about the natural history of M. schauinslandi has been learned during their time on land and little is know about their offshore habits (Bowen, Beck and Austin 2002; IFAW 2003).

Habitat and Home Range:
The Hawaiian monk seal lives in the tropical waters of the Central Pacific, primarily around the Northwestern Hawaiian Islands (NWHI). It uses the sandy beaches of the islands as haul-outs and for pupping. It forages among the nearshore coral reefs. Monachus schauinslandi must come ashore for birthing and the yearly molt (Berta 2002a). Only the Miroungas (elephant seals) and the Monachus experience a catastrophic molt in which large pieces of the upper layer of skin and fur are sloughed off. The rest of the pinnipeds molt individual hairs (Reeves, Stewart and Leatherwood 1992).

The largely uninhabited atolls of the Hawaiian Archipelago stretch 1,240 miles from the main Hawaiian Islands and are often referred to as the Leeward Chain (IFAW 2003). The chain includes the six major subpopulations sites: Kure Atoll, Midway Atoll, Pearl and Hermes Reef, Lisianski Island, Laysan Island and French Frigate Shoals (fig. 4).

Source: Marine Mammal Commission – Annual Report for 2000

Fig. 4 The Hawaiian Archipelago is the only place the Hawaiian Monk seal is found. The six Northwestern Hawaiian Islands (NWHI) underlined in red provide pupping beaches for the major breeding colonies of Hawaiian monk seals.

What historically were pristine white sand beaches and clear blue green waters with healthy reef systems, have been spoiled by such things as: tons of trash, plastics and discarded fishing nets. The seals must content with these foreign objects in their habitat (fig.2). According to the Marine Mammal Commission (2002), between 1998 and 2001 approximately 116,000 kg of net debris was removed from the islands and surrounding reefs and the following year another 118,000 kg of garbage was removed. Entanglement rates for Hawaiian Monk seals are the highest for any pinniped species (fig. 5). Garbage is an unfortunate component in the habitat of M. schauinslandi and will likely continue to be a significant threat for the species.

Most of the human inhabitation of the NWHI has been by the U.S. Military. In particular, Midway Atoll has large runways on both Eastern and Sand Island and had a military presence from the early 1950’s through the early 1990’s. Kenyon (1972) observed that monk seals avoid beaches frequented by humans on all the inhabited atolls. During the mid 1950’s when Kenyon (1980) was studying M. schauinslandi on the Midway Islands, he observed a small breeding population of about 70 seals on Eastern Island and no seals on Sand Island, which at that time was home to over 2,000 naval personnel. Since the Navy has departed Sand Island in the mid 1990’s and approach limits to the seals have been strictly enforced, the population has begun to recover (Reeves, et al. 2002)

Hawaiian monk seals are non-migratory and generally stay near the island where they are born. Only about 10 percent will move to another island during their lifetime (Gilmartin and Forcada 2002).

Source: Marine Mammal Commission – Annual Report for 2000

Fig. 5 The number of Hawaiian monk seals found entangled annually, 1982-2000. Additionally, 8 seals became entangled in 2001(MMC 2002). Entanglement by debris remains a significant hazard for the species.

Diet:
Hawaiian monk seals are opportunistic feeders and eat reef-dwelling fish, cephalopods and invertebrates including lobsters. Gilmartin and Forcada (2002) report M. schauinslandi has a prey base of at least 40 species. The NMFS (2002) state that four fisheries may interact with Hawaiian monk seals. After repeated urging by the Marine Mammal Commission and legal efforts by a consortium of environmental groups led by Earthjustice, the lobster fishery was closed in 2000 and remains closed (EarthJustice 2000). However, the longline, recreational, and parts of the bottom fisheries remain open and continued competition for M. schauinslandi prey (NMFS 2002).

Foraging dives are generally short, averaging four minutes or less and the species appears to forage mostly on the benthic coral reef environment at depths of 60 meters or less (Reeves et al. 2002). According to Bowen, Beck and Austin (1999), little is know about the foraging of pinnipeds. In the past, research on this subject has been limited to scat analysis, stomach contents of deceased animals, and observation of foraging dives. Technological improvements including data loggers, telemetry, and video footage are making it easier to measure what free ranging pinnipeds eat (Bowen, Beck and Austin 2002).

Mating and Reproduction:
Hawaiian monk seals mate in the water and have a serially monogamous breeding system (Reeves, et al. 2002). A male will often bite the female on the neck during the process. In some cases where the sex ratio has become skewed in favor of the males, the repeated biting can cause wounds or even death to the female. This behavior is known as mobbing and is thought to be a major impediment to recovery at Lisianski and Laysan Islands (NMFS 2002).

Pupping can occur year round, but most pups are born on beaches between December and August with an increase in births between March and April (IFAW 2003). The mother will not forage during the four to six weeks that she nurses the pup while the pup will gain approximately 1.4 kg a day from the high fat-content milk (Bowen, Beck and Austin 2002). After weaning, the mother will return to sea to forage for the first time since giving birth and the pup will then be on its own (Reeves, Stewart and Leatherwood 1992). Accidental exchanges between M. schauinslandi pups and their mothers, a behavior called alloparenting, can occur leading to serious consequences when the pups are of differing ages (Berta and Sumich 1999). A mother will lactate for only 4 to 6 weeks and if the pup switch results in her receiving a pup that is much younger than her own, the pup will be forced to stop suckling early, reducing its chance of survival (Gilmartin and Forcada 2002). The male has no role in raising the pup (Bowen, Beck and Austin 2002).

Hawaiian monk seals do not become sexually active until four or more years of age and give birth to one pup a year (IFAW 2003; Reeves et al. 2002). Estrus occurs postpartum, near the end of lactation. However, in all pinnipeds implantation is delayed for approximately three months to allow the mother to return to foraging and regain her strength. Fetus development is around nine months (Hammill 2002). Monachus schauinslandi can live as long as 30 years, but 20-25 years is more common (Reeves, Stewart and Leatherwood 1992).

Distribution:
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Except for the Indian Ocean, phocids are found throughout the world’s major oceans (Hammill 2003). There are 18 extant species of pinnipeds included in the family Phocidae. Five live exclusively in the southern hemisphere around Antarctica in the Southern Ocean; four live only in the North Pacific Ocean or the Bering Sea; three are restricted to the North Atlantic Ocean; one is widely distributed in coastal areas of the North Pacific and North Atlantic Oceans and is amphioceanic; another is found in the Mediterranean Sea with a range that extends into the North Atlantic Ocean; two are found primarily in the Arctic; and the last two have localized population in the Caspian Sea and Lake Bikal (Reeves et al. 2002).

Phocids are split into two or sometimes four subfamilies including the Monachinae, which is comprised of the southern seals (Berta 2002b). The Miroungas (elephant seals) include one species found in the North Pacific and another is found in the Southern Ocean. The Lobodontini (Antarctic seals) have a circumpolar distribution around Antarctica in the Southern Ocean. The genus Monachus consists of two extant monk seals species, M. schauinslandi and M. monachus found in subtropical and tropical waters (Forcada 2002; Reeves et al. 2002). A third member of this genus, M. tropicalis, has not been seen since 1952 and is now considered extinct (Reeves, Stewart and Leatherwood 1992; Reeves et al. 2002; MMC 2002; Gilmartin and Forcada 2002).

Previous distribution of M. tropicalis included the waters of the northern and western Caribbean Sea and the Gulf of Mexico with haul-out sites possibly restricted to the sandy beaches of remote atolls (Reeves, Stewart and Leatherwood 1992). Monachus monachus, survives in fragmented populations in the Mediterranean Sea and the southeastern North Atlantic. Possibly the world’s most endangered pinniped, the largest remaining colony (numbering 100 in 1997) of Mediterranean monk seals is found at the Cabo Blanco Peninsula on the coast of Western Sahara, Africa (Reeves et al. 2002).

Monachus schauinslandi is non-migratory with a continental slope and oceanic distribution in the central Pacific Ocean primarily around the NWHI (Forcada 2002; Gilmartin and Forcada 2002). Little is know about the habits of M. schauinslandi including its offshore distribution (Bowen, Beck and Austin; IMMA 1999).

There are also an estimated 100 animals living at Necker and Nihoa islands located in the middle of the archipelago and occasionally animals are seen at Johnston Atoll (NMFS 2002). Additionally, there are a small but increasing number of animals (including a breeding population) making their home on the sandy beaches and in the tropical waters around the main Hawaiian Islands (Gilmartin and Forcada 2002). According to the Marine Mammal Commission (2002), increasing populations on the main islands are promising for the recovery of the species. However, they also create new wildlife management challenges when monk seals haul out on popular swimming beaches (fig. 6 and 7). There is a risk of disease transmission to the Hawaiian monk seal population from feral animals and pets and there have even been a couple of reports of swimmers bitten by seals (MMC 2002).

Kenyon, a wildlife research biologist for the U.S. Fish and Wildlife Service, suspected M. schauinslandi must have lived on the main islands 2,500 years ago when the ancestors of the Hawaiian-Polynesian first settled there. Furthermore, he believes that the seals must have been quickly extirpated (Kenyon 1980). In the 2000 Annual Report to Congress, the Marine Mammal Commission (2001) concurred with this opinion. Hunted to near extinction by early settlers, sightings of M. schauinslandi were rare by the late 1800’s (IFAW 2003).

No one knows for sure how M. schauinslandi came to live in the Hawaiian Archipelago. Its closest extant taxonomic relative, M. monachus, lives on the opposite side of the world. LeDuc (2002) speculates that the spread of the monk seal to Hawaii must have occurred prior to the closing of the Central American seaway. The Isthmus of Panama has separated the Caribbean and Pacific basins for three million years.

	Fig. 6 Sign warning beachgoers to leave the monk seals alone. It is a violation of state and federal law to disturb the Hawaiian Monk seal, one of the most endangered marine mammals. Volunteers and wildlife management officials post signs like this one on beaches favored by humans when monk seals haul-out.


© Dominique Martel, photos used with permission
Fig. 7 Hawaiian monk seal on Kauai island beach. Increasingly monk seals are hauling out on beaches used by humans, despite a historic fear of people that has led them to abandon similar beaches in the past. This poses new wildlife management problems for popular beaches on the main islands (MMC 2002).

Population:
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The most current estimation of the Hawaiian monk seal population is 1,463 individuals. This estimate comes from the official stock status report released by the National Marine Fisheries Service (2002) and is the sum of the Northwestern Hawaiian Island (NWHI) subpopulations counts, an extrapolation of Necker and Nihoa Island counts, and counts on the main Hawaiian Islands.

Monk seals are very sensitive to human disturbance and this is reflected in the temporal patterns of the M. schauinslandi subpopulations in the NWHI (Kenyon 1972; Kenyon 1980; Schmelzer 2000; MMC 2001; MMC 2002). The most current report to the U.S. Congress by the Marine Mammal Commission states that:

Kenyon (1980), who spent years observing M. schauinslandi, wrote that the “mere presence of human intruders among these ‘tame’ seals appear to cause mortality among newborn pups” (48). The total population was reduced by 60 percent from the mid 1950’s through the early 1990’s (MMC 2001). Until the mid 1990’s the population recovery on the French Frigate Shoals seemed promising. However, since then the population has once again declined (MMC 2002; Schmelzer 2000). Monachus schauinslandi, and indeed, the entire genus Monachus remain perilously close to extinction.

Evolution:
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Phylogeny is the evolutionary history of a group of organisms (Berta and Sumich 1999). According to Berta and Sumich (1999) “the biogeographic history of Monachus is conflicting since there are differing hypotheses for its phylogenetic position relative to other phocids” (123). Because it is not understood how this genus relates to other members of the Phocidae family, there remains a great deal of conflicting thought on the evolution of the Hawaiian monk seal. Based on primitive and unspecialized skeletal and vascular anatomy, Gilmartin and Forcada (2002) state that M. schauinslandi is considered the “most primitive of living seals,” citing Repenning and Ray (1977). Additionally, they offer the hypothesize that the species originated in the North Atlantic during the middle Miocene epoch, 15 million years ago with M. schauinslandi likely descending from the extinct Caribbean species, M. tropicalis (fig. 8a) with M. monachus the basal taxon (Berta and Sumich 1999).

However, Berta and Sumich (1999) offer another hypothesis that concurs with molecular studies done by Arnason et al. (1995). It identifies M. schauinslandi as the sister taxon, or closest relative, to the other monk seals implying the converse of the previous idea and suggesting that M. tropicalis descended from M. schauinslandi or that M. tropicalis and M. monachus evolved concurrently (fig. 8b). Molecular sequencing data supports this theory (fig. 8c) but Monachus data is limited to the Hawaiian species and therefore leaves unanswered questions (Berta and Sumich 1999). Currently there are two alternative phylogenies for the phocids (fig.8) and this leaves open the question of how the Hawaiian monk seal evolved.

Source: Marine Mammal Evolutionary Biology (Berta and Sumich 1999)

Fig. 8 Alternative phylogenies for the Phocidae

Taking a step back and looking at the evolution of the Phocidae family, provides more unanswered questions but a generally agreed upon hypothesis. The debate here is whether pinniped evolution is diphyletic or monophyletic referring to whether the group is connected evolutionarily to one or two families – Ursidae and Mustelidae are the suggested families (Berta and Sumich 1999; Berta 2002b). In the monophyletic hypothesis, all pinnipeds are thought to descend from the arctoids including Mustelidae (fig. 9a). The alternate diphyletic hypotheses (fig. 9b) split the pinnipeds with Otariidae (sea lions and fur seals) and Odobenidae (walrus) descending from Ursidae and Phocidae (seals) descending from Mustelidae (Berta and Sumich 1999; Berta 2002b). Berta and Sumich (1999) state that pinniped monophyly is a “well-supported hypothesis based on both morphologic and molecular data” (45).

Questions remain about how the species M. schauinslandi, thought to be 15 million years old, came to inhabit the 6 million years old Hawaiian Islands with its closest extant taxonomic relative found only on the other side of the world in the North Atlantic and Mediterranean Sea (Berta and Sumich 1999). Did the species evolve in the Pacific or Atlantic and is it endemic to the Hawaiian Islands or did a founder population reach the islands at a later date? Is it the oldest or the youngest of the Monachus seals? What role did the Central American or Panamanian Seaway that existed between 12 and 3 million years ago play in the dispersal of the species?

The Central American Seaway is thought to be the “most likely” route taken by the monachine seals (Berta and Sumich 1999, 121) Based on acceptance of a monophyletic evolution, the species is hypothesized to have evolved in the Pacific and is the ancestor of the other two members of the genus Monachus. It remains unanswered when the Hawaiian Islands became their home and the literature does not offer conclusive evidence of fossil records. In many ways the evolution of the Hawaiian monk seal remains unresolved.

Source: Marine Mammal Evolutionary Biology (Berta and Sumich 1999)

Fig. 9 Two cladograms depicting alternative hypotheses for relationship among pinnipeds.
(A) Monophyly with Ursidae as the closest pinniped relatives.
(B) Diphyly in which phocids and mustelids are united as sister taxa as are otariids, odobenids and ursids
The monophyletic hypothesis is most widely accepted today (Berta and Sumich 1999).

Conservation Status:
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Considered the most endangered seal in U.S. waters, M. schauinslandi was listed in 1976 as depleted under the Marine Mammal Protection Act of 1972 and Endangered under the Endangered Species Act of 1973 (MMC 2001). In 2000, President Clinton established the Northwestern Hawaiian Islands Coral Reef Ecosystem Reserve that protects most of the M. schauinslandi habitat excluding Midway Atoll, which is protected under the Midway Islands National Wildlife Refuge. All federal waters within 50 nautical miles of major Hawaiian monk seal breeding colonies are protected and being considered for national marine sanctuary status (MMC 2002). Additionally, M. schauinslandi is listed as endangered in the International Union of the Conservation of Nature and Natural Resources (IUCN) Red List of Threatened Species and as Appendix I of the Convention of International Trade in Endangered Species (IMMA 1999).

Bibliography:
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Berta, Annalisa. 2002a. Pinnipedia, Overview in Encyclopedia of Marine Mammals, eds. Perrin, William F., Bernd Würsig, and J. G. M. Thewissen. Academic Press. San Diego, CA. 903-911.

Berta, Annalisa. 2002b. Pinniped Evolution in Encyclopedia of Marine Mammals, eds. Perrin, William F., Bernd Würsig, and J. G. M. Thewissen. Academic Press. San Diego, CA. 921-929.

Berta, Annalisa, and James L. Sumich. 1999. Marine Mammals: Evolutionary Biology. Academic Press. San Diego, CA.

Bowen, Don W., Carrie A. Beck, and Deborah A. Austin. 2002. Pinniped Ecology in Encyclopedia of Marine Mammals, eds. Perrin, William F., Bernd Würsig, and J. G. M. Thewissen. Academic Press. San Diego, CA. 911-920.

Earthjustice. 2000. Newsroom: Court rules fisheries service illegally failing to protect monk seals in Hawaii; injunction issued. [Online] Available: http://www.earthjustice.org/news/display.html?ID=77 (Nov. 15, 2003).

Forcada, Jaume. 2002. Distribution in Encyclopedia of Marine Mammals, eds. Perrin, William F., Bernd Würsig, and J. G. M. Thewissen. Academic Press. San Diego, CA. 327-333.

Gilmartin, William G., and Jaume Forcada. 2002. Monk Seals in Encyclopedia of Marine Mammals, eds. Perrin, William F., Bernd Würsig, and J. G. M. Thewissen. Academic Press. San Diego, CA. 756-759.

Hammill, Mike O. 2002. Earless Seals: Phocidae in Encyclopedia of Marine Mammals, eds. Perrin, William F., Bernd Würsig, and J. G. M. Thewissen. Academic Press. San Diego, CA. 352-358.

International Fund For Animal Welfare (IFAW). 2003. Hawaiian monk seal. [Online] Available: http://www.ifaw.org/ifaw/general/default.aspx?oid=13125# (Oct. 30, 2003).

International Marine Mammal Association (IMMA) 1999. Monachus Profiles: Hawaiian Monk Seal (Monachus schauinslandi). [Online] Available:http://www.monachus.org/profiles/hawaseal.htm (October 14, 2003).

Kenyon, Karl W. 1972. Man versus the monk seal. Journal of Mammology. 53(4):687-696.

Kenyon, Karl W. 1980. No Man is Benign – The endangered monk seal. Oceans. 13(3):48-54.

LeDuc, Rick. 2002. Biogeography in Encyclopedia of Marine Mammals, eds. Perrin, William F., Bernd Würsig, and J. G. M. Thewissen. Academic Press. San Diego, CA. 99-102.

Marine Mammal Commission (MMC). 2001. Annual Report to Congress 2000. Marine Mammal Commission. Bethesda, MD. [Online] Available:http://www.mmc.gov/reports/annual/ (Oct. 15, 2003).

Marine Mammal Commission (MMC). 2002. Annual Report to Congress 2002. Marine Mammal Commission. Bethesda, MD. [Online] Available: http://www.mmc.gov/reports/annual/ (Oct. 15, 2003).

NOAA National Marine Fisheries Service (NMFS). 2002. Hawaiian Monk Seal (Monachus schauinslandi) Stock Assessment Reports. NOAA Fisheries. Bethesda, MD. Oct. 31. [Online] Available: http://www.nmfs.noaa.gov/prot_res/PR2/Stock_Assessment_Program/individual_sars.html (Oct. 15, 2003).

Reeves, R.R., B.S. Stewart, P.J. Clapham, and J.A. Powell. 2002 National Audubon Society Guide to Marine Mammals of the World. Alfred A. Knopf. New York, NY.

Reeves, R.R., B.S. Stewart, and S. Leatherwood. 1992. The Sierra Club Handbook of Seals and Sirenians. Sierra Club Books. San Francisco, CA.

Repenning, C.A., and C.E. Ray. 1977. The origin of the Hawaiian monk seal. Proc. Biol. Soc. Wash. 89:667-688.

Schmelzer, Isabelle. 2000. Seals and Seascapes: Covariation in Hawaiian Monk Seal Populations and the Oceanic Landscape of the Hawaiian Archipelago. Journal of Biogeography. 27:901-914.

Introduction

Physical
Characteristic

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Taxonomic classification: Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Carnivora Family: Phocidae Subfamily: Monachinae Genus: Monachus Species: Monachus schauinslandi
	Photo © David B. Fleetham/Seapics.com, used with permission.
	Fig. 1 Hawaiian monk seal