Large diverse family of fish

Sharks or halu’u in CHamoru, and rays or hafula’ in CHamoru, belong to a family of fish referred to as Elasmobranchs. It is a very diverse group of fish that includes approximately 1,000 species. They are found in all of the world’s oceans and in many types of marine environments, from shallow sandy banks to 5,000 ft. deep in the open ocean. Elasmobranchs are characterized by having a skeleton comprised of cartilage (like human ears) instead of bone.

Also, instead of typical fish scales, these animals have a different skin covering called dermal denticles, or “skin teeth.” They have complex sensory systems and can detect electric pulses in the water using tiny gel-filled pores called Ampullae of Lorenzini.

Some are incredibly fast and feed on prey such as large sea lions. Others move slowly, filtering plankton from the water, and still others sit lazily in the sand sifting tiny shrimp and snails into their mouths.

Pose little threat to humans

While often perceived as dangerous, blood-thirsty monsters, these animals pose very little threat to humans. In fact, people are more likely to be killed by a vending machine than a shark.

However, humans are their biggest threat. Fishermen catch them accidentally or target them for their fins, leading to the decline of many species of elasmobranchs. Around Guam, there is a variety of species that inhabit the shallow lagoons, coral reefs, and deeper waters. Each is unique in its preferred habitat, behaviors, and choice of prey, yet encountering any of them leaves visitors with a unique a memorable experience.

Here are the most common elasmobranchs around Guam:

Black Tip Reef Shark (Carcharinus melanopterus)

Figure 1. Black Tip Reef Shark

Classification and distribution

Black tip reef sharks grow to a maximum of two meters long and are characterized by their yellow-brown color, blunt snout, and pronounced black tips on each of their fins. They live on coral reefs, reef flats, and drop-offs transitioning from reefs to open ocean. These sharks can be found year-round but often follow tides towards and away from shore. Most typically, they are solitary though are rarely found in small groups, especially when they are young.

Life history and behavior

Black tips are non-aggressive. They mainly eat fish but also consume invertebrates such as shrimp, squid, and muscles. This shark gives live-birth (referred to as vivipary) to two to four pups, each about 50 centimeters long, after a gestation period of eight to nine months, though gestation can be as long as 16 months.

Local significance

In some locations in the Pacific, this shark is fished for shark-fin soup and for meat due to its preference for nearshore habitats. However, in 2010, Guam banned the possession, sale, and distribution of any shark products or parts, protecting this species. Now, this species can be seen by divers and snorkelers exploring Guam’s reefs without the threat of being hunted.

Tawney Nurse Shark (Nebrius ferrugineus)

Figure 2. Tawney Nurse Shark

Classification and distribution

Tawney Nurse Sharks can grow to be 3.2 meters long, though are more commonly around 2.5 meters from their nose to the end of the tail. They are brown with angled fins and a relatively short tail. Nurse sharks live in lagoons, on reefs, and at the outer edges of reefs, preferring to hide in crevices and caves. They are often found lying stationary on the sand or in small caves, and use a feature called a spiracle to pump water over their gills so that they don’t have to continuously swim. Typically, they are solitary.

Life history

These sharks are extremely docile. Their diet consists of small fishes and invertebrates found near the bottom of the water column, where the shark lies. Nurse sharks exhibit ovovivipary which is a form of reproduction in which eggs develop in the uterus of the female shark until they hatch. Litters of approximately 8 pups are typical and gestation time is unclear, though some researchers suggest 6 months.

Local significance

This species is considered threatened because of its behaviors and habitats. Its inshore preference makes it vulnerable to decreases in prey availability due to fishing pressure. Most importantly, because of this shark’s calm disposition and docile attitude, it can be negatively affected by divers and snorkelers who crowd, touch, or harass this animal. Fishing of this animal is banned by Guam’s laws.

White tip reef shark (Triaenodon obesus)

Figure 3: White Tip Reef Shark. Photo Credit: Dave Burdick

Classification and distribution

White tip reef sharks can grow to a maximum of 2.3 meters but are commonly 1.6 meters long. They are a slender, grey shark with a short, broad snout, and white tips on its dorsal and caudal fins (up-facing fins). These sharks prefer lagoons and outer reefs though are often found resting on ledges and in caves utilizing spiracles, just like the nurse shark, to pump water over their gills.

Life history

These sharks are sluggish and calm. Their diet consists mainly of fishes, octopuses, lobsters, crabs and other animals living on the ocean floor. They give live birth (vivipary) to 2-5 pups after a gestation period of five months or more and can live to be about 16 years old.

Local significance

This species could be considered threatened because of overfishing of its prey species. Because they are close to shore and loyal to relatively small locations, they are vulnerable to population declines to do limited prey supply, destruction of inshore reefs, and harassment by tourists. However, they are protected by Guam’s laws, as are other sharks.

Whale Shark (Rhincodon typus)

Figure 4: Whale Shark. Photo Credit: Heather Peterson

Classification and distribution

Is it a whale or a shark? The whale shark is the largest fish in our ocean. Since all sharks are fish and all whales are mammals, that makes the whale shark a fish. It can grow to be approximately 18 meters long and weigh 50,000 pounds. They are easily recognizable by identifying white spots in between pale white stripes which cover the entire body. They can occasionally be spotted in the deep waters outside of Guam’s reefs, most commonly around the southern end of the island and near Blue Hole off of Orote Point.

Life history

This gentle giant only feeds on plankton (tiny animals such as krill or fish larvae), by filtering them from the water. They are ovoviviparous (like the nurse shark) though not much else is known about their reproductive process. However, it is estimated that they live to be 80 years old or longer.

Local significance

It is rare to see these sharks on Guam. However, local residents and visitors alike occasionally do spot one passing by near Blue Hole. Whale sharks are considered very valuable for their popularity in ecotourism though they are classified as endangered because of their migratory nature and overfishing in other parts of the world.

Manta Ray (Mobula alfredi)

Figure 5: Mobula alfredi (Reef Manta Ray). Photo Credit: Victor Snyder

Classification and distribution

Manta rays can grow to have a wingspan of 8.8 meters and body length of 7 meters, making it the world’s largest ray. They have a broad head with fins projecting forward on either side of their mouths, called cephalic fins. Mantas can be found on the surface of open water all the way down to 120 meters, and prefer coastal areas. While Guam has a resident population year-round, they are most commonly spotted between late February and early May, when mating occurs. For the best chance of spotting the Manta, snorkel near dawn or dusk.

Life history

Despite its size, the Manta feeds on plankton by filtering them through gill rakers. Mantas exhibit ovovivipary with a litter size of 1-2, though not much else is known about their reproductive biology. During mating months, it is possible to see groups of Mantas congregating and even swimming in a funnel-shaped pattern as they try to attract mates. Their lifespan is estimated to be over 40 years.

Local significance

Manta sightings on Guam are rare despite the knowledge that there is a resident population of these animals. For some, they are a symbol of conservation in Micronesia.

Ocellated Eagle Ray (Aetobatus ocellatus)

Figure 6: Ocellated Eagle Ray. Photo Credit: Whitney Hoot

Classification and distribution

The Ocellated Eagle Ray is a large pelagic ray (meaning that it swims higher in the water column instead of along the ocean floor) that is found in coastal habitats. Fully grown, they typically have a wingspan of 1.6 meters, though wing spans of 3.3 meters have been observed. These rays are characterized by their dark backs with distinct white spots and a white underside. While sometimes spotted in Cocos Lagoon or within reefs, they are most commonly found around Blue Hole alone or occasionally in schools.

Life history

These rays pose no risk to humans and spotting one gliding through the water is certainly a remarkable sight. Eagle rays feed on small fishes, crustaceans (such as shrimp and crabs), octopuses, worms, and other mollusks (such as mussels and snails). They exhibit ovoviviparity, giving birth to 1-4 pups after a 12-month gestation period, and live to be about 12 years old.

Local significance

The Ocellated Eagle Ray, which is found in the Indo-West Pacific, has recently been given a species classification of its own after being distinguished from the White-spotted Eagle Ray (Aetobatis narinari), which is now only found in the Atlantic.

About the author

Christy Starsinic was a graduate student at the University of Guam Marine Lab. She studied the resilience of coral reefs against stressors throughout Micronesia.

Editor’s note:This author was beginning graduate student in 2018 taking a course in scientific writing at the University of Guam. This article was assigned to provide the student with practice in communicating science to non-scientists. The student chose the topic which is related either to their thesis project or work experience. The instructor in the course is Dr. Laurie Raymundo, a UOG Marine Laboratory faculty member.

For further reading

Froese, R., and D. Pauly, eds. “Search FishBase.” Fishbase. Last modified 21 February 2020.

Mahr, Krista. “Guam Now One of the Shark-Friendliest Places on Earth.” Time USA, 25 February 2011.

National Oceanic and Atmospheric Administration Fisheries. “Giant Manta Ray.” Species Directory.

University of Guam Marine Laboratory. The Inshore Sharks of Guam: Methods of Small-Boat Shark Fishing. By Patrick G. Bryan. Technical Report 4: 1-26. Mangilao: UOGML, 1972.