Pacific Nurse Shark - Ginglymostoma unami

Basic Body Shape

The Pacific nurse shark has a robust, elongated body with a rounded cross-section. The body shape appears stocky and sturdy. This build is adapted to a demersal lifestyle and enables calm, energy-efficient movement close to the substrate. The torso transitions smoothly into a powerful caudal peduncle.

Head Structure and Snout

The head is broad and flattened. The snout ends bluntly just before the mouth. Two well-developed barbels are present on the underside of the snout. These serve as tactile sensors and aid in locating prey in the sediment. The nostrils are positioned far forward and are partially protected by skin folds.

Mouth Position and Jaw Structure

The mouth is distinctly subterminal. This position is typical for bottom-oriented sharks. The jaws are strong but not specialised for rapid biting movements. Instead, they allow controlled suction and gripping. The mouth opening is wide and extends laterally beyond the eyes.

Dentition Structure

The teeth are small, densely arranged and slightly curved. Each tooth crown ends bluntly and lacks sharp cutting edges. This dentition is not designed for slicing large prey but for gripping and holding. Tooth rows are regularly replaced to compensate for functional wear.

Eyes and Sensory Organs

The eyes are comparatively small and positioned laterally on the head. A nictitating membrane protects the eye surface. Vision plays a secondary role. Olfaction and electroreception are more significant. The ampullae of Lorenzini are distributed across the snout and enable the detection of weak electrical fields.

Fin Arrangement

The Pacific nurse shark has two dorsal fins, which are positioned far back. The first dorsal fin begins behind the pectoral fins. The second dorsal fin is slightly smaller but similarly shaped. The pectoral fins are broad, muscular and rounded. They serve as support when resting on the seafloor. The pelvic fins are positioned relatively far back and contribute to stable positioning.

Anal Fin and Caudal Fin

A well-defined anal fin is present. The caudal fin is asymmetrical with an elongated upper lobe. The lower lobe is reduced. This shape favours steady, slow swimming movements and is less suited to rapid acceleration.

Skin Structure and Scales

The skin is thick and covered with fine placoid scales. These are small, smooth and evenly distributed. The surface feels leathery and reduces drag during slow movement. The skin also provides high mechanical protection against abrasive substrates.

Colouration and Markings

The base colouration ranges from grey-brown to olive. The belly is lighter. Juveniles often display distinct dark spots or bands, which fade with age. The inconspicuous colouration aids camouflage on sandy or rocky substrates.

Internal Anatomical Features

The liver is large and rich in oily lipids. It significantly contributes to buoyancy and reduces the energy expenditure required for swimming. The gill openings are relatively small and located laterally behind the head. The digestive tract is short and robustly structured, suited to processing compact food.

Muscle Structure and Locomotor System

The musculature is designed for endurance rather than speed. The torso and tail feature evenly developed muscle strands. These enable smooth, undulating movements. Short bursts of speed are anatomically possible but are not a primary aspect of the locomotion strategy.

Adaptation to a Demersal Lifestyle

The entire body structure of the Pacific nurse shark is functionally adapted to a calm, demersal lifestyle. Flat body sections, robust fins and a specialised sensory system enable efficient orientation and feeding without high energy costs.

Geographical Distribution in the Eastern Pacific

The Pacific nurse shark is found exclusively in the eastern tropical Pacific. Its range extends along the Pacific coast of Central America. Records exist from Costa Rica, Panama and adjacent marine areas. The species also occurs around several oceanic island groups in this region. The distribution is clearly regionally limited and does not overlap with that of the Atlantic nurse shark.

Coastal Habitats

The preferred habitat lies in shallow coastal zones. The Pacific nurse shark is mostly found in bays, lagoons and sheltered coastal areas. It is particularly common in areas with low currents and structurally complex substrates. These habitats provide shelter, stable environmental conditions and suitable retreats.

Reef-Associated Habitats

Coral reefs and rocky reef structures are key habitats. The shark uses overhangs, crevices and caves as resting places. During the day, it often lies motionless on the seafloor or concealed beneath structures. Its close association with reef systems makes it vulnerable to habitat changes.

Substrates and Seafloor Types

The Pacific nurse shark prefers sandy, muddy or rocky substrates. Sightings frequently occur on sandy areas in close proximity to reefs. Its body structure allows it to rest directly on the substrate without needing to swim actively. Seagrass beds are also used, provided they offer sufficient cover.

Depth Distribution

The species primarily inhabits shallow depths. Most observations are from depths between one and thirty metres. Individuals have occasionally been recorded at greater depths, but the focus remains clearly on shallow coastal areas. Its association with well-lit zones is pronounced.

Site Fidelity and Home Range

The Pacific nurse shark exhibits high site fidelity. Individual animals use the same resting spots over extended periods. The daily home range is comparatively small. Long-distance migrations are rarely documented. This spatial fidelity makes local populations particularly vulnerable to human interference.

Environmental Conditions

The habitat is characterised by warm water temperatures. The species avoids cooler currents and prefers stable tropical conditions. Good water quality is crucial for the long-term survival of the species, as the shark is strongly dependent on intact coastal ecosystems.

Importance of Habitat for the Population

The narrow specialisation in coastal and reef-associated habitats limits the Pacific nurse shark’s ability to relocate. Changes caused by coastal development, sediment input or reef damage directly affect the usable habitats. The preservation of structurally complex shallow-water zones is therefore central to the long-term survival of the species.

Lifestyle

The Pacific nurse shark lives predominantly near the seabed and exhibits a distinctly calm activity pattern. During the day, it usually remains motionless on the seafloor, preferring sheltered areas such as rock crevices, reef overhangs, or caves. These resting phases can last several hours. Its body posture is flat and stable, often with propped-up pectoral fins.

The main activity period is during the evening and night hours. In this phase, the shark leaves its resting places and moves slowly across the seafloor. Movement is steady and energy-efficient. Rapid swimming is rarely observed. The species is site-faithful, with individual animals using the same retreats over long periods.

Social behaviour is limited. Occasionally, several animals rest close together without apparent interaction. Active group formation or coordinated behaviour has not been documented. Intraspecific aggression is rare.

Diet

Foraging primarily occurs at night. The Pacific nurse shark uses a combination of smell, tactile stimuli, and electroreception. The barbels on its snout play a central role in detecting prey in sediment or between rocks.

Feeding occurs near the seabed. Prey is sucked in or secured with a controlled jaw closure. The dentition is designed for gripping, not cutting. The diet consists mainly of benthic invertebrates and small vertebrates.

• Crabs and other crustaceans
• Molluscs such as squid
• Small benthic bony fish
• Occasionally sea urchins or other hard-shelled organisms

Food is usually swallowed whole. Larger prey is adjusted through repeated suction and repositioning in the mouth. Energy requirements are comparatively low, which aligns with its generally calm lifestyle.

Reproduction

The Pacific nurse shark is ovoviviparous. Embryos develop inside the female’s body from yolk-rich eggs. There is no placental connection. Reproduction likely follows fixed temporal cycles, though exact seasonal patterns are poorly documented.

Mating occurs near the seabed. The male secures the female by gripping her pectoral fin or body with its mouth. Copulation occurs via a clasper organ and can last several minutes. Bite injuries are possible but are considered a functional part of mating.

After a gestation period of several months, the female gives birth to live young. Litter size is limited, ranging from low single digits to low double digits. The young are fully developed and independent at birth.

Birthing sites are likely in protected shallow-water areas. Juveniles prefer very shallow zones, where they are protected from larger predators by low water depth and structural complexity. There is no parental care.

The slow reproductive rate and low number of offspring result in a low reproduction rate. Populations are therefore sensitive to increased mortality or habitat loss.

Current Threat Status

The Pacific nurse shark is considered a regionally threatened shark species. The main causes lie in direct and indirect human use of coastal habitats. Due to its benthic lifestyle and high site fidelity, the species is particularly vulnerable to local pressures.

A central risk is coastal fishing. The Pacific nurse shark regularly ends up as bycatch in gillnets, bottom trawls, and longlines. Due to its low escape activity, it often does not survive capture. Targeted commercial fishing is rarely documented, though individual animals are locally taken for meat or skin.

Additionally, the loss of suitable habitats directly affects populations. Coastal development, harbour facilities, tourism infrastructure, and sediment input lead to the destruction or fragmentation of reefs, mangroves, and lagoons. These areas serve as resting and breeding grounds for the Pacific nurse shark.

Further pressures arise from environmental pollution. Pollutants, microplastics, and organic waste accumulate in coastal zones, long-term affecting the health of individual animals and local populations.

Biological Factors of Vulnerability

The reproductive strategy of the Pacific nurse shark exacerbates its vulnerability. The species has a low reproduction rate. Litter sizes are limited, and sexual maturity is only reached after several years. Losses of adult animals can therefore only be slowly compensated.

High site fidelity means that local populations react in isolation. A decline in one area is not quickly offset by migration from other regions. As a result, regional populations can be severely depleted without this being immediately visible at a higher level.

Legal Protection Status

The Pacific nurse shark is classified as vulnerable by the International Union for Conservation of Nature. The assessment is based on a limited distribution range, ongoing habitat loss, and documented population declines.

There is no internationally uniform protection status. National protection regulations vary widely between the coastal states of the eastern Pacific. In several countries, species-specific legislation is lacking, so the shark is only indirectly covered by general fishing regulations.

Existing Conservation Measures

An effective tool for protecting the Pacific nurse shark is marine protected areas. In several coastal regions of Central America, known habitats of the species lie within designated protected areas, where fishing is either restricted or completely prohibited.

In some regions, shark fishing bans have been introduced that also include the Pacific nurse shark. However, these regulations are not comprehensive, and their enforcement is sometimes inadequate.

Research projects to record population sizes, site fidelity, and breeding grounds are increasingly providing reliable data. This information forms the basis for more targeted management plans and the designation of further protected areas.

Conservation Needs and Management Approaches

Long-term protection of the Pacific nurse shark requires the preservation of structurally rich coastal habitats, including coral reefs, mangroves, and shallow lagoons. Reducing destructive fishing methods and better control of bycatch are key measures.

Educating local fishing communities plays an important role. Since the Pacific nurse shark is not a commercially significant target species, conservation measures can often be implemented with minimal conflict when communities are appropriately involved.

Without targeted conservation strategies, the species remains at long-term risk due to its biological characteristics and limited distribution range. The preservation of local populations is crucial for the stability of the overall population.