Sharks inhabit every ocean on the planet; this is a testament to both the adaptability and the variety found amongst shark species as the oceans themselves provide very diverse conditions. Constant variation in water temperature, concentrations of dissolved oxygen and salts, light levels and movement of water masses are all encountered periodically by sharks.
Benthic and Pelagic Environments
The benthic (seabed) habitat has a varied topography from flat and gentle undulations to seamounts, sheer drop-offs and canyons. The seabed itself can consist of algae, mud, sand, gravel, pebbles and large rocks. The pelagic (open water) habitat can be categorised vertically by depth and light abundance and horizontally by latitude.
1. The Photic (Epipelagic) Zone extends from the surface down to 200m, within this zone the ocean is relatively bright and sunlit allowing photosynthesis to occur. This results in a plentiful supply of algae which underpins the oceanic food chains and sustains the high biodiversity of this zone. The top surface of the water is largely influenced by changing weather patterns and subsequent temperature fluctuation.
2. The Mesopelagic zone ranging from 200-1000m is the start of the aphotic zone where animals either; vertically migrate for their food or they rely on material sinking from above, or an alternative energy source such as hydrothermal vents. This region is occupied predominantly by teleosts, cephalopods and sharks, animals inhabiting these depths usually have larger eyes to capture the limited supply of light levels.
3. Below the thermocline of 10-12°C (700-1000m) are 3 main zones; the bathypelagic (700-4000m), the abyssopelagic (4000-6000m), and the hadalpelagic (6000-11033m). The water pressure is around 69 bar (700m) to 1082 bar (11033m) respectively (sea surface equals 1 bar), animals found at these depths have unique physiological adaptations which allow them to survive at such high pressures. The deepest dwelling shark recorded to date is the Portuguese dogfish (Centroscymnus coeldepis) at 3700m. Temperatures and currents are less varied in these zones. Despite these depths constituting the largest area of the marine environment, very little has been explored.
Generally speaking most shark species inhabit temperate or tropical seas rather than the polar seas, however some utilize a wider range of water temperature’s during either horizontal or vertical migration (e.g.: the white shark has been found to occupy a temperature range of 3.4-24°C ).
The tropical seas are the warmest of all the oceans typically exceeding 20°C and remaining relatively constant year round. The tropical waters are fundamental in regulating the earth’s climate and large scale weather patterns with the heating of the sea surface (the sun is directly overhead at least once during the year) being drastically different from the polar seas, resulting in heat driven convection currents in the atmosphere and oceans. Vertical circulation of heat and nutrients also occur during downwelling’s and upwelling’s allowing deep ocean mixing to take place.
Many of the sleek predatory sharks thrive in the tropics such as the oceanic whitetip (Carcharhinus longimanus) and the blue shark (Prionace glauca) found in all of the tropical oceans (Atlantic, Indian and Pacific).
Temperate waters vary in accordance with latitude and season however the averaged water temperatures are between 10-21°C. Mako sharks (Isurus sp.) are quite often found in temperate regions.
The polar sea temperatures range between <10 to -2°C depending upon latitude and time of year. The colder water tends to occur at the surface (coldest deep water recorded at 0°C) because colder water is more dense it will sink and thus plays a large part in driving deep ocean mixing. The Greenland shark (Somniosus microcephalus) is known to inhabit Arctic and SubArctic waters with a temperature range of 7 down to -2°C.
Proximity to Shore
The pelagic zone can be split into two subregion’s the neritic zone and the oceanic zone. The neritic zone encompasses the water mass directly above the continental shelves, while the oceanic zone includes all the completely open water. In contrast the littoral or intertidal zone covers the region between low and high tide.
The classification of shark species in terms of habitat in relation to landmass can be considered subjective. This is largely as a result of human occupancy or research having been conducted reasonably close to shore and hence any shark encounters within these areas have been recorded. Just because a shark hasn’t been seen in a particular place does not definitely mean it isn’t there.
A classic example of this is the white shark (Carcharodon carcharias) considered (the females in particular) to be a coastal water species. However in November 2003 a female white shark was tagged off the coast of South Africa, she then made a transoceanic migration to the North East coast of Australia and then migrated all the way back to the site of tagging, resulting in a round trip of 20,000km in just under 9 months!
The use of specific habitats
Quite often specific habitats are associated with the particular behaviour of a shark species.
Lemon sharks (Negaprion brevirostris) give birth in shallow, mangrove fringed, grass flats. The juveniles are highly site attached and aggregate into groups within these “nursery” areas. When they reach maturity at around 12 years of age they leave the shallow flats and migrate to deeper water. Reproductive females show strong philopatry to nursery grounds, returning to these sites to give birth.
Juvenile Caribbean reef sharks (Carcharhinus perezi) aggregate at coral reef based cleaning stations where they adopt a position that shows they pose no threat and want to be cleaned. Specific fish species such as the Yellownose Goby (Elacatinus randalli) then remove parasites from the skin, mouth and gills.
The abundance of a food source in an area of ocean often dictates the migration patterns and habitat use of sharks. Basking sharks (Cetorhinus maximus) for instance are found in UK waters most notably from the latitudes of the Southern tip of Cornwall to the Shetland Isles from late March through to October. They are usually seen to be feeding on the zooplankton whose presence is influenced by the Gulf Stream. There is also the classic example of the abundance of white sharks (Carcharodon carcharias) increasing notably during the seal pupping season, with the sharks patrolling areas where the young seals first venture out to sea.
Diel (daily) rhythms have been described in several shark species. For example the megamouth shark (Megachasma peliogios) shows diel vertical migration with a proximity to the sea surface during the night following an upward migration at dusk and downward migration at dawn, with light levels providing the sensory cue for the sharks ascent and descent. Horn sharks (Heterodontus francisci) exhibit diel activity patterns with increased activity at night (most likely in association with foraging/feeding behaviour) with sharks in the daytime found sheltering under rocky overhangs and crevices.
Thus rhythmic activity by sharks also results in variability of habitat use on a regular basis, showing that not all migrations are undertaken across large time scales such as seasonal or annual migrations.