Both seas and oceans are, without a doubt, one of the sources richest in biodiversity on the planet Earth. Its interior houses countless hosts that make it a fascinating place. Hosts that vary markedly in their shape, size, color, habits, reproductive strategies, and ways of feeding.
Obviously, aquatic ecosystems are very different from each other. Their characteristics can vary greatly, which has a very specific impact on their capacity to be inhabited o no.
Logically, it is not the same to live in shallow waters or near the coast. There, the light is more abundant, the temperature fluctuates more, and currents and water movements are more frequent and sometimes dangerous. However, as we descend into the depths, we encounter a totally different panorama: darkness, high pressure, food shortages, and thermal stability. For this reason, living beings vary greatly depending on the area of ​​the ocean or sea in which they live.
This is where two key words appear: pelagic y benthic.
Pelagic and benthic
Pelagic refers to the part of the ocean that is above the pelagic zone. That is, to the water column which is not in contact with the bottom and extends from the surface into the sea, either on the continental shelf or in the open sea. Benthic, on the other hand, is the opposite: it is related to everything linked to the sea and ocean floor, including the surface sediment and the layers immediately below.
Broadly speaking, aquatic living beings, among which are fish, are divided into two large families: pelagic organisms y benthic organisms.
Definition of pelagic organisms
When we talk about pelagic organisms, we refer to all those species that live in middle waters of the oceans and seas, or near the surface. Therefore, it is clear that these types of aquatic life limit contact with the bottom, although some species travel to considerable depths in their life cycle.
They are distributed in well-lit spaces that can range from the surface itself to 200 meters deep. This layer is known as photic zone (or euphotic), where light allows photosynthesis. Below, the light and temperature gradient changes markedly.
To better understand the pelagic domain, it is usually subdivided bathymetrically into zones with distinct environmental conditions:
- Epipelagic zone (0–200 m): illuminated, with strong primary productivity and seasonal temperature variations. It is the most populated region.
- Mesopelagic zone (200–1.000 m): Very low light; pronounced thermal transition. Strategies such as daily vertical migrations and bioluminescence.
- Bathypelagic zone (1.000–3.000 m): permanent darkness, cold temperatures and high pressure.
- Abyssopelagic and hadopelagic zone (> 3.000 m): extreme depths, high pressures and stable, low temperatures.
In the last three zones, absolute darkness, very high pressures and constant temperatures predominate; therefore, both the diversity and biomass They usually decrease drastically with respect to the surface layer.
From a morphological and functional point of view, many epipelagic species have a body fusiform and hydrodynamic, powerful muscles and fins capable of sustaining high speeds. The typical coloration is a dorsal-ventral contrast (blue-green or dark above and silver/white on the flanks and belly), which favors camouflage seen from above and below. Some very active species show red musculature and regional thermoregulationIn extreme cases, the swim bladder may be absent, necessitating continuous swimming (e.g., certain tuna) and ventilation by ram-ventilation in the case of some elasmobranchs.
It should be noted that a great enemy of many of these organisms is the indiscriminate fishing, which reduces populations and alters food webs.
There are three main groups of pelagic organisms according to their ability to move and their relationship with the surface: nekton, plankton and neuston.
necton
It is home to fish, turtles, cetaceans, cephalopods, and others. These are organisms that, thanks to their movements, can counteract ocean currents and actively move in search of food, reproduction or migratory routes.
Plankton
It is characterized by having small, sometimes microscopic dimensions. It can be of plant type (phytoplankton) or animal (zooplankton). Due to its anatomy and buoyancy, the currents do not overcome and are dragged by them, although they have very efficient vertical movements and flotation strategies.
neuston
They are those living beings that inhabit the surface microfilm of water (the air-water interface), where they take advantage of the unique resources of that environment.
Pelagic fish
If we focus on the group that comprises pelagic fish as such, we can make another subdivision that depends on the aquatic zones they inhabit:
Coastal pelagics
Coastal pelagic organisms are usually fishes of small size that live in large schools that move around the continental shelf and near the surface. Examples of this are animals such as the anchovy, sardine’s most emblematic landmarks, the anchovy’s most emblematic landmarks, the mackerel and mackerelThey tend to be gregarious species, with rapid life cycles, great trophic plasticity and strong dependence on productivity pulses.
Oceanic pelagics
Within this group are species of medium and large size that they usually do migrations powerful. They share anatomical features with their coastal relatives, but differ in their eating patterns and in the spatial scale of their movements. Despite having rapid growth and high fertility, the density of their populations is lower, and its recovery is slower, largely due to the mass fishing. Fish like the tuna and the nice They are typical specimens of oceanic pelagic organisms; species such as the melva and little tunny in some regions.
Synonym for pelagic organisms
Since the term "pelagic" describes the life in the water column, there is no strict synonym that completely replaces it. Sometimes related terms are used, such as "oceanic» (in open sea) or «neritic» (on the continental shelf). It is important to clarify that «abyssal» is not a synonym for pelagic; abyssal refers to a depth specific within the ocean and can refer to both waters of the abyssal zone and the abyssal floor, so its use as a synonym is incorrect.
Definition of benthic organisms
Benthic organisms are those that cohabit in the aquatic ecosystems background, unlike pelagic organisms. They include both those that live on the substrate (epifauna) like those who inhabit it below (infauna). In shallow environments where some light still reaches, benthic primary producers appear photosynthesizers (macroalgae, seagrasses and phytobenthos microalgae).
Already immersed in the aphotic background, lacking light and located at great depths, are the consumer organisms that depend on the organic remains and the microorganisms that gravity drags from surface levels. A peculiar case is bacteria chemosynthesizers and symbiotic, which thrive at sites such as hydrothermal vents at mid-ocean ridges, supporting complex communities without the need for light.
To order them spatially, the benthic domain is usually divided into:
- Coastal zone: coastal area affected by tides; communities adapted to exposure/re-emergence.
- Sublittoral zone: from the lower limit of low tide to the edge of the Continental platform.
- Bathyal zone: slope of the continental slope; decrease in light and temperature changes.
- Abyssal zone: extensive deep plains; stable cold and high pressures.
- Hadal zone: deepest ocean trenches; extreme conditions.
At first glance, it might seem that benthic creatures are less familiar to us, but nothing could be further from the truth. There is a very famous group associated with them: the coralsCoral reefs are one of nature's jewels, although they are also among the most threatened by practices such as trawl nets and other human impacts.
Many other living beings are part of the great benthic family, such as echinoderms (stars and sea urchins), the Pleuronectiformes (sole and related species), cephalopods (octopuses and cuttlefish), bivalve y molluscs various, in addition to numerous seaweed and seagrasses.
Benthic fish
As previously mentioned, within the benthic organisms we find those fish belonging to the order Pleuronectiformes, which includes plaice, roosterfish and sole.
These fish are characterized by a very particular morphology. Their body, considerably compressed laterally to form a flattened shape, does not leave anyone indifferent. From fry, they possess bilateral symmetry, with one eye on each side; as they develop, one of the eyes migrates to the other side. The adults, which rest on their sides, have a flat body and eyes on the upper side.
As a rule, they are carnivores and predators that capture prey by means of stalking hunt, camouflaged on the substrate. The best-known species in gastronomy and fishing are the sole and the turbot. To these are added other charismatic benthic species with soft or rocky bottoms, such as different monkfish and certain stripes, which, depending on their ecology, can be considered benthic or demersal.
Demersal fish (intermediate between pelagic and benthic)
Fish demersal experi near the bottom of the littoral, eulittoral and continental shelf zones, usually reaching depths of up to a few hundred meters. They remain in the strata close to the substrate, with moderate movements on it, and can perform migration movements according to their life cycle or their nutritional needs.
Among the best-known demersals are the hake, blue whiting and the red mullet, among others. Although they are not strictly benthic (they do not spend their entire life in contact with the bottom), they share with the benthos certain trophic affinity and adaptations to take advantage of the resources of that stratum.
Patterns of diversity and biomass: a comparative note
In general terms, the pelagic domain contains a fewer species than the benthic, but concentrates a very high number of individuals, especially in the epipelagic zone. For example, in seas such as the Mediterranean, it is estimated that, although a high percentage of the known species are benthic, a significant fraction of the total weight of the catch comes from pelagic organisms. This contrast reflects the enormous productivity of the surface layers and the key role of the small pelagic fish in food webs and fisheries.
Key adaptations: pelagic vs. benthic
To survive in such different environments, species have developed differentiating traits:
- Pelagic: bodies hydrodynamic, countershaded (dark back, light belly), schools for defense and feeding, high swimming efficiency, developed swim bladder (or adaptive loss in persistent swimmers), migrations and, in mid-water, bioluminescence and vertical migrations.
- Benthic: camouflage and cryptic coloration, flattened bodies or with structures for attachment to the substrate, reduction or absence of swim bladder, territorial or solitary habits, feeding by lurking predation or filtration, intensive use of the microhabitat.
Human impacts and conservation
In both the pelagic and benthic domains, human pressure exerts significant effects. overfishing on pelagic fish (especially small ones, the base of many food chains) alters the population structure and resilience of ecosystems. In the benthos, techniques such as bottom trawl remove sediment, damage fragile habitats—including coral reefs—and reduce biodiversity. Adaptive management, seasonal closures, marine protected areas, and selective fishing gear are all essential tools for sustainability.
Quick glossary and terminology notes
- Pelagic: life in the water column, far from permanent contact with the bottom.
- Benthic (benthos/benthic): bottom-associated life (on or within the substrate).
- Epipelagic/mesopelagic/batypelagic/abyssopelagic/hadopelagic: depth zones within the pelagic domain.
- Littoral/sublittoral/bathyal/abyssal/hadal: areas of the benthic domain according to depth and position.
- Nekton: active swimmers; plankton: life adrift; neuston: superficial microlayer.
- Demersal: species that live near the bottom but not strictly in it.
Nature is a fascinating world, and aquatic ecosystems deserve a chapter of their own. Discussing pelagic and benthic organisms ranges from the changing light of the epipelagic to the stillness of the abyss, and from seagrass meadows that capture carbon to flatfish that lurk invisibly above the sediment. Understanding their differences, Zones, adaptations and relationships with fisheries allows us to better appreciate how life is organized in the ocean and why its protection is key to the balance of the planet.