Benthic & Pelagic Fishes: Defining Oceanic Lifestyles

Describing pelagic fishes is easy: They swim, feed and just hang out in the open ocean, a pretty consistent pattern across many ocean-going species. Describing benthic fishes is something else. They live at the bottom of the ocean but they go about their lives in a bunch of differing ways – above, on and sometimes in the seafloor. Some may engage in all three approaches.

yellowhead jawfish hovering
A true benthic fish, a friendly yellowhead jawfish risks emerging from its burrow to pose for a headshot.

IF TOLSTOY HAD WRITTEN ABOUT MARINE LIFE, he might have said that all pelagic fishes are alike but each benthic fish is benthic in its own way.

  • Pelagic fishes have one immediate imperative: To swim fast in a straight line through featureless waters, feeding on other organisms on the go. Think: tunas, mackerels, wahoos, manta rays, whale sharks, blue sharks and great white sharks.
  • Benthic fishes, especially reef fishes, live in complex landscapes with confined spaces, bulky coral shapes with overhangs, ledges and lots – lots! – of crevices, cracks and holes, sponges, sea fans and other stuff. In this environment, each species must find its own niche. Think: All of the reef fishes you know, from flounders to barracudas.

To be clear, for easier discussion this post focuses on fishes – and particularly on reef fishes. But these terms apply to invertebrates as well – pelagic organisms like copepods, krill and bacteria, benthic ones like corals, sponges, crabs, clams, featherstars – and more copepods.

BONUS WORD FACTOIDS

  • “Demersal” is another word for benthic, although it’s probably used more in reference to fishes targeted in commercial fisheries. Bottom-feeding fishes are often termed “groundfish.”
  • “Bottom” in this context covers not just the sandy seafloor but substrates like coral structures, sponges, wrecks and other stuff that sticks up above the seafloor – and upon which stuff lives.

DEFINING PELAGIC FISHES

yellowfin tuna swimming
Yellowfin Tunas: Definitively Pelagic fish

Pelagic fishes are the guys that race compulsively through the open ocean, away from land. Biologists describe them into two categories:

  • “Oceanic pelagics” are fishes like the aforementioned tunas and wahoos that swim their way across the briny deep, outside the limits of the continental shelves – that is, in waters with depths of more than 660 ft./200 m.
  • “Coastal pelagics” are fishes like cods, flatfishes, sardines and anchovies, bar jacks and spadefishes, that are likely to reside within the reach of the continental shelves.

PELAGIC FISH DESIGN

  • Pelagic fish mostly follow a common body pattern ideal for fast, straight-line swimming: streamlined torpedo-shapes, silvery upper body colors, white underbody colors, smooth skin, forked tails. Fishes like whales sharks and manta rays, of course, are prominent exceptions.
  • Plenty of pelagic fishes are sometimes-visitors to the reefs – oceanic voyagers like those whale sharks and manta rays, coastal pelagics like horse-eye jacks, bar jacks, spadefishes, boga, blue runners.

DEFINING BENTHIC FISHES

benthic fish - scorpionfish on coral
As benthic ambush experts, scorpionfishes rely on colors and fleshy body parts to blend into the reef while they wait for unsuspecting prey to come within range.

Benthic refers to fishes and invertebrates that actually live at the seafloor – above it, on it or in it. They can be small, slender jawfishes and shrimp gobies that live in burrows in the sediment, pancake-flat flounders that patrol the seafloor, blue and brown chromis that station themselves above the corals.

Size, shape, fin anatomy and other body aspects play into reef fishes’ ability to maneuver safely in this environment, to dart into crevices, hover amidst the branches of rope sponges or find cover among sea plumes.

As examples, the flat bodies of flounders and stingrays minimize their profiles and aid them in burying themselves under the sand when they need to go into hiding. The fleshy appendages of scorpionfishes help them disappear into their surroundings. The more torpedo-shaped bodies and fins of grunts, snappers, damselfishes, chromis and wrasses give them the maneuverability to position themselves for feeding and to retreat into safe places and hide behind coral heads when needed.

TRUE BENTHICS & BENTHOPELAGICS

Naturalists categorize benthic fishes into two types:

  • “True benthics” are the fishes that live on or in the seafloor.
  • “Benthopelagics” are fishes that swim freely in the water column as part of the reef habitat.

True benthics, whose lifestyle plants them on and in the seafloor, have denser body masses. As larvae concerned with surviving as plankton in the water column, they tend to have swim bladders that assist them with buoyancy. But, as they morph into adults, the bladders disappear.

Benthopelagics maintain functional (and useful) swim bladders. Actually, most reef fish are benthopelagic. While they may appear to be free swimmers, their lives are tied to the reef habitat, and, once settled as larvae, many of them live their lives in surprisingly small territories.

 A TRUE BENTHIC FISH SAMPLER

TRUE BENTHIC FISH STRATEGIES INCLUDE:

  • Burrowers: Fishes like garden eels, yellowhead jawfishes, shrimp gobies, dragonets and pikeblennies occupy spaces in the sediment from which they can safely extend themselves to capture plankton moving past in the currents. Garden eels excavate their burrows with tail action. Shrimp gobies have shrimp roommates to help dig and maintain their shared abodes.
  • Hole-in-the-Wallers: Fishes like roughhead and spiny blennies, find abandoned wormholes in the coral to occupy as their safe refuges. Toadfishes back their way into crevices or holes, facing themselves at the entrances to croak from a safe refuge. Spotted drums and other fishy friends are often sighted keeping within protected areas in the coral.
  • Ambushers: Flounders, sand diver lizardfishes and scorpionfishes are among the sneaks that lie quietly on the seafloor waiting to strike unwary passing prey. Scorpions rely on blending in. Flounders, masters at blending in, also take to burying themselves in the sand, sometimes with only their eyes showing. Sand divers sometimes bury themselves with only their heads visible.
  • Hiders: Stingrays bury themselves in the sand for concealment, but as carnivores committed to crunching on mollusks and crustaceans they find in the sediment, they’re hardly ambush predators.

A BENTHOPELAGIC FISH SAMPLER

BENTHOPELAGIC FISH STRATEGIES INCLUDE:

  • Slow swimming: Piscivores like Nassau groupers, jacks, snappers and Spanish wrasses stealthily move slowly in the water column to stay alert for victims – and strike fast. Barracudas patrol slowly, and also hover in place waiting for a chance to shoot like a bullet at vulnerable prey.
  • Hovering: Plantivores like blue and brown chromis hover in the currents above coral heads to sweep in passing plankton.
  • Speedy swimming: Creole wrasses and boga swim in schools along the reef grabbing plankton as they progress. Eagle rays periodically drop to the sandy bottom to scrounge for benthic crustaceans, worms and mollusks.
  • Hanging out: Fishes like bicolor damsels, fairy basslets, butterflyfishes, sergeantfishes, tangs, filefishes and bluehead wrasses motor around coral heads looking for their preferred delectibles. Herbivores like three-spot damselfishes actively cultivate little gardens of microalgae, hanging in the water around coral heads – and expending much time and effort warding off raiders.
trunkfish feeding in sandy bottom
This short Poseidon’s Web video shows the diverse feeding strategies of several bottom-feeding benthopelagic fishes, including trunkfish, southern stingrays, goatfish and spotted eagle rays. Click on the screenshot to play.

MIXED STRATGIES

  • Grunts, eagle rays and trunkfishes are benthopelagics that hang above the reef, not on it, but earn their livings as bottom-feeders scrounging in the sediment for worms, small mollusks and copepods and other crustaceans.
  • Small groupers like tigers and yellowfins, red hinds and coneys swim above the bottom but often can be found resting on corals and sponges.
  • Seahorses go a step farther and rather than “rest” simply anchor themselves to a sea rod, sponge or other handy substrate and wait for prey in the form of planktonic copepods to come to them. This may seem dumb but they are in fact highly effective predators.
  • Flying gurnards and sea robins can swim but often “walk” around on the seafloor as they forage.

FOR THE ETYMOLOGICALLY OBSESSED:

  • “Pelagic” and “Benthic” are adjectives with no nouns in sight. They’re properly used attached to other words, as in “pelagic fish” and “benthic organisms.”
  • This post uses “pelagics” and “benthics” as shorthand, since the whole point is the creatures that make up each part of the ocean. But they’re not in the dictionary.
  • Benthic does have a noun in “benthos,” which means the biogeographical zone of the seabottom and/or the organisms that live there. I use the terms benthic and benthos to refer to the ocean bottom but they’re also used in reference to lakes and rivers.

PRINCIPAL SOURCES: Marine Biology, Peter Castro, Michael Huber; Marine Biology, An Ecological Approach, James Nybakken; Reef Fish Identification Florida, Caribbean, Bahamas, Paul Humann, Ned DeLoach; What are pelagic fish? Oxford Dictionary of Zoology, Michael Allaby, Oxford University Press; National Oceanic and Atmospheric Administration; Demersal Fish, Benthic Fish, Tethys, Pacific Northwest National Laboratory; Benthos, Dictionary.com; Pelagic Fish, Demersal Fish, et al., Wikipedia.

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