Category: Fish

The Ocean Deep

Seventy percent of the Earth’s surface is under the ocean, and only a small portion is in shallow water along continental shelf areas.  The ocean is divided into five zones; the sunlight zone, above 650 feet; the twilight zone, extending to 3300 feet; the midnight zone, down to 13,000 feet; the abyssal zone, down to 21,300 feet; and the hadal zone, extending to the bottom of the ocean.  Below 3,000 feet, no light penetrates from the surface; the water temperatures are freezing; and oxygen levels are very low.  The pressure exerted on any surface at sea level is 14.7 pounds per square inch (psi), and humans can withstand three to four times this amount, or 45 psi to 60 psi.  At 3,000 feet deep, the pressure is 1,345 psi.

Ocean zone chart by DonArnold, 2023
Delta submersible by Robert Schwemmer, CINMS, NOAA, Oct 2010

Pressure keeps sea water from becoming solid ice, even when it is at freezing temperatures.  It also makes exploring the ocean difficult, requiring special submersible vehicles.  Everything moves more slowly under pressure, and a submersible can take an explorer deep into the ocean given enough time and oxygen.  Deep sea creatures can survive because the structure of their bodies is lean and contain no air spaces.  Let’s take a look at a few of the wondrous creatures that live in these habitats.

Humpback anglerfish by August Brauer in Public Domain

In the darkness of deep areas, many species find food and mates by touch.  Others may use bioluminescent communication – the ability to create light from a chemical reaction between luciferin and oxygen within an organism’s body.  Lights show up as spots of various colors on different parts of the body and may be used for attracting a mate, confusing a predator, or attracting prey.  Lanternfish have several spots located on their heads, undersides, and tails.  Female anglerfish and dragonfish have a shaft that protrudes from their head out in front of their mouths with a glowing tip attracting prey within striking distance.  Gulper eel use the lighted tip of their tail as a lure to attract a meal.

Tripod fish by NOAA Okeanos Explorer Program, Public domain, 2010
Fangtooth fish by Sandra Raredon-Smithsonian Institution, Public domain

Tripod fish have two pelvic fins and a tail fin that are elongated and can be filled with fluid to stiffen them, allowing the fish to stand on the ocean bottom while conserving energy.  Two long, wavy pectoral fins act as  arms to detect prey floating or swimming by and push the prey toward their mouths.  Fangtooth fish is another species that can detect vibration and nearby movement.  This species is only six inches in length, but have the largest teeth in proportion to their body size of any fish in the ocean.  The long lower fangs fit into pouches in the roof of their mouths so their teeth do not pierce their brain cavity.

Dragonfish teeth array by UC San Diego Jacobs School of Engineering, May 2019

The dragonfish, another small fish about six inches long, has a frightening array of teeth located on its jaws and tongue.  Their teeth are transparent and their bodies are black, making them impossible to see even in depths where there may be some light.  The jaws are loosely hinged, allowing the fish to open its mouth wider than normally hinged jaws would allow.  Prey swimming nearby may not notice the wide opening, nor be able to detect the transparent teeth, making the dragonfish a top predator of invertebrates, squid, and shrimp.  In addition, when biting, they inject a highly toxic poison to paralyze and kill their prey.  This toxin can be dangerous to humans getting bitten or attempting to eat this species.

Giant spider crab by Michael Coghlan, Jan 2014; Gulper eel by Claf Hong, Mar 2005; Lanternfish, Myctophum punctatum1 by Emma Kissling

Some fish practice diel vertical migration, a pattern of movement where a species feeds near the surface at night and moves back into deeper waters to rest during the day.  Lanternfish come up to feed on zooplankton and fangtooth fish feed on squid in shallow waters.  Some species, including anglerfish and gulper eels, have elastic-like skin for their mouths and stomachs.  Mouths can be opened extremely wide to swallow prey larger then themselves.  The food is contained in a similarly elastic stomach which shrinks slowly as digestion progresses.  Several different species of spider crabs roam the ocean depths scavenging for meat or plant material that falls to the bottom.

Deep-sea animals are seldom seen, but are not immune from climate change and human impacts.  Scientists continue to study changes in deep-sea food webs caused by overfishing, ocean acidification, and expanding low-oxygen zones.  Learn more about the habitat and species of these areas at the Jurica-Suchy Nature Museum at Benedictine University or your local nature museum or aquarium.

Chinook Salmon

At this time of year, many species are migrating south to spend the winter months in warmer climates.  But, a few species are moving north including the Chinook salmon, Oncorhynchus tshawytscha.  The largest species of Pacific Ocean salmon, they are blue-green on the head and back and silver on the sides.  Their tails, backs, and upper fins have black spots, and the tooth gum line is black.  Salmon average three feet in length and weigh thirty pounds, but can grow past five feet and 110 pounds.

Oncorhynchus tshawytscha, Chinook salmon by Michael Jeffries at Hiram M Chittenden Locks, Seattle

Chinook salmon, sometimes known as king salmon, are native to the northern Pacific Ocean and freshwater streams and rivers in the Pacific northwest.  They are born from eggs laid in autumn’s cool waters that hatch in springtime when those waters start to warm.  Young salmon feed on insects and small crustaceans in freshwater habitats until they become adults between one and two years of age.  An anadromous species, meaning they live in both freshwater and saltwater habitats, as adults, they travel downriver to the ocean, where they will spend the next one to five years following the warmer ocean currents, eating a diet of nutritionally-rich prey including herring, sandlance, and squid, allowing salmon to reach their large size.

Oncorhynchus tshawytscha, Chinook salmon by Dan Cox, USFWS 2013

Salmon are also known as semelparous, meaning they spawn once and then die.  As salmon reach maturity, they return in autumn to the rivers where they were born.  Females dig a nesting hole, called a redd, in which she deposits 3,000 to 14,000 eggs.  Males fight to be the one that pairs up with each female and fertilizes her egg nest.  A benefit to being big is that larger females lay more eggs and dig deeper redds.  A deep nest protects eggs better by not exposing them to faster, surface water.  Both parents guard the eggs for the first few weeks, but the parents will die before winter, months before the eggs hatch.

Oncorhynchus tshawytscha, Chinook salmon by Mike, iNaturalist

Returning salmon and their eggs offer a buffet for more than 125 species that depend on them as a food source just before winter.  Marine mammals including whales, orcas, sea lions, and harbor seals hunt large schools of salmon as they converge on the Pacific northwest coastal areas.  As salmon enter the river systems, they attract eagles, seagulls, herons, kingfishers, murres, and puffins.  A great variety of land mammals also make their way to the waterways including bears, wolves, minks, martens, weasels, and fox.  Shorebirds wade into deep water to snack on any eggs that may escape a redd.  Migrating birds on the Pacific flyway feast on salmon that are dying after spawning.  Insects feed on the decaying bodies, and in turn, are food for the smaller birds migrating south.

Chinook Salmon (Oncorhynchus tshawytscha) spawning in Los Gatos Creek by Robin Agarwal, 2020

Chinook salmon populations have declined in recent decades and some populations of Chinook salmon are federally listed as endangered.  Loss of snowpack and shrinking glaciers have reduced the amount of water in streams and rivers.  Along with shallower waterways, warmer summers are raising stream temperatures leading to habitat where parasites and disease grow and attack more easily.  Severe storms and forest fires contribute to less undergrowth and more soil eroded into rivers causing siltation that constricts the passage of water and river inhabitants.  Supporting improvements in land use practices and saving habitat will greatly benefit Chinook salmon as well as the many other species they live with.