What Are Sponges? Maybe the Future of the Reef.

What are sponges? They’re animals, perhaps animals that stretch our conception of the term, but animals for sure. It’s easy to dismiss them as just backdrop scenery to more exotic stuff on the reef. But ocean sponges come in amazing shapes and colors, chug along with body features unique in the animal kingdom and perform important reef-related roles. 

Sponges were among the earliest arrivals in the ocean. And, if corals disappear, sponges may be the future of reefs.

srawberry vase sponge
A strawberry vase sponge (Mycale laxissima), blood red and often found on walls, as here in Roatan, Honduras.  

NOBODY DIVES ON THE REEF TO SEE SPONGES, but they ought to. Invariably, sponges are there wherever we dive. They’ve been there for many millions of years, among the oldest members of the animal kingdom.

Marine sponges are found in every habitat in every ocean in the world, from shallow reefs to deep arctic seas, anchored to corals and boulders and soft sediments alike.They range in size from tiny – less than an inch across, to huge, up to six feet in height. The largest sponge in the world is reputed to be one in Hawaii the size of a small truck.

So, what are sponges?


  • Sponges are animals. At one time it was thought they might be plants because they live attached to a substrate and don’t move. Well, hardly ever.
  • They are, however, very singular members of the animal kingdom. Sponge bodies totally lack the tissues and organs found in nearly every other animal. Instead, they consist of loosely associated individual cells held together by a gelatinous material called mesohyl.
  • Most sponges are cavity-shaped. Their many forms resemble vases, bowls, barrels, ropes, “smokestacks,” “cannonballs” and the substrate they lie on. Those last  would be non-cavity forming encrusting sponges, which spread across corals and rocks like blankets.
  • Ocean sponges earn their livings by filtering bacteria, diatoms and other microscopic organisms from surrounding seawater, drawing it in through the pores on their surfaces and expelling it through the cleaned H2O through their central cavities.
  •  They get additional boosts from sugar they absorb from seawater and/or from photosynthetic algae or bacteria embedded in their bodies.
  • And, it develops that some sponges are carnivores, feasting on very tiny crustaceans.


How much do we take sponges for granted? I try to take note of all life I see and I was surprised to find that almost all my sponge shots were from the Caribbean, none from my Pacific trips. It’s impossible to depict all 8,000 species. Here’s a sampling of types:

  • They come in an array of striking colors – blood reds and bright reds, dark and pale blues, azures and lavenders, brash yellows, subdued oranges, plain brown. Sometimes the plain brown turns out to be bright red when you shine a light on it. Sometimes, it’s just plain brown.
  • They clean up algae-laden waters, provide hiding places and habitats for many other reef denizens, and contain chemicals and glassy fibers that researchers are anxious to understand.
  • It’s always worth looking inside an ocean sponge’s tube or bowl. Small fishes, brittlestars, banded coral shrimps and other critters often hang out there. Larger fishes like coneys and schoolmasters rest on or dawdle above barrel sponges. And black and white damsels and other critters make their homes in the bumpy outer surfaces of large barrels.
  • Of the 8,000 or so species of sponges believed to exist on our planet, almost all are marine animals. For the record, there are some 150 species found in fresh water habitats.
  • Sponges were on the planet before the corals came along and have proven themselves remarkably tolerant of stress and adaptable. Some researchers think that in a future of warmer and acidic oceans, they’ll succeed corals as mainstays of the reefs.



To be clear, the sponges you see in the oceans are animals, albeit singular animals with a very ancient lineage. The first sponges were in the oceans at least 550 million years ago. At one time, scientists thought that they might have been an early step in the road of evolution that led to modern animals, including us humans. However, they now believe that they were an evolutionary dead end – a sidestep, albeit, one with staying power.

Perhaps ocean sponges’ early arrival in the on the stage of life explains their unique body make-up. Rather than the tissues or organs that describe nearly all other animals, they form collective structures of essentially independent cells that work together with specialized functions, held together in amorphous matrixes of a gelatinous matter called mesohyl.


They’re supported by internal “skeletons” of transparent, needle-like spicules embedded throughout their bodies. Varying in size and shape, spicules give strength to the sponge body. Some 75 to 90 percent of sponges are” demosponges,” with of spicules of either silica or calcium. Often also adding strength is a tough, elastic fiber made from a protein called spongin.

sponge spicules
A scanning electron microscope image displays the diversity of spicule shapes and sizes that support many demosponges. (Image credit:  Rob W. M. Van Soest, Nicole Boury-Esnault, Jean Vacelet, Martin Dohrmann, Dirk Erpenbeck, Nicole J. De Voogd, Nadiezhda Santodomingo, Bart Vanhoorne, Michelle Kelly, John N. A. Hooper, via Wikimedia Commons.)


A smaller percentage are “glass sponges,” separate from the silicon-spicule sponges considered demosponges. The scientific term “hexactinellid” reflects their propensity for silicate spicules with four or six points. They’re found in oceans around the world,  predominantly in Antarctic and Northern Pacific waters. But they’re relatively rare, and generally reside in deep ocean habitats. 

Glass sponges’ silicate skeletons provide them with a unique ability to rapidly conduct electrical impulses, making them an object of research for scientists focused on fiberoptics. Although they have the capability of developing into distinct “sponge reefs,” such systems have been assaulted by commercial fishery and offshore oil and gas industries. At present, the only known glass sponge-based sponge reef is located on the northern Pacific’s western Canadian continental shelf.


For sponges as a whole, the extensive variety among the 8,000 or so species of sponges reflects millions of years of adaptation. As a result, they’re found everywhere: deep, shallow, cold, tropical, solid substrates, muddy bottoms.

Even sponges in the same species can adopt different characteristics. Colors and shapes are affected by their living conditions as well as their species. Factors include temperature, access to light and currents in the water column.

Sometimes, identifying species definitively can be difficult without laboratory examination, and most field guides describe them based more on shape, color and other traits than on taxonomic labeling. And that can be off: A “yellow” sponge is not necessarily brown or yellow. And a lot of brown specimens turn out to be blood red when a light is shown on them.

Species estimates actually vary between 5,000 and 10,000, and usually end with the notation that scientists believe that half the existing species have not be discovered yet.


Sponges are sessile. They mostly live their lives attached to a substrate and don’t go anywhere. Well, actually, under stress from, say, environmental challenges, some sponges can migrate – at an astonishing rate of a few millimeters per day.

Some do travel faster by virtue of being kidnapped by decorator crabs, hermit crabs or sea urchins to provide them with camouflage.

sponge-varigated- sea urchin
One way sponges can travel is to be picked up for camouflage (along with shells and debris) by varigated sea urchins, like this one.


To start off with, most sponges are cavity-shaped, taking the form of bowls or barrels or tubes. This is important to note, since they function by drawing nutrient-rich water in through the pores on their exterior sides (ostea), filtering it through internal feeding chambers and expelling it through the central cavity (the osculum).

In some sponges, like cannonball and encrusting varieties, the osculum may be not a large central cavity but one or more large excurrent openings (to you Latin purists, oscula).

sponge anatomy
This image depicts the basic anatomy of three variations of vase-like sponge bodies – where water enters through the ostea pores and exits through the osculum, or central cavity. Yellow bits indicate pinacocyte cells, red the choanocytes, all imbedded in gelatinous mesohyl.

In any event, plankton-laden water is drawn through the ostea into canals by the beating tails of flagellum-equipped choanocytes, or “collar cells” that line them. Cells in internal feeding chambers absorb oxygen for respiration and capture food particles sustenance. The collar cells’ hair-like tails propel the spent water on into the osculum (or oscula) and back into the water column. Sometimes, you can place your hand just above the top of a smokestack sponge and feel the exiting current.


Sponge cells are individual units but they assume specific functions within the animal’s  architecture. Plate-like cells called pinacocytes form a one-layer external skin that covers all parts of the sponge not fronted by collar cells. Pinaccocytes also anchor the sponge to the substrate and digest food that’s too big for the ostia to take in.

Besides choanocytes, other cell types include cells that slowly migrate through the mesohyl producing collagen fibers, oocytes and spermatocytes for reproductive functions, sclerocytes that produce the spicules that support many sponges, other sclerocytes that produce spongin, “grey” cells that more-or-less serve as an immune system.

If this assignment of duties sounds rigid for an animal that doesn’t have organs or tissues, sponges still have superpower, shapeshifting capabilities. Through a process of buddings, they can reconstruct and regrow their bodies from smaller pieces. Additionally, sponge cells are totipotent. This means that they’re like the stem cells found in more advanced animal bodies – any sponge cell can transform into a different type of cell, if need be. In many ways, sponges are awesome.


The edibles that marine sponges filter from the passing waters include bacteria, diatoms, protozoans and other microscopic organisms, often so tiny a microscope would be needed to see them. In the ocean food web, this represents a successful niche, since these are prey not useful to other marine animals.

sponge oscula
In a demonstration of sponges’ filtering feeding system, red food dye exits a ball sponge’s excurrent oscula.

The multiplicity of cell types plays a role here. Mostly, drifting prey are drawn in through the ostia for processing by the choanocytes and other internal cells. But anything greater than 0.5 micrometers – let’s just say much, much thinner than a strand of human hair – is too large to enter an ostia pore. In that case, the pinacocytes forming the external skin stand ready to engulf and digest the food particle.


Scientists have recognized that along with the usual planktonic organisms ocean water is loaded with dissolved organic matter in the form of microscopic bits of sugar and carbohydrates. The importance of sugar to sponges has been recognized for several years but new research has identified marine sponges in the Indo-Pacific basin that absorb that sugar directly in a process the scientists call “cell drinking” (it had been surmised that bacteria living with sponges did the processing).

tube sponges
Resembling trumpets sounding a fanfare, brown tube sponges (not necessarily brown) hang off a wall.

At least some species of sponges also benefit from the presence of photosynthetic zooxanthellae and bacteria. In the case of boring sponges, research has suggested that sponge-embedded zooxanthellae help them break coral down to establish a foothold rather than build them up, as is the case with coral.

And some marine sponges, it turns out, are carnivores, albeit carnivores of very small crustaceans and perhaps other tiny creatures. They do this through the use of tiny velcro-like hooks that snag passing prey. Most live in deep waters and as best as is known, the crustaceans in question are in the smaller-than-1-mm-range.


Most sponges are hermaphroditic, meaning they can play both male or female roles. The male-acting sponge releases sperm into the water that travel toward the female-acting cells to begin a multi-step process leading to fertilization. Most hold onto their eggs until they hatch, releasing tiny larvae into the water column.

But sponges can also reproduce asexually, by the budding process noted earlier. Broken-off pieces of a sponge’s body may be carried by the currents to a substrate where they can anchor themselves and grow into a full sponge.

tube sponge community
Stands of sponges provide excellent habitats for communities of fishes and critters.


As sightseeing visitors, we might regard sponges as just ornamental afterthoughts on the reef. But in fact they play vital roles in their environments, like providing shelter and habitat for a broad range of fishes and critters.

Look under any rope sponge and you’re likely to find grunts and other fishes hanging out, hoping to remain safely out of sight. Look inside any vase sponge and you may find pairs of banded coral shrimps, brittlestars or even (at dawn, say) sleeping fishes. Look around the knobby exterior of large barrel sponges and you’ll often whole communities of small fishes and other critters. These creatures rely on sponges as hosts on which to hunt prey, plant egg nests and maybe just remain safe.

angelfish eating sponge
The silica and calcium spicules that provide strength to sponge bodies also deter predators – but not grazers like this French angelfish, who circled around taking chunks out of this encrusting sponge on a pier piling.


Beyond this, sponges are important to the health of the ocean environment, from filtering particulate from the water to, as physical structures, enhancing the flow of currents to the benefit of other filter feeders. Some scientists refer to sponges as civil engineers of the reef.

A large ocean sponge may filter as much as 1,500 liters of water per day, creating a cleaner, healthier habitat. And not only do they snatch bacteria from the passing currents, they’re loaded with other bacteria that permanently reside within their bodies.

Some of these guest bacteria absorb phosphorous, sharing this valuable nutrient with any manner of other animals on the reef. The photosynthetic zooxanthellae embedded within sponges’ cells assist their host sponges, but they also they make significant amounts of oxygen and nutrients available to the reef community as a whole.


Along with simple spongin and gelatinous mesohyl, sponges are loaded with lots of potentially useful stuff. Scientists and engineers are always looking for ways to apply the secrets of glass sponges’ silica-based spicules to the development of new fiberoptic communications cables and other devices.

Research has identified more than 100 types of microbes living in the bodies of sponges (but not in surrounding waters). Pharmaceutical researchers look enviously at them, searching for compounds to replicate in the laboratory for human and animal use. Promising potentials include compounds for treating cancers, fighting viral diseases and eliminating infectious bacteria like MRSA, a serious problem in hospitals.

elephant ear sponge
A spotted eel,, a long-spined sea urchin and an elephant ear sponge walked into a reef…


It’s a given that, due to climate change, warming seas and increasing acidification of the oceans, our planet’s rich, robust coral reefs are under significant threat to their continued existence. If that doomsday prospect should occur, what is likely to be left of reefs?

Some scientists think that the future of reefs is sponges. In the millions of years that sponges  have been present in our planet’s oceans, they’ve adapted to almost every oceanic environment.

“Sponges are important components of coral reefs and there is increasing evidence that may sponges may be  more tolerant to impacts of climate change than corals,” say a team of New Zealand and Australian scientists in a paper published in 2018.

Earlier research had found that lipid and fatty acids offset impact of warming temperatures on sponge cells. While algae has tended to replace dying corals in some areas, in others, sponges proliferate.

Reefs dominated by ocean sponges don’t support the same abundance and diversity that their coral predecessors do, but they do represent viable habitats for much life. But what those habitats  will specifically look like is still an open question, the researchers note.


PRINCIPAL SOURCES:  Marine Biology, Peter Castro, Michael Huber; Marine Life, Caribbean, Bahamas, Florida, Marty Snyderman & Clay Wiseman; Reef Creature Identification, Florida, Caribbean, Bahamas, Paul Humann, Ned DeLoach; Coral Reef Animals of the Indo-Pacific, Terrence Gosliner, David Behrens, Gary Williams; Marine Life of the North Atlantic, Andrew Martinez; Elucidating the sponge stress response; lipids and fatty acids can facilitate survival under future climate scenarios, Global Change Biology; Climate change alterations to ecosystem dominance: How might sponge-dominated reefs function? Ecology; Symbiotic zooxanthellae enhance boring and growth rates of the tropical sponge Anthosigmella varians forma varians, Marine Biology; The Life of a Sponge, Tree of Life Web Project; Secrets of Our Ocean Planet: The Not-So-Simple Sea Sponge, National Geographic Newsroom; Secrets of Our Ocean Planet: Saving Sponges to Keep Marine Ecosystems Healthy, National Geographic Newsroom; Secrets of Our Ocean Planet: Sponges as Civil Engineers and Pharmacists, National Geographic Newsroom; New Killer Sponges Found in the Deep Sea, National Geographic Newsroom; The Sponge Guide, spongeguide.org; Seawater Is Filled With a Sugary Feast. Here’s How Sponges Eat It, New York Times; As corals decline, a new kind of reef emerges, Anthropocene; Sponge, et. al., Wikipedia, the free encyclopedia.