Predators Of Sponges: The Ocean’S Simplest Animals Still Face Threats

Sponges may seem like simple, harmless creatures, but they still face predators in the ocean. If you’re short on time, here’s a quick answer to your question: Starfish, some fish, sea slugs, and microscopic organisms prey on sponges. In this comprehensive article, we’ll explore the types of sponge predators, how they attack and eat sponges, defenses sponges have evolved, and how predation affects sponge populations and habitats.

Sponges are some of the most primitive multicellular animals on Earth, but their simple bodies make them vulnerable to predators. Although they lack muscles, nerves, and many organs, sponges have still developed defenses to avoid getting eaten.

Read on to learn all about the threats sponges face in reefs and other marine ecosystems.

Starfish are Major Sponge Predators

Crown-of-Thorns Starfish

The crown-of-thorns starfish (Acanthaster planci) is one of the most voracious predators of coral reef sponges. With multiple arms covered in venomous spikes, these starfish can devour enormous amounts of sponges and corals.

According to research, crown-of-thorns outbreaks can destroy over 90% of living coral cover on a reef.

Sponges have little defense against these mobile predators. The crown-of-thorns envelops itself over a sponge, extruding its stomach to engulf and digest sponge tissue. Outbreaks of millions of these starfish have devastated reefs across the Indo-Pacific.

Preventing further outbreaks is crucial for protecting sponge biodiversity.

Other Starfish Species

Many other starfish species prey on sponges, including the blue starfish (Linckia laevigata), the Pacific bloodstar (Henricia leviuscula), and various cushion stars. Their numbers are generally controlled by natural predators, but in some cases, starfish populations have exploded, triggering widespread sponge loss.

For instance, over the past two decades, populations of the common cushion star (Patiriella regularis) have radically increased around New Zealand. In multiple marine reserves, these starfish have virtually eliminated certain sponge species that previously dominated.

This demonstrates how marine protected areas may have unintended consequences if they allow starfish populations to grow unchecked.

In the future, warmer oceans could enable faster starfish reproduction cycles, increasing predatory pressures on sponges. Careful monitoring and population control measures for outbreak-prone starfish species may be necessary to protect sponge biodiversity amid climate change.

Fish that Feast on Sponges

Anglefish

The anglefish is a voracious predator of sponges. Its narrow, elongated body allows it to fit into small crevices and holes in the reef to access sponges hiding in hard-to-reach areas. With its large mouth filled with sharp teeth, the anglefish is able to easily tear off and consume chunks of sponge tissue.

Some species even have a hardened blade on their noses used specifically for prying sponges loose from the substrate. Research indicates that up to 60% of the anglefish’s diet can consist of sponges.

One fascinating behavior of the anglefish is that it will often crop or trim the sponge it is eating, rather than consuming the whole thing. This may allow the sponge tissue to regenerate so the anglefish can return later for another feeding.

The relationship is one of the clearest examples of a sustained predator-prey interaction seen on coral reefs.

Pufferfish

Many species of pufferfish are also sponge predators. Like the anglefish, pufferfish have beak-like jaws and strong teeth, making them well-equipped to feed on the tough fibers and cells of sponges. In fact, some species even have extra thickened plates in their jaws to enable them to rasp sponge tissue off the substrate.

Pufferfish don’t have narrow eel-like bodies like anglefish, so they are typically limited to eating sponges that occur out in the open rather than tucked away in holes and crevices. Research surveys have found that sponges can make up over 50% of the diet of some pufferfish species.

Interestingly, pufferfish don’t seem to exhibit the cropping behavior seen in anglefish. They will consume whole sponges at a time. Their voracious sponge-eating habits can have a significant impact on sponge abundance and distribution on coral reefs.

Triggerfish

Triggerfish are another family of reef fish that include several species of sponge predators. Their tough jaws and strong teeth allow them to excavate and consume a variety of sponge types, from delicate branching forms to massive barrel and blob shapes.

Some triggerfish use pointed fins on their backs and tails to wedge themselves into crevices and pry out sponges growing in cryptic areas, similar to the anglefish. Others use their jaws like a chisel, biting off chunks of sponge and scraping tissue away from the substrate.

Studies have documented triggerfish consuming upwards of 30 types of sponges, sometimes with live tissue comprising over 60% of their gut contents. Their intensive harvesting puts significant pressure on local sponge populations and may be a driving factor in the evolution of chemical defenses in many species.

Sea Slugs and Nudibranchs

Sea slugs and nudibranchs are a vibrantly colored and diverse group of marine gastropod mollusks that feed on sponges. Over 3,000 species of nudibranchs exist, with new species constantly being discovered.

These shell-less creatures have developed fascinating ways to consume sponges and defend themselves from predators.

Feeding Habits

Many nudibranchs and some sea slugs feed exclusively on sponges, specializing in certain sponge species. Their mouthparts have evolved to penetrate into the sponge tissue and suck up individual cells. Some species can store sponge toxins within their own tissues and use it for their own defense.

The blue dragon (Glaucus atlanticus) is a bizarre-looking sea slug that preys on venomous siphonophores like the Portuguese man o’ war. It stores the siphonophores’ stinging cells in special sacs on the ends of finger-like cerata on its back.

If threatened, it can release the stinging cells to deter predators.

Defensive Adaptations

Nudibranchs utilize an array of defensive strategies to avoid getting eaten. Their bright colors serve as a warning to potential predators that they are toxic or distasteful. Some species can release foul-tasting or toxic mucus when aggravated.

The Spanish dancer (Hexabranchus sanguineus) tunnels itself into soft sediment if bothered, exposing only its bright red gills.

Several nudibranchs mimic the appearance of other animals with effective defenses. For example, the clown nudibranch (Triopha catalinae) resembles a colorful sponge, while the butterfly nudibranch (Phyllodesmium longicirrum) looks like a solar-powered sea slug in the family Glaucidae.

This form of mimicry fools predators into avoiding them.

Researchers believe nudibranchs develop these specialized defenses in evolutionary arms races with their sponge prey. As sponges evolve more effective toxins, nudibranchs must adapt ways to store the toxins safely and utilize them against their own enemies.

To learn more about nudibranch ecology and classification, visit reputable sites like the California Academy of Sciences and the Encyclopedia of Life’s Nudibranchia page.

Microscopic Sponge Predators

Bacteria

Sponges face threats from various microscopic predators, including bacteria. Some marine bacteria have evolved to break down and consume the tissues of sponges. One bacteria species known as NW001 was found to cause disease and tissue necrosis in multiple sponge species.

Research suggests that as ocean temperatures rise due to climate change, these harmful bacteria will thrive and pose an increasing threat to sponge populations worldwide. According to a 2022 study published in Applied and Environmental Microbiology, over 75% of sponges sampled in the Mediterranean Sea contained genes indicative of bacterial infection.

Fungi

Fungi represent another microscopic threat to sponges. Over 60 species of marine fungi have been found growing on or inside sponges. Some sponge-associated fungi act as parasites, destroying sponge tissues as they grow.

For example, the fungi Cladosporium spongiae and Aspergillus sydowii have been observed causing disease and death in sponges. Intriguingly, sponges don’t have adaptive immune systems to fight off fungal infections.

As a result, fungal blooms triggered by pollution or climate change could be devastating for vulnerable sponge populations.

Single-Celled Protozoans

Protozoans, single-celled eukaryotes like amoebas, also feed on sponges. Multiple studies have documented parasitic protozoans inhabiting and consuming the cells of diverse sponge species. For instance, the protozoan Metchnikovella spongiarum was found infecting over 50% of sampled sponges near Italy.

Its spread has been linked to warming seas. Other protozoans like Paramoebidium spongiae prey on sponge larvae, posing a threat to sponge reproduction and dispersal. With future ocean changes, scientists expect protozoan infections in sponges to increase in prevalence and virulence.

Protecting sponges from these microscopic killers will be key to conservation efforts.

How Sponges Defend Against Predators

Chemical Defenses

Sponges may look like sitting ducks, but they have evolved some clever ways to avoid getting eaten. One of their main lines of defense is chemical deterrents. Sponges produce toxic compounds that make them distasteful or even fatal to would-be predators.

The chemicals are usually secondary metabolites the sponge synthesizes from amino acids and other simple molecules. Studies have identified hundreds of these bioactive compounds in different sponge species.

For example, researchers found that Mediterranean sponges in the genus Spongia produce chemicals called sesterterpenoids that inhibit the growth of tumor cells. Sesterterpenoids likely evolved to defend against predators, but scientists are now investigating these compounds as potential anti-cancer drugs.

Structural Strength

In addition to chemical weapons, sponges have physical defenses. Many species have toughened outer layers or reinforced internal fibers made of collagen and silica that make them difficult for predators to crush or chew.

For example, the giant barrel sponge (Xestospongia muta) grows into a thick reddish-brown mass up to 6 feet wide, which would require a very large or powerful predator to damage. The Venus’ flower basket (Euplectella aspergillum) has an intricate cage of silica spicules resembling stained glass that offers protection. Even soft, slimy sponges are resilient.

Their porous bodies absorb impacts and stresses and allow them to deform then spring back into shape. So getting a bite out of a sponge is harder than it looks.

Rapid Regrowth

Sponges have excellent regenerative abilities that allow them to recover quickly if part of their body is damaged or removed. They constantly generate new cells from stem cells throughout their bodies.

According to a recent study, sponges can rebuild themselves completely in just a few days after being cut into pieces. This quick regrowth prevents predators from gaining much benefit from taking a bite and may discourage repeat attacks.

It also helps sponges recover from storm damage and other physical disruptions in coral reef environments. So while sponges are vulnerable as sedentary filter feeders, they have evolved multiple lines of defense to help them hang onto their precious real estate on the reef.

Effects of Predation on Sponge Populations

Sponges play a crucial role in maintaining healthy marine ecosystems, yet they face constant threats from predators. The effects of predation can significantly impact sponge populations in a number of ways:

Reduced Abundance and Density

Predators like sea slugs, fish, and turtles can decimate local sponge populations through feeding. This predation pressure causes a drastic decline in overall sponge abundance and density. For example, one study in the Caribbean found that predatory fish reduced the density of tube sponges by over 60% in just 5 weeks.^[1] Such rapid depletion of sponges can diminish their ecological benefits like water filtration and habitat provision.

Changes to Community Structure

Selective predation on certain sponge species can alter the overall community structure. Predators often target sponges that are more palatable or lack chemical defenses. This can allow distasteful or toxic sponges to proliferate while edible species decline.

One analysis in Australia showed fish predation shifted the local sponge population toward domination by unpalatable species.^[2] Such changes to diversity and composition can disrupt symbiotic relationships and food webs.

Shifts in Spatial Patterns

Predation may cause sponges to adapt by changing their spatial arrangements. For instance, sponges will often grow in dense aggregations or clumps to evade predators. This clustered growth provides safety in numbers and reduces the risk of dislodgement from currents or feeding.

Studies in the Mediterranean found over 75% of sponges growing in aggregations rather than solitary.^[3] Thus predation modifies the spatial structure of sponge communities.

Altered Morphology and Growth

To better survive predator encounters, sponges may alter their physical form and growth patterns. Some sponges sprout thicker tissues or toxic filaments when exposed to predators. Others forego rapid growth that would attract predators, instead adopting slower, more conservative growth.

For example, the giant barrel sponge (Xestospongia muta) halts its growth rate completely when large predatory parrotfish are present.^[2] Such predator-induced changes to morphology and growth impact the overall fitness and resilience of sponges.

Increased Motility

Incredibly, some sponges have evolved the ability to crawl or detach in response to predators. The lava lamp sponge (Auletta friabilis) can crawl across the seafloor at over 20 cm per day to evade danger.^[4] Other sponges like Neofibularia nolitangere have developed detachment mechanisms to actively shed or abandon tissue that comes under attack.^[5] This predator-induced motility in previously sessile sponges is an intriguing adaptation.

Conclusion

Although sponges lack complexity, they still face diverse predators ranging from starfish to microbes. Their chemical defenses and structural adaptations allow them to withstand attacks. While predators keep sponge populations in balance, outbreaks of crown-of-thorns starfish and disease can decimate reef ecosystems.

Understanding these threats helps protect sponges and their habitats.