Corals and sponges are aquatic animals (not plants, as most people believe). Al-Biruni (a Persian polymath) classified corals and sponges as animals, pointing out their response to touch as a reason. However, no one heeded this classification until centuries later. Only in the eighteenth century did William Herschel prove corals have cell membranes – a characteristic of animals and other scientists began observing the animal characteristics of sponges.
Sponges and corals are immobile invertebrates and indicators of the oceanic ecosystem’s health. The similarities end there. These organisms differ in just about everything else, be it their feeding method, habitats, or structures. Sponges have pores all over their bodies, whereas corals are made of polyp colonies.
Sponges Vs. Corals
Sponges belong to the phylum Porifera, whereas corals are members of the phylum Cnidaria.
Difference Between Sponges And Corals In Tabular Form
|Parameters of Comparison||Sponges||Corals|
|Habitat||Sponges are found in freshwater, too, though most species are predominantly in the ocean.||Corals require saltwater to survive, meaning they cannot be found in freshwater.|
|Tissue Presence||They do not have tissues.||Corals have tissue layers. Their stinging cells are present in the tentacles and outer tissues.|
|Food||Sponges obtain food and oxygen when water flows through their pores. Their flagella helps them trap bacteria and other organic matter to feed. Some species capture crustaceans with their spicules.||Corals predominantly derive nutrition when photosynthesis occurs in the dinoflagellates (single-celled eukaryotes) that live within their tissues. Sometimes, they use their stinging cells to capture plankton and small fish (look at that, even corals have better luck catching fish than most humans.)|
|Reproduction||Sponges reproduce sexually and asexually.||Sexual reproduction is predominant in corals (though asexual reproduction does occur).|
|Threats||Sponges do not face many threats, as they are not tasty (hey, some animals have a delicate palate. Do not judge.). Hawksbill sea turtles and nudibranchs are the only organisms that eat them.||Coral mining, pollution, blast fishing, increasing sea temperature, pH changes in the ocean, and so on are major threats to corals. (Hope those poor things catch a break.)|
What Are Sponges?
Sponges are multicellular, heterotrophic (cannot produce their own food) aquatic animals. They lack cell walls, tissues, and organs. Sponges have unspecialized cells that differentiate into specialized cells. Most of these cells are exposed to water flow and aid in nutrition acquisition and waste disposal, rendering nervous, digestive, or circulatory systems unnecessary. Sponges have body shapes adapted for maximum water flow and are sessile (lack self-locomotion) animals. However, they are motile (can move independently) in the larval stage.
Demosponges (a class of sponges including nearly 8800 sponge species) have endoskeletons (inner fibrous skeletons) made up of spicules (fragments of silica or calcium carbonate) or spongin (modified collagen protein type). Some species have calcium carbonate exoskeleton (outer skeleton). Sponges reproduce sexually and asexually. In sexual reproduction, the sponges release sperm (male reproductive) cells into the water or retain them to fertilize the ova (egg cell). The fertilized eggs develop into larvae (juvenile form before metamorphosis into next life stage). These larvae swim off and settle in different places.
Broken-off sponge fragments regenerate to form new sponges (asexual reproduction). Sponges produce gemmules (internal buds involved in asexual reproduction) when facing unfavorable environmental conditions. They act as survival pods, enabling the unspecialized cells to remain dormant until the prospects improve. New sponges form once the conditions improve.
Sponges work similar to chimneys in a way; they allow water to get in at the bottom and out through the osculum at the top. Though sponges are sessile animals, some species are capable of moving at the speed of 1 – 4mm per day. Their pinacocytes (flat cells on the sponges’ surface and inner canals) are the reason for such movements. The sponges in temperate regions live for a mere few years, but those in tropical regions can live for a few hundred years easily. Moreover, some species have been around for 5000 years.
These aquatic animals do not have complex immune systems but can reject foreign material with the aid of their grey cells. (No, these grey cells are not the ones Hercule Poirot refers to often when talking about his brains and deduction methods.) The grey cells produce a chemical to stop the movement of other cells and release toxins to kill the foreign cells that enter the internal transport system.
All this information about sponges is fascinating, but know what is more awesome? Some sponges are small enough to be mistaken for rocks, and others are taller than humans. Moreover, they occur in a variety of colors and shapes. Forget cloud watching and take a dive into the ocean (with scuba diving equipment if people want to see deep-dwelling sponges, too).
Apparently, the sky is not the only fascinating thing to study; the ocean is proving to be just as great! Another interesting fact about sponges is that dolphins use them as tools for foraging. The bottlenose dolphins use sponges to protect themselves from sharp rocks they may come across when sifting/digging the seafloor looking for prey.
Types Of Sponges
The following are the various sponge types:
Demosponges are also known as coralline sponges because their layered skeletons are similar to reef corals. The most economically significant demosponge species is the Spongia officinalis (better known as bath sponges). Divers harvest these sponges from the ocean. However, these sponges can be grown commercially. The harvested sponge is bleached and marketed. They are well-known for their softness. The first demosponge is believed to have appeared near the ‘Snowball Earth’ period’s end. Spongillida (a family included in demosponge) is the only sponge species that lives in freshwater.
The distinguishing feature of calcareous sponges is their spicules. The spicules are made of calcite or aragonite (carbonate minerals) and mostly triradiate. However, some species have two or four-pointed spicules. Calcareous sponges lack tiny spicules called microscleres found in other sponges. These sponges are found in shallow waters (mostly tropical) and are filter feeders (they strain food particles from water to feed).
These sponges are more popularly known as glass sponges (a catchier name) and are believed to be the longest-living animals in the world. They can live up to a maximum of 15,000 years (what? That’s amazing). Sponges are such primitive things, but look at how long they outlive most other species. Perhaps people should take a leaf from their books and lead a laid-back life instead of worrying about the future. They may not live to outlast a sponge, but they may add a few more years to their lifespan.
Glass sponges are found in deep water (450 – 900 meters below sea level) and are abundant in the Antarctic and Northern Pacific Oceans. However, the species Oopsacas minuta belonging to this class is present in shallow water. The choanosome (part of a glass sponge’s body) acts as the mouth for feeding, while the inner and outer canals connected to the choanosome form the consumption path. The interesting thing about these sponges is they form sponge reefs that rival a coral reef’s beauty.
What Are Corals?
Who has not heard about these close cousins of jellyfish? Corals are colonial organisms composed of hundreds to thousands of polyps. They mostly feed at night and use nematocysts (stinging cells) to capture food. Fortunately, humans are not much affected when corals sting (anyone who experienced a jellyfish’s sting will know why that is a huge relief). Corals prey on a variety of things ranging from planktons (ocean organisms unable to propel themselves against a current) to small fish using their stinging cells, besides gaining energy and nutrition from the photosynthetic dinoflagellates of the genus Symbiodinium.
The dinoflagellates (colloquially known as zooxanthellae) are responsible for the coral’s color that people love so much. The photosynthetic pigment chlorophyll causes the corals to be green or brown. Phycoerythrin is another photosynthetic pigment that causes a coral to be a bright orange. Moreover, corals produce chromoproteins (reflective proteins) that make them appear red, blue, purple, mauve, etc. The fluorescent pigments present in corals absorb light and emit it in a different color. The corals’ color controls how much sunlight filters through, thus protecting them from UV/damaging sunlight.
Unfortunately, corals, similar to humans, do not like stress. Coral bleaching (the corals turn white) occurs when the corals are stressed due to high temperatures, toxins, etc. The corals turn white as they expel the zooxanthellae that live in their tissues in distress (much like how humans throw a temper tantrum and break or throw things, right?). Chromoproteins increase a coral’s resistance to bleaching, but even that is not enough sometimes.
Synchronous spawning is typical in broadcasters (corals that release gametes into the water). The darkness present between sunset and moonrise appears to be the cue for coral spawning. On the other hand, brooders only release sperms, which sink into the egg carriers to fertilize. Corals may reproduce asexually by budding or dividing. In the former process, a small polyp splits from an adult and forms body parts. The latter involves the polyp splitting longitudinally or transversely and forming into two polyps (they become as large as the split polyp).
Corals found in shallow water are mostly reef-building, whereas deep-sea corals predominantly do not form reefs. Instead, they form massifs, thickets, and groves resembling reefs but differing in structure and function. However, some stunning coral reefs have been discovered in deeper and darker parts of the ocean. Deep-water corals do not need zooxanthellae to survive. These corals were discovered around 250 years ago.
Types Of Corals
The most common coral types are as follows:
Soft corals are not considered ‘true’ (stony) corals. They are found in the deep sea, tropics, subtropics, and polar water. In contrast to stony corals, these corals thrive under less intense light (which explains their survival in the deep sea). Their major energy source is the dinoflagellates; however, they eat any free-floating food (hey, food is food. They are happy as long as they derive nutrition and energy from it). Black or thorn corals are a popular order of soft-deep sea corals, named thus because of their black skeletons despite their color being white, yellow, red, etc.
Scleractinia (stony or hard corals) are marine animals that build themselves a hard skeleton through calcium carbonate secretion. Solitary hard corals predominantly reproduce by budding, while colonial stony corals reproduce sexually and asexually. Stony corals may be hermatypic corals that formed most of the coral reefs in the world or ahermatypic (non-reef building). Reef-forming corals are mostly colonial, whereas non-reef-forming corals are colonial or solitary.
Main Difference Between Sponges And Corals (In Points)
- Coral necklaces and other jewelry are famed and prized among people (especially, things made of red corals). On the other hand, people can use natural sponges (the synthetic sponges abundant in stores are not made from these) for bathing and cleaning if they manage to get their hands on one.
- The water flows through the sponges’ central cavity to deposit food/nutrition and exits through the osculum (excretory structure resembling a large opening). On the other hand, food enters the corals’ stomachs through the mouth, and the waste excretion takes place through the same opening (gross!).
- Sponge loops (recycling of dissolved organic matter by sponges) aid corals in thriving and forming colorful, diverse reefs, whereas corals play no role in sponge reef formation.
- 75% of hermatypic (reef-building) corals release sperm and eggs into the water in synchronization to facilitate successful fertilization. On the other hand, sponges release only the sperms into the water, which enters a female sponge’s body through its pores.
Corals are known for their vibrant colors that are stunning to look at. After all, they are capable of attracting thousands of tourists with their mere appearance. However, sponges are exceptional looking, too. Though people have not studied sponges as nearly as much as corals, sponges have proven to be worth knowing about. They may be primitive compared to corals, but they are an invaluable part of the marine ecosystem.