Sponges and corals are two different types of marine organisms.
Coral vs Sponge - Anatomical Differences
Anatomy of sponges
Sponges do not have nervous, digestive or circulatory systems. Instead most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes, and the shapes of their bodies are adapted to maximize the efficiency of the water flow.
Anatomy of corals
While a coral head appears to be a single organism, it is actually a head of many individual, yet genetically identical, polyps. The polyps are multicellular organisms that feed on a variety of small organisms, from microscopic plankton to small fish.
Polyps are usually a few millimeters in diameter, and are formed by a layer of outer epithelium and inner jellylike tissue known as the mesoglea. They are radially symmetrical with tentacles surrounding a central mouth, the only opening to the stomach or coelenteron, through which both food is ingested and waste expelled.
Sponge vs. Coral - Physiological Differences
Corals can catch small fish and animals such as plankton using stinging cells on their tentacles. However, they obtain most of their nutrients from photosynthetic unicellular algae called zooxanthellae. Consequently, most corals depend on sunlight and grow in clear and shallow water, typically at depths shallower than 60 m (200 ft). These corals can be major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, such as the enormous Great Barrier Reef off the coast of Queensland, Australia. Other corals do not have associated algae and can live in much deeper water, with the cold-water genus Lophelia surviving as deep as 3000 m. Examples of these can be found living on the Darwin Mounds located north-west of Cape Wrath, Scotland.
Corals coordinate behaviour by communicating with each other.
Sponges do not have distinct circulatory, respiratory, digestive, and excretory systems – instead the water flow system supports all these functions. They filter food particles out of the water flowing through them. Sponges have minute pores lined with flagellated cells all over their body. The flagella help sponges take water and food particles in through their pores.
Reproduction in sponges vs corals
Reproduction in corals
Corals can be both unisexual and hermaphroditic, each of which can reproduce sexually and asexually. Reproduction also allows coral to settle new areas.
Sexual reproduction in corals
Corals predominantly reproduce sexually, with 25% of hermatypic corals (stony corals) forming single sex (gonochoristic) colonies, whilst the rest are hermaphroditic. About 75% of all hermatypic corals "broadcast spawn" by releasing gametes - eggs and sperm - into the water to spread offspring over large distances. The gametes fuse during fertilisation to form a microscopic larvum called a planula, typically pink and elliptical in shape; a moderately sized coral colony can form several thousands of these larvae per year to overcome the huge odds against formation of a new colony.
Corals that do not broadcast their eggs are called brooders, this is the case for most non-stony corals. These corals release sperm but harbour eggs, allowing larger, negatively buoyant, planulae to form which the polyp later releases ready to settle. The larva grows into a coral polyp and eventually becomes a coral head by asexual budding.
Asexual reproduction in corals
Within a coral head the genetically identical polyps reproduce asexually to allow colony growth. This is achieved either through gemmation (budding) or through division. Budding involves a new polyp growing from an adult, whereas division forms two polyps each as large as the original.
Reproduction in sponges
Sexual reproduction in sponges
Most sponges are hermaphrodites (function as both sexes simultaneously), although sponges have no gonads (reproductive organs). They produce both sperm and eggs. Each egg generally acquires a yolk by consuming "nurse cells". During spawning, sperm burst out of their cysts and are expelled via the osculum. If they contact another sponge of the same species, the water flow carries them to choanocytes that engulf them but, instead of digesting them, metamorphose to an ameboid form and carry the sperm through the mesohyl to eggs, which in most cases engulf the carrier and its cargo. A few species release fertilized eggs into the water, but most retain the eggs until they hatch.
Glass sponge embryos start by dividing into separate cells, but once 32 cells have formed they rapidly transform into larvae that externally are ovoid with a band of cilia round the middle that they use for movement, but internally have the typical glass sponge structure of spicules with a cobweb-like main syncitium draped around and between them and choanosyncytia with multiple collar bodies in the center. The larvae then leave their parents' bodies.
Asexual reproduction in sponges
Sponges have three asexual methods of reproduction: after fragmentation; by budding; and by producing gemmules. Fragments of sponges may be detached by currents or waves, and perhaps by predators. These fragments re-attach themselves to a suitable surface and then rebuild themselves as small but functional sponges over the course of several days. While very few species of sponge reproduce by budding, some sponges reproduce via gemmules or survival pods when dying. The gemmules then become dormant, and in this state can survive cold, drying out, lack of oxygen and extreme variations in salinity. Freshwater gemmules often do not revive until the temperature drops, stays cold for a few months and then reaches a near-"normal" level.