It's a Spongy World
Elyssa Quinton
Marine sponges may not look like much, but these animals (yes, animals) are fascinating organisms. Today we delve into their morphology, how they reproduce and what they eat. Not only do sponges create homes for a variety of different organisms, but they also provide compounds for pharmaceuticals. But these incredibly resilient creatures are under threat. Read on to find out why…
A species frequently overlooked, once thought to be a plant, and one whose DNA sequences are present in every animal below and above the surface - the marine sponges (phylum Porifera) are one of the simplest, yet most fascinating organisms that I have come across in my biological studies! They arrived here from an ancient world, surviving the Precambrian seas (the earliest eon in the world's history). They comprise considerably more than half the biomass of a reef, giving the reef colour, alien-like shapes, and textures to discover. Although they are superbly intriguing, they also play crucial roles in the ecology of a reef - and definitely a species worth knowing about!
Check out how colourful the world of the marine sponges is! They come in all different shapes, textures and colors. Photo: Hedvika Michnova.
So what exactly are marine sponges?
Named purely by their appearance, sponges are from the phylum Porifera. There are over 11,000 described species and 9000 living species. Around 150 of these live in freshwater and the rest are found in the ocean or brackish water. Genetic analysis has revealed they are the most primitive (still have very similar characteristics found at the beginning of their evolutionary history) animal group alive today, and are probably some of the first animals to ever appear on the earth.
How do we know this? A team of scientists led by Dr Gordon Love analysed fossils of sponges for traces of a steroid biomarker produced by a common class of sponges (the demosponges). The fossils containing this biomarker were dated to be at least 635 million years old! It’s believed that they dominated the oceans as the main reef builders 400 million years ago (a responsibility hard corals have taken over in present day). How can soft animals be reef builders you may ask. Well, they eventually fossilised into hard rock and were used to build castles in the middle ages.
All sponges are aquatic (found in water), completely sessile (non-motile/ do not move) and filter feeders. However the larvae of sponges are motile and free swimming in the plankton, until they settle on a substrate, turn themselves inside out and begin to grow. Most are marine and are found from the intertidal to abyssal zone (at depths of 3000- 6000m) which is in complete darkness).
Quick facts
Size range: ½ inch → 6 ft. tall (The Loggerhead Sponge being the largest!)
Shape: Round / flat / encrusting / vase-like
Colour: Often brightly coloured, given by the pigments in the surface cells!
Body plan:
Their body is a network of pores, canals, and passageways
They have no ‘mouth’
They have no nervous system, so they are the only animals that don't react when disturbed
Water is pumped into holes called ostia to filter out the nutrients
Water exits through larger openings called oscula
The large chamber is called the spongocoel
They have to filter 1 ton of water to get 1 oz of food (the size of 2 tablespoons!)
They control the flow of water by constricting the osculum at night and opening it in the day when food is more plentiful
They also reverse the flow to clean out their canals after a storm
This is me and a giant barrel sponge on a shipwreck in the Solomon Islands! Photo: Elyssa Quinton.
Cell types
Sponges have a variety of different body cells, and each have their own function - this is key to their success! Learning about their cells and what they do gives us a better understanding of how the overall sponge works.
Choanocytes (collar cells)
These cells are the most distinctive and important of all sponge cells, as they are totipotent cells, which means they can change their form and function. Each of these cells has a flagellum (a ‘tail’) and is surrounded by a sieve-like collar which acts as a strainer. The flagellum beats to draw water currents into the sponge, particles are strained through the collar and then the particles are absorbed by phagocytosis (where cells use their plasma membrane to ingest small particles).
The flagellum (‘fla’) and the choanocytes (‘cho’) and where they are situated in the layers of the sponges body. The flagellum is on the inside of the sponge, in the spongocoel, and the ectoderm is the outside of the sponge. Photo: American Museum of Natural History Library.
Pinacocytes (tissue-like cells)
These are flat, thin cells that form the outer part of the sponge.
Porocytes
These are tubular cells which form the pores. The channels which the water is inhaled through are called the ostia.
Archaeocytes
These cells move about in the mesophyll matrix, receiving the food particles from choanocytes for digestion. They also digest old cells, and can differentiate into any other type of cell.
The different sponge body structures. Colour coding: yellow = pinacocytes, red = choanocytes, grey = mesophyll, pale blue = water flow. Photo: Philcha
Skeleton - the most fascinating part!
All these cells are arranged around a skeleton of spicules. Spicules maintain the sponges shape, keep the pores and canals open and come in a variety of pretty shapes! Spicules are made of:
Calcium carbonate
Silica
Spongin (a form of collagen; a protein ONLY found in animals), the spongin gives the sponge its flexibility
The spicules are how different species of sponges are identified, and this can only be done under a microscope. Two sponges of the same species may look completely different to the human eye, i.e. growing in a different pattern or have a different colour, but will have the same spicules. The colour of the sponge is a consequence of water chemistry, depth, light and the presence of algae living on the skeleton (sponges form a symbiotic relationship with algae - just like corals).
Scanning electron micrograph images of a selection of sponge spicules. Not to scale - sizes vary between 0.01 and 1 mm. Photo: PLoS ONE.
Reproduction
Sponges reproduce both asexually and sexually.
1) ASEXUAL
Cells clone themselves to create asexual buds, which either break off and form new colonies of cells (a new sponge) or remain attached to increase the size of the sponge. Internal buds are also formed (called gemmules) which are dormant masses of encapsulated cells that are formed during harsh conditions. They are retained in the ‘parent’ sponge, and then as the parent dies the gemmule falls to the bottom and lays dormant, ready to recolonise when conditions improve. Pretty smart really!
2) SEXUAL
Specialised sex cells form the eggs and the sperm. Sperm is released into the water and drawn into the female sponge through the ostia, to fertilise the egg. Free swimming larvae are released (they are ciliated - small hairs along their structure which enables the larvae to move). These larvae swim in the plankton for a while, and then turn themselves inside out and settle to become a sessile adult.
So how do sponges eat?
Sponges are mostly filter feeders and they eat detritus, plankton, viruses and bacteria. They also absorb dissolved nutrients directly from the water through their pinacocyte cells; each cell is responsible for getting their own food! One species is a little different; it is a predatory sponge called Cladorhiza corona and is found in caves in the Mediterranean. As they are in stagnant water, there isn’t much to filter, so C. corona has developed tentacles that grow around the victim to engulf and digest it. There are also parasitic sponges, such as boring sponges, which encrust shells and corals, creating hollow tubes and passageways. This eventually causes death of the unlucky host. The boring sponge is a problem on oyster and mussel farms as they destroy and weaken their shells.
This close-up could almost pass for a flower in your garden, but is in fact a predatory cave sponge! Photo: NOAA Okeanos Explorer Program.
Ecological significance
Sponges create homes for a variety of different organisms (e.g. snails, shrimps, brittle stars, and fishes) and protect them from predators. For example, one sponge found was 2 meters tall and had 16,000 shrimps inside!
An arrow spider crab seeks shelter in a sponge. Photo: Mildred Heredia https://mildredheredia.com/.
Sponges grow on other animals (e.g. molluscs, barnacles, corals and crabs) which they use as camouflage, for example decorator crabs. Sponges also have mutualistic associations with microbes such as bacteria, micro algae and fungi. The microbes have a protected environment and in return carry out a whole range of survival functions for their host sponge, including metabolic function and removal of waste products. The microbial community within a single sponge can make up 40% of the sponges volume!
Sponges are also an important food source. There aren’t many creatures that can eat a tough-skinned, fibrous, spicule-packed sponge. But the short-list of animals who choose to eat sponges includes some of the most delightful animals on the reef! The most famous animals to feast on sponges are hawksbill turtles, whose diet consists of nearly 80% sponge material. Other animals include many nudibranchs, most angelfish, and several species of filefish and occasionally some parrotfish. Sponges have few predators, as many synthesise toxic substances that make them unpalatable. But turtles seem to know which are chemically undefended. These chemical compounds formed in sponges have been widely researched and used in pharmaceuticals.
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The hawksbill turtle's beak-like mouth enables them to tear and rip sponges off the rocks to feed on them successfully. Photo: Hedvika Michnova.
Human impacts on sponges
A big concern for the decline in marine sponges is the bath sponge trade. Sponges have been used to help wash skin since the Bronze Age 4000 yrs ago! A sponge can hold up to 35 times its weight in water but takes up 5 years to grow to a marketable size. In the 1940s the Florida sponge fleet in Key West had >350 ships and employed ~1400 people. The sponge harvest ceased in the 1940s due to over-collection, toxic algae blooms and a fungal disease that wiped out the sponge beds. Due to this decline, synthetic sponges (made of plastic) are now used by the global market.
As we all know, the main threat to our oceans is climate change. But seeing as sponges have survived through countless fluctuations in climate over millions of years, why do current changes in our oceans pose such a threat? It’s in fact not the sponge itself at risk, but the symbiotic microbes who the sponges have become so heavily dependent on. These microbes are susceptible to slight increases in temperature, which suddenly turn them into harmful, disease causing parasites. Research by Dr. Nicole Webster from the Australian Institute of Marine Science highlights this.
Who would have thought that the spongy masses we frequently see in the underwater world would be so fascinating and have their own story to tell? When you’re next underwater, keep your eyes peeled and see if you can find a member of the Porifera phylum! Send your photos in, we would love to see them!