Sponges: Those adorable squishy creatures of the sea

Dr. Maria Marra, Guest Blog Author.

Sponges in the CÚRAM exhibit at Galway Atlantaquaria. The picture is a close-up of the sponge tank showing the different species hosted (the sponges are the orange and pale-yellow organisms!).

The CÚRAM exhibit at Galway Atlantaquaria. The picture above shows the part of the exhibit dedicated to the sponges, but there are other organisms and their fascinating applications to discover too, like the barnacles and the beautiful diatoms and coccolithophores!

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Over the last few weeks, we explored the rockpools along the Wild Atlantic Way and we encountered some of their most typical residents, like the sea anemone, the limpet, the hermit crab, and even the seaweed. In our journey, we have had the opportunity to learn a lot about biology and ecology of these organisms as well as how they are used by humans, and even sometimes the role they play in Irish folklore. However, the rockpools host many other types of organisms, some of which are less known or not known at all by most people, and the perfect example of this are the sea sponges.

Some of you might say: what does a sponge even look like? When they hear the word ‘sponge’, many people immediately think of the sponges they use to clean their dishes after a meal, some would remember the yellow soft sponges that are used especially to give babies a bath (and mind, that actually is a correct answer!), and possibly most of us would think that we are just talking about some sort of weird plants! Actually, there is a bit of truth in all of these ideas, but let’s proceed one step at a time.
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Sponges are the ancestor of all animals, us human beings included, believe it or not! They are extremely simple organisms made of a bunch of different types of cells immersed in a gelatinous substance. The outer surface of sponges is delimited by a skin-like layer, and their body is pierced by multiple pores called ostia which lead to a more or less complex system of internal canals, also lined with a similar skin-like layer. This structure of the body is what gives these animals the scientific name of ‘Porifera’, that means ‘pore bearers’. Through the ostia, the sea water can get into the body of a sponge and circulate inside through its canals, from where the water gets in contact with specialised cells called choanocytes, which are sometimes organised in distinct chambers. These curiously shaped cells (have a look at the diagram below to get to know them) have a number of projections arranged like a skirt that act like a sieve, filtering from the water all the bacteria and other small organisms and organic material the sponge feeds on. Once well filtered, the water is expelled from the body of the sponge through larger openings on its surface called oscula. The presence of this complex water system makes the sponge body very porous, which explains why they can absorb so much water and why they have been used for so long by humans to wash surfaces, from our own skin to our cars. 

Diagram showing how the sponge body is organised. The choanocyte, beside the ‘skirt’ of cell projections called microvilli, has also a much longer projection called ‘flagellum’, which is located centrally within the ring of microvilli and, with its whip-like movements, creates the water flow in through the ostia and outside the sponge body through the oscula.

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The apparent simplicity of these animals and their nature as filter-feeders (the term we give to animals that filter from the water the tiny organisms they feed on) might make you think that sponges are harmless, but you’re wrong if you think so! In fact, despite the lack of protection given by shells, scales, thorns, teeth or any other possible type of physical defence, sponges are on the forefront of intense competition for the substratum that occurs on the seafloor without us knowing it. So how do they compete if they can’t attack their rivals? The answer is actually pretty cool: sponges have become perfect chemical weapons! Similarly to the plants they are often confused with, which cannot pull out their roots from the soil and run away from danger, sponges live attached to the seafloor, so they have evolved an arsenal of more or less complex compounds that can be extremely toxic to the taste of potential predators or the contact with unfortunate neighbours. Don’t worry though, very few sponge species can be considered harmful for humans, and none of them is found in Ireland! And that’s not all: sometimes, the toxic compounds the sponges use for their own defence are not produced by the sponges themselves, but by some microorganisms like bacteria or fungi living in the sponge body. In fact, another fascinating aspect of sponge biology is that these animals act like living condos inhabited by the most diversified communities! Just imagine it, not only a large number of other invertebrates, like small crustaceans and worms, but even seaweed choose to live inside the sponge bodies and to use their canals as a shelter, but sponges can also host inside their tissues a plethora of bacteria, viruses, fungal spores, yeasts and microalgae that sometimes are so crowded they make up 35% of the sponge volume! And microbes can produce an incredibly diverse range of unique chemical compounds, as we know from the high number of pharmaceutical drugs available on the market that are based on them, so sponges can cleverly take advantage of this precious skill of their tenants for their own defence in exchange for nutrients and shelter.
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It’s not surprising then that in the last few decades scientists have been busy studying the compounds produced by the sponges and the microorganisms living in them because it was discovered that these molecules often have pharmacological properties that can be used to treat a large range of human diseases, cancer included. One of these groups of scientists are based in the National University of Ireland Galway and it’s called CÚRAM, and they are studying the slime produced by some Irish sponge species to fight cancer and harmful microbes. In 2021 the CÚRAM team worked on the establishment of a new public exhibition based in Galway Atlantaquaria showcasing how marine resources can aid medical device research. The exhibit is located on the upper floor of the National Aquarium and is fully accessible. Visitors can browse information panels, tanks and models of marine resources that are used in medical device research. This new exhibit investigates how marine-inspired medtech research can heal the body and it also reinforces the message of the importance of Ocean health and conservation, tying in perfectly with the Galway Atlantaquaria’s education programme. One of the main attractions in the CÚRAM exhibit is the sponge tank, which give the opportunity to the general public to get to know these fascinating animals along with other very interesting groups of organisms of large biomedical potential like seaweed, barnacles and microalgae.

The elephant hide sponge, one of the six targets of the ‘Sponge Safari’ citizen science project currently carried on by Seasearch Ireland (photo courtesy of Tony and Rory O’Callaghan).

Now that you have learned so much about sponges, you probably wonder where you can find them. As said at the beginning of this blog post, sponges can be found on the shore when the tide is low: pulling up rocks (but don’t forget to put them back in the same position as you found them!), you might discover underneath a sheet of soft tissue of different colours: orange, red, yellow, green, even pink. To tell which sponge you’re looking at, though, is not necessarily an easy task because there are around 400 different species of sponges that have been identified in the waters of Ireland and the UK, and just a few of them can be reliably identified by eye. However, don’t let this stop you from discovering the diversity of sponges you have at your fingertips: there are great resources out there to start getting familiar with the most common species of sponges, as for instance the ‘Sea squirts and sponges of Britain and Ireland’ guide published by Seasearch in 2018. Also, if you are a diver you can join two great citizen science projects currently carried on by Seasearch Ireland. One is the Sponge Safari, which aims at increasing the number of records of six of the common and easily recognisable sponges that can be reliably identified to species in the field, i.e. the chocolate finger sponge, the crater sponge, the black tar sponge, the boring sponge, the elephant hide sponge, and the purse sponge (all super cool names, aren’t they?!). Seasearch Ireland is also carrying on in collaboration with us at Galway Atlantaquaria a Sponge recording project which aim is to help map the distribution of sponges in Irish waters, particularly cryptic and difficult to identify species, by marrying citizen science observations with microscopy analysis of sponge samples collected by the Seasearch divers.
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We hope you are intrigued now and want to learn more about these super interesting animals! Keep following our blog and come and visit us at Galway Atlantaquaria to learn more about sponges and many other fantastic marine creatures. Keep Discovering!