We start off this year’s first blog with an abundance of color! Sponges (Phylum Porifera) are amongst the most ancient animals living in the world’s oceans, occurring all over the planet in a myriad of shapes, sizes and colors. Sponges are also some of the world’s simplest animals. Despite this simplicity they are known to form highly structured habitats known as sponge grounds, aggregations, gardens or reefs. These naturally ‘engineered’ habitats play key ecological roles in the worlds oceans and seas:
➜ “Serving as shelter and a nursery;
➜ Providing food for numerous other species of invertebrates and fish;
➜ Mediating the transfer of energy between the benthic (seabed) and pelagic (open sea) systems;
➜ Participating in biogeochemical cycling processes” .
Sponges have no heads, eyes, tails, or mouths, and they live most of their lives rooted in one spot. Their bodies are full of channels and tiny holes called pores. They do not have nervous, digestive or circulatory systems. Instead, sponges rely on maintaining a constant water flow through the channels and pores to filter out small particles of food . They can filter an amount of water 100,000 times their size each day! That means a basketball-sized sponge could filter an entire residential pool in a single day . Sponges also use the channels and pores to replenish their oxygen supply and to remove wastes.
The size and shape of sponges varies immensely with some species growing bigger than a fridge and living for more than 100 years. Some deep-water sponges are even known to live to be over 200 years old! Sponges often use chemicals to deter predators from eating them. Scientists have discovered that some of these chemicals may have potential to treat cancer and HIV .
Globally there are between 5.000 to 10.000 sponge species. Approximately 700 of these are known to occur in the Mediterranean Sea. The first written records of sponges for the Aegean region were provided by the celebrated Greek author Homer and philosopher Aristotle. To help you get started identifying sponges in this part of the world we present below ten different species commonly found in the Aegean. Their ranking below is a rough representation of the depths at which you will find them in Porto Rafti (starting with the shallow sponges) as well as their overall occurrence in these waters.
Bath Sponge - Spongia officinalis
Depth range: 1-100m
Habitat: rocks, sand, walls
Did you know that the use of bath sponges for bathing and other purposes originated in Greece? From here it spread to the rest of Europe during the Middle Ages and then the whole world! The term ‘sponge’ derives from the ancient Greek word σπογγοσ (spóngos) and sponge fishing has been in practice in the Mediterranean since ancient times. Aristotle even wrote of it as far back as around 350 BC. Traditionally, sponge fishing was practiced by freedivers (descending on a single breath) who dove underwater to collect sponges.
It therefore only makes sense that we start our Top 10 with the sponge that was at the heart of Greece’s fame, the Bath Sponge. Individuals grow in large lobes with small openings and are light grey to black in color. Aside from using sponges for washing they were also used for padding in Roman soldiers helmets, as absorbent material during surgeries, as medicine to help digestive issues, and as a primitive ‘contraceptive sponge’. “Today, sponges are still used for washing and are also used for recreational purposes, like sponge painting.” 
Yellow Tube Sponge - Aplysina aerophoba
Depth range: 1-20m
Habitat: sand, rocks
The Yellow Tube Sponge is a yellow, tube-forming or encrusting sponge commonly occurring in the shallower waters of the Mediterranean Sea and with a clear preference of sunlit locations. The surface of this sponge is slippery to the touch and its texture is firm and rubbery. It can form colonies over one meter in diameter with individual tubes growing up to 4 cm long and 2.5 cm wide. When removed from water, this sponge turns blue, leading to its specific name "aerophoba" (Greek: "fear of air") . It’s bright color and beautiful structure make it a favorite among underwater photographers and videographers.
Kidney Sponge - Chondrosia reniformis
Depth range: 1-30m
Habitat: rocks, stones, mollusk shells, calcareous algae, generally away from sunlight
The Kidney Sponge is a commonly occurring sponge in the Mediterranean that can grow up to 4cm thick covering large surface areas. Its color varies with the light intensity that it is exposed to. The brighter the light the darker its surface will be. The Kidney Sponge is very rich in collagen, the main protein providing structural and biochemical support to the cells making up our body’s tissues. Collagen is therefore a prized material for many applications dealing with human health and wellbeing, including regenerative medicine (in other words wound healing, drugs and gene delivery/carrier; cosmetic, food industry, etcetera).
Black (Leather) Sponge - Sarcotragus spinosulus
Depth range: 1-300m, especially common between 5 and 25m
Habitat: rocks, sand, walls, seagrass, caves
The Black Leather Sponge is an often large, horizontally flattened sponge occurring either in black or dark grey-brown (light-brown inside). It commonly takes on a rounded massive form, often with a slightly retracted base giving it a subspherical appearance. In Porto Rafti you’ll find some approaching the size of car tires occasionally with fish resting on their surface overlooking their surroundings. In 2011 chemical investigation of this sponge led to the isolation of a new chemical compound along with two already known compounds that can be used to treat leukemia (blood cancer).
Orange-Red Encrusting Sponge / Oyster Sponge - Crambe Crambe
Depth range: 1-60m
Habitat: rocks, stones, corals, generally preferring dimly-lit areas
The Orange-Red Encrusting Sponge is endemic (native) to the Mediterranean Sea varying from dark orange to red in color. Its colonies can be commonly found in the shallower waters forming thin orange to orange-red plates with a very rough surface perforated by raised openings found along the ‘breathing’ channels. These colonies can cover a surface of over 1m². Whilst preferring hard substrate this sponge is occasionally found growing on sea grass and mollusks. The Orange-Red Encrusting Sponge can be toxic to some animals living in the Big Blue.
‘Horny Sponge’ - Ircinia retidermata
Depth range: 1-300m
Habitat: sand, rocks, mud
Back in the 1980’s this species of ‘horny sponge’ was (still) described as a rarity in the Mediterranean Sea. These past few decades, however, this sponge has rapidly increased in abundance. Scientists account this leap from ‘few to plenty’ to environmental change (including climatological changes), suggesting that the conditions of the last few decades have been much more favorable for the growth and territorial expansion of this sponge. The outcome of the 2013 survey revealing the rapid territorial expansion has lead to scientists questioning the current common belief that rare species in semi-enclosed seas are prone to extinction.
Yellow Boring Sponge - Cliona celata
Depth range: 1-200m
Habitat: rock, stone, mollusks, seagrass
The Yellow Boring Sponge is a commonly occurring sponge that can be found all over the planet. It lives inside shells or calcareous rocks which it perforates (bores) into. Interestingly this species of sponge can be found in three different types of forms depending on its stage of development and the part of the world. In its earliest stage the sponge is barely visibly appearing only as specs in the limestone rocks that it ‘invades’. In its next stage of development it encrusts the surface of the rock eroding the material below forming a clearly visible yellow layer. In its largest form it appear as a more voluminous massive sponge. In the Mediterranean Sea it is mostly found as an encrusting sponge living inside shells or calcareous rocks which it perforates (bores). In the Atlantic Ocean the Yellow Boring Sponge mainly forms large and fleshy bright yellow masses, often isolated on poorly inhabited rocks.
Orange Crater Sponge / Orange Agelas - Agelas oroides
Depth range: 1-150m
Habitat: rocks, corals, crevices, caves
The Orange Crater Sponge is a common massive sponge in the Mediterranean Sea. It inhabits sheltered habitats with low light intensity. This sponge has an important ecological role, increasing the structural complexity of its surroundings and consequently providing a habitat for the invertebrates and fish that dwell in and on it.
The Orange Crater Sponge has also proven to be an important source of many new natural products, some with demonstrated bioactivity.
Stony Sponge - Petrosia ficiformis
Depth range: 1-80m
Habitat: rocks, stones, corals, generally preferring sunny places
The Stoney Sponge varies tremendously in shape and size, may be found both covering small and large surface areas (~1m2) and can grow up to 10cm thick. Its surface has a rather hard and tough feel to it and it is usually purple brown in color due to symbiosis with photosynthetic cyanobacteria. In absence of light the cyanobacteria will be much smaller in numbers (if any) resulting in a white coloration. Despite its name, this sponge is actually a fragile species. The Stoney Sponge is the main and preferred food of the Discodoris nudibranch (Peltodoris atromaculata) which eats away sitting on the sponges surface. The Discodoris accumulates chemical compounds of the sponge on their digestive tract as well as using the compounds for defense .
Common Antlers Sponge - Axinella polypoides
Depth range: 10-100m
Habitat: rock, stone, calcareous algae
The Common Antlers Sponge is a species of sea sponge that derives its common name from its close resemblance to the antlers of deerlike species. These antlerlike branches are often positioned in one plane. Whilst frequently occurring in the Mediterranean Sea and connecting Atlantic coastline it can also be found in the colder seas of Northern Europe. The branches of this sponge are rounded and its surface is smooth and has a yellow to orange color. The body length usually varies between 20cm and 25cm.
1] Maria del Mar Otero, Fabrizio Serena, Vasilis Gerovasileiou, Monica Barone, Marzia Bo, José Manuel Arcos, Antonio Vulcano, Joana Xavier (2019). Identification guide of vulnerable species incidentally caught in Mediterranean fisheries.
2] Eleni Voultsiadou (2005) Sponge diversity in the Aegean Sea: Check list and new information, Italian Journal of Zoology, 72:1, 53-64, DOI: 10.1080/11250000509356653
3] Sanibel Sea School (2015). 5 Facts About Sponges.
5] Global Biodiversity Information Facility.