Shape-shifters on Caribbean Coral Reefs


Corals and sponges create the vibrant and varied panoramas of Caribbean reefs. It is, however, the versatility and services of sponge organisms at the cellular level that hold the secret of sponges. Dr Anjani Ganase explains the marvellous science of sponges.

As abundant as corals on Caribbean reefs are marine sponges. Snorkelers and divers often mistake them for corals because of their high prevalence on coral reefs; they add structure to the reefscape and stand out from the background in bright purple, orange, yellow and blue against the dominant brown colouration of Caribbean corals. Marine sponges may appear to compete with corals for space and attention but they are intimately dependent on them for the infrastructure to settle and grow.

Marine sponges (Phylum Porifera) are relatively simple animals that lack a digestive tract and a nervous system. Sponges are impressive filter feeders, where some species can pump thousands of litres of water within an hour. This filter feeding power has significant impact on the water quality of their surroundings; they remove not just planktonic material – bacteria and particulates - but also dissolved organic matter from the water column. And they do more. Through the rapid replacement of sponge cells, sponges essentially convert inaccessible dissolved nutrients into a form that is available to reef fauna as food (dead sponge material). Researchers are now looking into the significance of their contribution from this simple process, which is now being referred to as the “sponge loop” (de Goeij et al 2013).

The physical formation of marine sponges is considered primitive, made up of very few “layers”. Between the inner canal of the sponge and the external layer, sponge cells roam around transporting nutrients, food and gases from one place to another. Yet, there is sophistication in this simplicity; and it is likely the reason that marine sponges occur in almost every marine benthic habitat around the world. Although a sponge is considered a colony of cells, individual sponge cells carry out specific roles based on their location for the overall function for feeding and survival as a whole. However, sponge cells easily change their roles, can relocate through the inner space of the sponge, and even undergo physical changes for new roles. This feature is referred to as totipotency, similar to stem cells: when a role is given to a sponge cell, it differentiates to carry out specific functions for the sponge. However, unlike stem cells, these roles can be reversed and so the cells can revert to the totipotent state before being assigned to other new roles. For this reason, sponges are not considered to have true distinct or permanent layers.
 
Several species of tube, vase and branching sponges stand out on a coral reef at Mayreau Gardens, St. Vincent and the Grenadines. Photo by Anjani Ganase courtesy XL Catlin Seaview Survey


On Caribbean reefs, sponges come in all shapes and sizes - massive, branching, tube and encrusting shapes. However, marine sponges can be quite amorphous, often driven by their physical environments, such as wave action, predation, sedimentation and even light. The combined effects of these factors govern the community composition of marine sponges and their morphology. Where water movement is minimal, the sponge morphology and the orientation of its canal system maximise filtration even in the calmest water.  Where there is a lot of sediment, such as deeper areas at the base of the reef slope, sponge species have adapted their shapes to vases or shapes that stand out from the substrate to avoid clogging of their canals. Sponges may be flatter in shallower environments with higher water movement to limit the frictional pressure on the body and to avoid being dislodged.

Some sponges, like corals, partner with photosynthetic symbionts to grow surfaces that maximize light capture. The skeleton framework of such sponges is structured by spicules – short interconnecting needle-like shards of glass or calcium carbonate. The formation of the spicules allows for plasticity in the shapes of certain sponge species. These flexibilities of marine sponges allow them to  occupy tropical coral reefs and reside in marine habitats all over the ocean from deep ocean seamounts to polar benthic communities. These traits also make marine sponges notoriously difficult to identify in the field because their physical traits can shift and change for their environment. Sponge species are identified by observing their skeleton structure as well as composition and cell characteristics.

Coral reef sponges serve crucial ecological functions. The structure of large barrel and tube sponges create habitat spaces for marine creatures to utilise and hide in. Other sponges grow between the spaces of coral rubble and substrate binding the coral fragments and sand particles together (Bell et al 2005). By doing this, these sponges stabilise reef substrate and allow other organisms to grow without being toppled by water movement. Another group, known as bio-eroding sponges, erode the reef by boring into the calcium carbonate skeleton framework of the reef and create spaces for others organisms. As sessile filter feeder animals, many sponge species house microbial communities that carry out important nutrient cycles, such as cyanobacteria that cycle nitrogen on the reef (Bell et al 2005). The diversity of ecological functions is reflected in the diversity of sponge species.

Researchers have recently discovered an untapped trove of chemical resources provided by sponges. This array of chemical compounds is produced naturally to serve as defences against predators and competitors. The unpleasant array of chemicals is capable of deterring most predators with the exception of a few animals such as the hawksbill turtle and nudibranchs. Scientists have discovered that many compounds produced by sponges have antibacterial, anti-viral and anti-tumor properties, useful in the treatment of HIV, malaria and cancer (Anjum et al 2016). Research in replicating sponge chemical compounds for use in pharmaceuticals is an exploding field.

Here in Trinidad and Tobago, we have unique and diverse communities of sponges on our coral reefs. Barrel sponges litter our northeast reefs and extend to great depths. Yet very little research has been done to understand the ecology of our marine sponges and their value. By protecting and managing our reefs, we’re not just conserving reef health but also securing the research potential for scientific discoveries.
 
Bright coloured sponge, Aplysina archeri shot in the dark on a Curaçao coral reef. Photo by Anjani Ganase

References:
Anjum K, Abbas SQ, Shah SA, Akhter N, Batool S, ul Hassan SS. Marine sponges as a drug treasure. Biomolecules & therapeutics. 2016 Jul;24(4):347.

Bell JJ. The functional roles of marine sponges. Estuarine, coastal and shelf science. 2008 Sep 10;79(3):341-53.

De Goeij JM, Van Oevelen D, Vermeij MJ, Osinga R, Middelburg JJ, de Goeij AF, Admiraal W. Surviving in a marine desert: the sponge loop retains resources within coral reefs. Science. 2013 Oct 4;342(6154):108-10.


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