No Free Lunch! Sponge Life on Coral Reefs
Updated: Mar 23
The Roles and Services of Sponges in Coral Reefs
M. C. Diaz
“Nature gives us hundreds of services for free." This is one of the messages that our beloved naturalist and myrmecologist (ant expert), Dr. Edward . O Wilson repeatedly quoted, letting “this fast human world” know that Ecology means Economy. I heard this quote from an interview to E.O. Wilson as part of “The Arts and Lectures” series, and it made me think about the role and services that marine sponges offer for free in all seas.
Figure 1. Can you see the twelve Sponge species I see in this picture of a Mexican reef?. Photo by Philippe_Hamilton (Ocean Image Bank). Objectively speaking sponges are conspicuous inhabitants in coral reefs.
SPONGES ARE EVERYWHERE
Sponges are not casual components of the fauna in coral reefs worldwide. From fringing coral reefs, atolls, and patch reefs, to deeper reef escarpments, reef walls, or mesophotic reefs (50-150 m deep), sponges surpass any animal phyla in terms of species richness and living biomass in most tropical coral reefs (Diaz and Rützler, 2001; Reed et al., 2018). In the Caribbean, the number of different sponge species at any reef ranges from 50-300 species, and a large difference on species composition can be found among reefs within “15 km” (Hooper, 2018).
Figure 2. An IntelliReef prototype in Sint. Martin after 20 months underwater. The massive ramose salmon sponge, Desmapsanma anchorata, cradling a recruit of a fire coral Millepora species. Photo by: The Nature Foundation
Either as emergent species with bodies built onto the water column as tubes, barrels, branches, and balls, or as thin extensive crusts that thrive on cryptic habitats (crevices, or caverns), sponges play a significant role in the quality of water surrounding a reef. Sponges filter large amounts (up to 1 l per cm3 per hour, sensu Reiswig, 1976) of sea water with the help of the myriad of microbes inhabiting their bodies. With an almost continued movement and transformation of the water processed by these organisms, sponges are known to be major players in the processing of main nutrients like Nitrogen (a major compound for proteins), Phosphorous (an element essential for energy transfer) and Carbon (the fundamental element of life compounds on earth). Using stable isotopic tracer methods, radio-labeled isotopes, and recovery of gene specific sequences for key enzymes, scientists have found that sponges mediate several microbial metabolisms, including Photosynthesis (Phototrophs >3:1 photosynthesis to respiration ratio), nitrification, N2 fixation, denitrification, sulfate reduction, and anaerobic ammonium oxidation (anammox) (Fiore et al., 2013). These processes are essential to the health and overall productivity of a reef's ecosystem.
Figure 3. An experiment where a seawater solution of Fluorescin dye is applied to a sponge surface and within seconds the dyed water is expelled through the oscula (Image taken from The shape of Life video I on Sponges).
Figure 4. The common Caribbean reef sponge Svenzea zeai, given its biomass in the Fore reef at Carrie Bow Cay in Belize, presents the highest potential Benthic nitrification rates at 6-10.9 mol/day m2 (oxidation of Ammonium to Nitrate) (Diaz and Ward, 1997)
SHELTER, PROTECTION, and FOOD
In addition to their filtering capabilities, sponges offer a home to hundreds of species of invertebrates and fishes by creating space for protection or “hiding” within and around their bodies (Lesser and Slattery, 2013). Many reef fish--mostly Holacanthus, Pomacanthus, Lactophrys, Cantherhines, and parrot fish (Wulff, 2020)-- also nibble on sponges for food, while seastars and turtles might eat big chunks of sponges as part of their diet. In Florida, Manatees have even been found eating specific specimens of Chondrilla caribensis.
The photo below encompasses the 400% increase in coral cover over a 12 month period, as well as the astonishing sponge species, doing the "Ocean Cleanup" work they are designed for in "The Invisible Loop: Sponges Allow Reefs to Flourish in a Marine Desert"
Figure 5. An intrepid hermit crab parks comfortably inside this gigantic Callyspongia plicifera, on a reef on the island of Martinica. Photo by; M.C.Diaz.
Predation is a key factor in the distribution and abundance of coral reef sponges.
For coral reefs in the Caribbean, major sponge predators are opisthobranch mollusks, hawksbill turtles, and a few fish species, including angelfish (Holacanthus and Pomacanthus spp.), trunkfish (Lactophrys spp.), filefish (Cantherhines spp.) and parrotfish. The presence of sponges in these habitats helps support a healthy food web by providing nourishment to a variety of different species.
Figure.6. Central station with many sponges being scrutinized. Photo by Philippe_Hamilton (Ocean Image Bank).
FINDING ROOM TO GROW
There is only so much room to grow on the foundational rock structures of reefs, making open space a precious commodity for many organisms. Sponges have shown unique strategies while competing for space on a reef by either growing in the water column, spreading over other organisms (epizoism) or capturing space and sequestering it through chemical defenses. (Aerts, 1998; Bell y Barnes, 2003). These chemical defenses and other types of secondary metabolites have given sponges significant potential for their use in Biomedical products.
Figure 7. Scopalina ruetzleri growing over an Agelas wiedenmayeri short tube (left). Many species on the reef avoid space competition by growing on the water column. Here we see a branching species of Agelas, and a white tube of a Calcarea species of the genus Leucetta (right).
WHAT WE KNOW, and WHAT WE DON'T KNOW
9,493 species of sponges have been scientifically classified in the phylum Porifera (de Voogd et. al 2022). However, experts believe that the total species number of sponges in the seas probably doubles our current recognized number (Van Soest et al., 2012), and most aspects of the biology and ecology of those species are unknown (Lesser et al., 2009). Each one of these sponge species represents its own evolutionary pathway that has given rise to a unique biological, physical and chemical architectures. As Dr E.O. Wilson stated during the opening talk at the first Biodiversity genomics conference in Washington DC (2015) We need a “Linnean renaissance,” combining classical morphologic and natural history observations with most modern techniques to discover all diverse forms of life and their unique living pathways. Similarly to much of the marine world, there's a lot we still don't know about sponges and all they are capable of. However, the knowledge we do have about the roles of sponges proves their importance to a healthy reef, because without them, the cycle of life, oxygen, and additional species "food banks" will be lost!
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