Story and photos by Darelle Snyman
Marine sponges are fascinating creatures that have existed for over 600 million years. These ancient creatures are common Caribbean citizens, gracing the underwater world in a staggering array of colors and shapes. The most compelling aspects of the life of these unassuming filter feeders are the intricate relationships they have forged with an array of marine organisms. Their diverse community of symbiotic partners makes them important role players within the marine ecosystem. So let’s dive into the world of marine sponge relationships.
The constant flow of water and abundant food in these porous critters makes them attractive real estate for a host of tenants. The giant barrel sponge (Xestospongia muta), with its large barrel-shaped body, is just the type of sponge that will attract tenants. It is for this reason that I love inspecting them for sponge dwellers when I encounter them. For a start, you might find small fish and shrimps inside them, scavenging for food particles. For shrimps, sponges not only offer shelter but also a banquet of delicacies. The nimble pincers of these tiny crustaceans pick away at parasites and debris clinging to the sponge surface, providing it with a dedicated cleaning service.
Some shrimp species set up permanent residence in their sponge hosts and certain sponges on coral reefs along the Western Atlantic house just such remarkable creatures. It is thanks to this article that I stumbled across the truly unique regal shrimp (Synalpheus regalis), a snapping shrimp species. It is not the fact that they associate exclusively with sponges that make them unique, but that they live in a society with a hierarchical setup that resembles that of bees and ants, making them one of the very few eusocial creatures in the marine world. A colony of these tiny shrimps that can number over 300 consists of a single breeding female, the queen. Her minions who take care of the colony forage for food and defend it. They do not cohabitate well and it is up to the larger individuals in the colony to fiercely defend their home, directing their wrath especially to other shrimp species.
Finding these translucent orange critters will not be easy as they are so small and well hidden within their host. Their penchant to only associate with one or a few species of sponges means that when you are looking for these critters you are basically looking for a limited set of sponges. In Belize, for example, it was found that they mostly associate with three sponge species: Neopetrosia proxima, N. Subtriangularis and Hyatella intestinal.
Shrimps are not the only crustaceans attracted to this prime real estate with its smorgasbord of food; spider crabs and pea crabs can often be found in the nooks and crannies of sponges. Some crabs remove pieces of a living sponge, decorating their carapace to blend into its surroundings. A Caribbean resident that employs this technique is the spongy decorator crab (Macrocoeloma trispinosum). This industrious creature not only glues pieces of sponge to its shell but will add hydroids and algae, creating a living collage of camouflage. These picky gardeners prefer brightly colored sponges that closely match their environment and replace their carefully selected decorations with fresh ones when they become worn out. This relationship is mutually beneficial as the sponges continue to survive and feed wherever the crab takes them, although I think the crab has the better end of the deal.
A spiny echinoderm that has forged a very lucrative relationship with sponges when it comes to feeding is the brittle star. One such aptly named species is the sponge brittle star (Ophiothrix suensoni). These can often be seen draped across sponges, and if you ever dived or snorkeled in Bocas del Toro, Panama, a sponge encrusted with multiples of these curious creatures, feeding on a sponge-provided feast of plankton and detritus, is not such an uncommon sight. Brittle stars do not only graze on the sponge surface, but they are also in the unique position to access the morsels hidden inside. Their long, squiggly arms are perfectly designed to explore the labyrinth of canals that make up the sponge’s body. The relationship between brittle stars and sponges is one of commensalism, where the one reaps all the benefits from the relationship and the other is not harmed.
Another curious creature that takes advantage of the hospitality of sponges to form an unlikely partnership is the sponge zoanthid (Umimayanthus parasiticus), a member of the phylum Cnidaria, which includes corals and jellyfish. I encountered many a sponge in Bocas del Toro encrusted with the tiny polyps of these unique critters, creating a living tapestry across the surface of the sponge. Their fleshy polyps consist of a central oral disc surrounded by two rows of tentacles that can give the sponge a fuzzy appearance. The sponge zoanthid can be recognized by its pale central disc with a brown circle inside. This successful partnership provides the zoanthid with a safe and supportive haven to feed from, while the sponge gets rewarded by sharing in the food bounty grabbed by the multitude of little tentacles that dot their surface.
Another common Caribbean zoanthid that frequents sponges is the golden zoanthid (Parazoanthus swiftii). I have seen them encrusting green finger sponges (Iotrochota birotulata), where their bright yellow or orange bodies contrast beautifully with the green of the sponge. Their vivid color advertises their toxicity and may deter predatory fish like the rock beauty (Holocanthus tricolor) from taking bites out of its sponge host.
Not all of these relationships are created equal and a sponge dweller that is not just a casual tenant but actually a parasite is the sponge threadworm (Haplosyllis spongicola). These tiny, white, segmented worms live parasitically on several sponge species around the world. In the Caribbean these tiny invaders are often associated with the touch-me-not sponge (Neofibularia nolitangere), where they appear as small white shapes embedded in the sponge tissue. They feed by burrowing into the sponge, sucking out the contents of individual cells. Despite their parasitic nature the worms do not kill their host, even though there can be tens of thousands of worms living in a single sponge. Their presence has enticed several species of small goby (Elacatinus spp.) to also set up residence inside the sponges where they feed almost exclusively on the worms.
The true nature of the relationship between a sponge and its squiggly residents is still not fully understood. Some scientists believe that the sponges do reap some benefit from the interaction; others believe that the worms are true parasites reaping all the benefit.
The most vital symbiotic associations in a sponge’s life are those that happen unseen to the naked eye. Sponges have established intricate associations with a host of microbial allies, from bacteria and archaea to algae, which play a crucial role in the sponge’s overall health. Of these, bacteria are the most common and can number in the trillions. Some break down organic matter into a form the sponge can absorb, others fix nitrogen, an essential but often scarce resource within many marine environments. Photosynthetic bacteria and algae in turn supplement the sponge’s filter-feeding diet with vital nutrients. The benefits of these collaborations, however, extend beyond basic nutrition. Some of these microbial marvels produce potent chemicals that not only deter spongivorous specialists such as turtles and some nudibranchs, but also provide protection against disease, and even help the sponge to compete for space. The sponge, in turn, provides its microbial partners with a safe haven and a constant flow of food.
The complex web of partnerships sponges have established are clearly amazing, giving us a greater appreciation for the remarkable ways animals have adapted to survive and thrive. So the next time you encounter these seemingly simple creatures underwater, take a closer look. You might even be lucky enough to witness some of these captivating interactions.
