By Samuel Georgian, Marine Biogeographer at Marine Conservation Institute
When you think of a ‘sponge’, you may immediately picture the dish sponge you use to wash dishes or mop up spills. Did you know that sponges are also animals that occur throughout our world’s oceans and in some freshwater lakes and rivers? In fact, it was these animals that were originally used as dish sponges, bath sponges (as far back as ancient Greece), and even rudimentary tampons. In modern times, your dish sponge is more likely to be made from some type of polyester, and you may never have seen a natural sponge. However, these animals are important members of healthy benthic communities, contributing significantly to habitat creation and ecosystem functioning.
Sponges are members of the animal phylum Porifera. They are the oldest extant animal phylum, evolving over 600 million years ago. As a result, they have a relatively simple structure comprised of two thin layers of cells surrounding a jelly-like substance called the mesohyl. Sponges use special pores and the motion of thousands of flagella to generate water flow that replenishes oxygen, removes waste, and helps the sponge feed. Most sponges are filter feeders, relying on the water flow they generate to feed on bacteria, plankton, and organic particles. However, some sponges also host photosynthetic symbionts that provide the sponge with sugars and fixed nitrogen, and a few species are even carnivorous – typically predating on small crustaceans. As the most primitive animal group, sponges do not have organs or circulatory or nervous systems, but rather specialized cells that catch and digest prey (choanocyte cells), regulate water flow (porocyte cells), and secrete tiny structural elements known as spicules (sclerocyte cells).
Sponges are divided into three groups based in part on the type and structure of their spicules: Hexactinellida (glass sponges) produce highly organized spicules made from silica, Calcarea (calcareous sponges) produce spicules made from calcium carbonate, and Demospongia (demosponges) produce spicules made from specialized proteins and in some cases silica. Despite their sessile nature and apparent lack of defenses, sponges are not commonly eaten by predators due to the presence of these spicule structures and in some species, potent toxins. These toxins, and some of the secondary metabolites produced by the bacteria they ingest, have also made sponges a focus for biomedical research due to their potential as antifungal, antiviral, antibacterial, and cancer treatments. However, some predators such as Hawksbill sea turtles and many nudibranchs have evolved the ability to safely digest sponges.
In the deep sea, sponges play an important role in creating three-dimensional habitat structures in otherwise sparse environments. Glass sponge reefs have been discovered off the coast of British Columbia that cover hundreds of square miles of seafloor and tower more than 65 feet over the seafloor. Originally, these reefs were only known from the fossil record, believed to have gone extinct over 40 million years ago. The British Columbia reefs were only discovered in 1987, and to date are the only known living example of this incredible habitat. One small glass sponge reef in the Straits of Georgia was found to filter 83,000 liters of water per second – that’s an Olympic sized swimming pool every minute!
Sponges are important carbon sinks in the deep sea – they are capable of filtering more than 90% of bacteria and organic matter from seawater, sequestering carbon at a rapid rate. On the Explorer Seamount in the Pacific Ocean, a Nautilus expedition discovered a huge quantity of large, trumpet-shaped glass sponges, with intricate silicate structures rising over a meter above the seafloor. These sponges were found to provide important habitat for a number of smaller animals including squat lobsters and fish.
Unfortunately, deep-sea sponge habitats are at significant risk from a growing number of threats including climate change, oil and natural gas extraction, seafloor mining, and fisheries. Bottom-trawling fisheries, which drag enormous nets along the seafloor and indiscriminately catch corals and fish, have completely destroyed these habitats in many areas. It is crucial to secure protection for sponge habitats given their role in supporting biodiversity and their slow growth rate and extreme longevity (one sponge was found to be over 11,000 years old – one of the oldest known animals!). However, conserving these ecosystems is extremely difficult because only a small percentage of the deep seafloor has been explored, and we do not know the full extent and distribution of sponge habitats.
Researchers at Marine Conservation Institute use a combination of field surveys and ecological models to increase our understandings of the distribution and biology of deep-sea sponges in and around seamounts. In the summer of 2018, we participated in two deep-sea expeditions off the coast of California in conjunction with Marine Applied Research & Exploration and the NOAA National Marine Sanctuary program. Over the course of these expeditions, we used a remotely operated vehicle (ROV) to visually observe the seafloor and assess the location and abundance of many benthic species – including sponges. By recording the current distribution of sponges, corals, and other key taxa, we can provide a strong, science-based rational to protect areas with high levels of biodiversity.
Since it is not feasible to adequately survey the entire ocean (deep-sea expeditions can cost more than $40,000 per day!), we couple these field observations with habitat suitability models that can predict the distribution of species across vast areas of seafloor using a small number of initial observations. The models link environmental data (such as seafloor features, water chemistry, and sediment type) with locations that are already known to be inhabited, allowing us to predict where we expect to find new communities in unexplored areas. Since these models can be developed using limited field surveys and remotely sensed data, and then used to predict distributions over large areas of the seafloor, they are particularly useful in improving our understanding of deep-sea distributions. Marine Conservation Institute has modeled the distribution of sponges and other deep-sea organisms across seamounts off the coast of New Zealand (see figure above), the glass sponge reefs off the coast of Canada, and the coast of California. These models are our best estimate of where we expect to find these taxa in completely unexplored areas, and will help better inform management and conservation decisions.