by Anshika Singh – Marine sponges belong to phylum ‘Porifera’, which means ‘pore-bearers’. They are generally known as sponges due to their soft, porous and squashy body. They look nothing like the famous cartoon character Spongebob Squarepants. Neither do they resemble the synthetic sponge which you might have seen in your kitchen or bathroom. They are very diverse in their sizes, shapes, and colours. You can easily complete a rainbow by putting different colours of sponges found naturally. While some sponges are quite bright, you also find sponges that are white or dull coloured. Some sponges are only a few millimeters in size, while others such as the Giant Barrel sponge, found in deep oceans, can easily accommodate a 6 ft-tall human adult inside its hollow body. They can be different shapes such as cylindrical, spherical, barrel, tubular, and fan-shaped. Some sponges can also be irregular and mat-like (i.e. encrusting sponges). There are about 9000 species of sponges known worldwide and about 500 species are present in India alone. They are distributed widely, right from the deep oceans to the rocky intertidal zones, the frozen poles to the sun-baked tropics, from brackish waters to freshwater ponds, tanks, and rivers.
Sponges are the simplest multicellular animals and lack any of the advanced systems (such as vision, locomotion, digestion, excretion, circulation and nervous systems). The sponge body has few cell types freely scattered in the mesophyl (which is a jelly-like structure between its inner and outer cell layers). Sponges are very efficient filter-feeders. They can filter more than twice their body volume in a minute. They obtain their nutrition and oxygen by filtering a large amount of the surrounding seawater. They feed on very small plankton and are purely vegetarians. However, one group of sponges growing in low nutrient regions have become non-vegetarians by capturing small crustaceans in their porous bodies and slowing digesting them with the help of symbiotic microbes.
Marine sponges have been reported to thrive successfully in areas with high anthropogenic pollution. This is because they act as the aqua-guards of our oceans. They can quickly soak up the pollutants such as heavy metals, crude oil and organic pollutants (polychlorobiphenyl contamination) from the surrounding water during their filter-feeding activity. Reports suggested that marine sponges can induce synthesis of metallothioneins (MTs) in response to heavy metal pollution which help in detoxification by binding to heavy metals (such as Cu, H, Pb, Cr, V, and Zn). Marine sponges can also accumulate trace metals (Ag, Cd, Co, Mn, and Se) by producing some bioactive ligands (secondary metabolites) within their bodies. Trace metals (Fe and Se) are required for the production of the skeletal system in the sponges and are hence utilized for their growth and development. Sponge symbionts play an important role in helping the host sponge to survive in polluted regions. Symbionts can transformation organic pollutants (PCB) into harmless by-products. Future research in this field will help to better understand the process of microbial biotransformation, identify the specific microbial species and enzyme systems responsible for the detoxification of persistent xenobiotics in sponges. One study conducted in Florida Bay showed that marine sponges are responsible for controlling the phytoplankton population. In the absence of marine sponges, there was a rise in phytoplankton bloom. However, different marine sponges have a different rate of bioaccumulation of pollutants, attributed to specific morphological features and/or to the specific bacterial community rather than their filtration abilities.
Marine sponges are good caretakers of our marine ecosystems due to their simple body structure, wide abundance, species richness, filter-feeding, and sessile (non-locomotory) lifestyle. They continuously filter a large amount of surrounding seawater and are able to remove a wide range of pollutants, right from heavy metals to organic pollutants. On top of this, they are extremely tolerant of these pollutants and have proper detoxification systems in place. Marine sponges also act as a very suitable system to monitor environmental health. It is important to identify monitoring parameters and to determine the most appropriate biological indicators for long-term monitoring. These parameters can be listed and designed in the form of user-friendly kits to allow the common public to take part in such environmental monitoring programs. Although marine sponges are doing their jobs very efficiently, it is the need of the hour not only to appreciate their role but also take necessary steps for their conservation. A suitable science-citizen program will allow the public to take responsibility, get connected to the marine world and understand the concept of
“ONE OCEAN -ONE HEALTH”
It is important to maintain our marine ecosystem to sustain our own existence on this blue planet.
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