There is a type of animal extant today that has been on Earth for a very long time. Long before the dinosaurs who walked the earth 100 million years ago and even longer than jellyfish who have floated around the oceans for 550 million years . This group of animals are sessile aquatic creatures and for a long time were classified as plants. If you’ve been swimming in the sea or even walked along the beach surveying what the ebbing tide has left behind, you have surely seen one. They are sponges.
Simple multicellular organisms that have successfully evolved to colonise all the seas and oceans of the world, including the deep-sea. They can even be found in freshwater aquatic environments such as ponds, lakes, and streams. Sponges are filter feeders who use flagella lined cells to create a current that pushes water into their internal canals, where they remove small organic particles from the water. Today there are over 11,000 described species of sponges and as many as 8,500 thought to be awaiting discovery .
The true masters of survival
A study by a team of scientist led by researcher Dr. Gordon Love from the University of California analysed the fossil record for traces of a steroid biomarker produced by a common class of sponges . This class of sponges is the demosponges and they make up over 80% of known sponge species. The presence of this steroid biomarker in fossils of known ages confirmed that sponges are one of the evolutionary oldest animals present on our planet today, having survived at least the last 635 million years .
These ancient creatures have survived so long because of their simple structure which allows them to adapt and evolve quickly, to many different environments. The cells of sponges are capable of differentiating into functional cell types, however, sponges lack the ‘true’ tissues, organs, and systems associated with more complex animals .
Given their long evolutionary history, coupled with a simple organisational structure, it is unsurprising that sponges have developed a large number of symbiotic relationships with aquatic microbes . Aquatic microbes are microscopic organisms present in the water such as bacteria, microalgae, and fungi. These mutually-beneficial relationships have allowed sponges to evolve into many new distinct species with an astonishing variety of shapes, sizes, and forms.
The relationship between sponges and microbes is mutually-beneficial as the microbes receive a protected environment and in return carry out a whole range of survival functions for their host sponge, including metabolic function and removal of waste products. The microbe species community within a single sponge is incredibly diverse and abundant, with microbes often accounting for over 40% of the sponge’s volume.
The reason why sea sponges are threatened today
It is known that the main threat to our oceans and the marine animals is climate change. But given that sponges have survived through countless fluctuations in climates over millions of years, why do these future changes in our oceans pose such a threat?
It is, in fact, these tiny microbes, who the sponges have come to so heavily depend on, that are susceptible to slight rises in temperature . Research by Dr. Nicole Webster from the Australian Institute of Marine Science highlights how the delicate equilibrium of the symbiotic relationship between marine hosts and their microbes will be affected by rising sea-surface temperatures.
Sea-surface temperatures of 33 degrees Celsius or higher cause a breakdown in the relationship between the sponges and their microbes, where these once helpful microbes become harmful disease causing parasites to their sponge hosts . Based on current climate change projections and the resulting degeneration of microbe-host relationships, the future survival of sponges is an uncertain one at best.