by Matthew Norton
The ability to move through one’s environment is key the existence of virtually all life on earth. For any multicellular organism many of these movements need to be coordinated to benefit the organism. Most animals achieve this through a system of nerves, which transmit information as electrical signals, and a ‘central processor’ (e.g. a brain) to interpret this information. Some animals groups achieve coordinated movements with very simple nervous systems, but one group, the sponges (Porifera) has no ‘true’ nervous system at all.
Sponges, so called for the pores around their bodies, consist of two cell layers with a middle gelatinous layer which may contain ‘loose’ cells and pieces of carbonate, spongin (protein fibres), or silica (glass), depending on the class of sponge, for structural support. Water flows in through the pores, where specialised feeding cells, choanocytes, filter out particles of food, and then back out through a collective opening, the osculum, along with any waste. They lack any tissues or organs (e.g. mouth, muscles, heart), instead functioning through the individual actions of specialised cell types. These do not include nerve cells, yet many species appear to overcome this limitation and perform behaviours only achievable through cell to cell coordination.
Sponges can contract specific body parts, such as to close their osculum (see above image), and their entire bodies, dramatically shrinking their size in the process, in response to certain stimuli. These stimuli include electric and chemical stimulation, changes in light levels (i.e. changes from day to night and vice versa) and touch, although only the last two have been observed in the wild. Some species can also crawl along surfaces with the use of ‘crawling’ cells on their underside. This movement is slow, but still allows them to perform some remarkable feats, for example the carnivorous sponge, Asbestopluma hypogea, can surround and engulf their prey in a matter of hours.
How can sponges move like this without even a basic nervous system? Apparently by relying on other mechanisms for cells to communicate with each other, the use of chemical messages, such as hormones, remains a viable alternative for all sponge species. These are used in all animals (e. g. the control of blood sugar with the release of insulin), but the nervous systems are still used for quick and coordinated movements as chemical messages are too slow to take effect. However, in sponges their slow movement and simple body structure, with fewer cells to communicate between, could mean that fast acting electrical signals are not needed.
Glass sponges, which possess silica shards for structural support, may be an exception, possessing a primitive system for transmitting electrical signals between cells. Normally nerve cells rely on the movement of sodium (Na+), and to a lesser extent potassium (K+) ions to move the electrical signal through the nerve cell and specialised junctions, called syanpses, to transfer this signal to the next cell. In glass sponges each signal appears to be transmitted through direct channels between cells by changes in calcium ion (Ca2+) concentrations.
There has been a lot of interest in sponges from an evolutionary perspective as they are thought to closely resemble the common ancestor of all animal life, and so can provide unique insights into how certain features evolved. Nervous systems are no exception, with the coordinated behaviours and primitive signaling systems that facilitate them giving us an idea of the starting position from which ‘true’ nervous systems later evolved.
From a human perspective
Two groups of sponges (Hippospongia and Spongia species), commonly called ‘soft sponges’ have been used for centuries for various purposes, such as helmet padding, paint applicators and bathing sponges.
More recently there has been interest in many of the chemical compounds produced by sponges, and their associated microorganisms (bacteria, fungi etc). Sponges use these compounds to protect themselves from predators, fouling organisms and disease, but they have been effective in treating a number of human ailments. These include viral infections, such as HIV, certain cancers, especially leukaemia and breast cancer, and could also be a valuable source of antibiotics.
Unfortunately extracting these valuable chemical compounds from the sponges has proven difficult. Harvesting wild populations is not feasible due to low population densities, which has been exacerbated by previous overfishing and disease outbreaks. Furthermore each individual sponge contains these chemicals is relatively low concentrations. Cultivating sponges in captivity would appear to be the most logical alternative, but this comes with its own issues to overcome. ‘Open’ cultivation systems, which involve keeping captive animals within their natural(ish) environment, are unsuitable due to variable environmental conditions. In ‘closed’ cultivation systems environmental conditions can be tightly controlled, yet there have been few occasions where sponges have been successfully cultivated in such condition.
A third option would be to acquire the chemical compounds without the sponges. In theory, once we know the molecular structure we could then recreate these compounds from their core components. Unfortunately, once again, this is easier said than done as the structure of many of these compounds are complex and thus difficult to recreate.
We have used sponges, as well as other marine organisms, for a variety of purposes, and we continue to find new ways to exploit them as our technological capabilities advance. I would however, ask that we remain cautious, it is a sad truth that we often learn how to exploit the environment far quicker than we how to do so sustainably.
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