Sponges - complicated yet simple animals

Sponges (Porifera) are some very simple, yet super interesting aquatic organisms. They have been around for a long time already – for some species fossil records date back approximately 600 million years to the earliest (Precambrian) period of Earth’s history [1]! It is assumed that sponges were first to branch off the evolutionary tree from the common ancestor of all animals, making them the sister group of all other animals.

However, some scientists also think that that animals' nearest evolutionary relatives are choanoflagellates, which are single-celled organisms that are very similar to sponges' choanocytes [2] This, however, would imply that most Metazoa (multicellular eukaryotic organisms) evolved from very sponge-like ancestors, meaning that sponges may not be monophyletic (a group of taxa that share a common ancestry), as the same sponge-like ancestors may have given rise both to modern sponges and to non-sponge members of Metazoa [2].

Negombata magnifica, the toxic finger-sponge

Sponges are multicellular organisms but have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. The mesohyl is a jelly-like substance made out of collagen and structured by a dense network of fibers. Its inner surface is covered with the so-called choanocytes, which are cells that can create a wave-like motion with their whip-like flagella, driving water through the sponge's body. They do not have nervous, digestive o,r circulatory systems. Instead, sponges mostly rely on maintaining constant water flow through their bodies to obtain food and oxygen and to remove wastes. All sponges have Ostia, which are channels that lead into the interior through the mesohyl. In most sponges, these Ostia are controlled by tube-like structures that form closable inlet valves. Pinacocytes, form a single-layered external skin over all other parts of the mesohyl that are not covered by choanocytes and also digest food particles that are too large to enter the Ostia. The large opening to the outside through which the current of water exits are then called Oscula (or ‘Osculum’ when you are talking about a single one).

Closeup of an encrusting sponge and its Oscula

Due to this filtering, sponges are also of great importance for coral reef ecosystems, as they can affect the water quality on the reef. By filtering the water, sponges collect bacteria and process carbon, nitrogen, and phosphorus. This is especially important in the nutrient-depleted waters around tropical coral reefs, were some sponge species are thought to make carbon biologically available by excreting a form of “sponge poop” that makes carbon available which other organisms can feed on [3]. Thereby they are fueling the productivity throughout the ecosystem and protect the reef against extreme fluctuations in nutrient density, which is beneficial for the survival of other reef organisms.

The tube sponge Haliclona fascigera

Although adult sponges are sessile animals, some marine and freshwater species are known to be able to move! They can reach up to 1–4 mm per day, by the aid of their pinacocytes and other cells that can perform amoeba-like movements [4]. Some species can also contract their whole bodies, or close their Ostia and oscula (the pores through which the sponge excretes the water again) and do this often in a very regular manner, e.g. with a day/night rhythm [5].

References:

[1] Li, C.W. et al. (1998). “Precambrian sponges with cellular structures”. Science, 279: 879-882. [2] Borchiellini, C. et al. (2001) "Sponge paraphyly and the origin of Metazoa." Journal of Evolutionary Biology 14: 171-179. [3] De Goeij, J.M. et al. (2013) “Surviving in a marine desert: the sponge loop retains resources within coral reefs” Science, 342: 108-110. [4] Galtsoff, P. S. (1923) “The amœboid movement of dissociated sponge cells’ The Biological Bulletin, 45: 153-161. [5] Nickel, Michael (2004) "Kinetics and rhythm of body contractions in the sponge Tethya wilhelma (Porifera: Demospongiae)." Journal of Experimental Biology 207: 4515-4524.