Sensory organ discovered in sponges helps them respond to their environment despite having no nervous system

by Amanda Kahn

Sponges are animals, but they do not have the features we’re used to seeing when we think of animals: no gut, no head or tail, no nerves, and no stomachs or other organs.  And yet despite not having a nervous system, sponges are able to respond to their environment by changing the canal sizes in their filter-feeding system, in an action called the “inflation-contraction response.”  It’s basically akin to what we do when we sneeze.  This was observed in the mid-1900’s, but scientists have only been able to speculate what could be helping the sponges sense and coordinate various cells in their body when there are no nerves or sensory organs observed.  Danielle Ludeman, one of the authors here at the Madreporite, has just published an article describing the sensory organ that she and her coauthors, Nathan Farrar, Ana Riesgo, Jordi Paps, and Sally Leys, discovered in many different species of sponges: primary cilia used to detect changes in water flow.  Check out the time-lapse video below to see how responsive sponges are to irritants (in this case sediments) in the water.

Danielle tested if those cilia are used to detect changes in water flow by using drugs that target and knock out the cilia.  When the cilia were knocked out or knocked down, the “sneeze” response couldn’t be initiated.  If cilia were permitted to grow back following treatment, the “sneeze” response could be initiated.  In our kidneys, primary cilia are used to detect water flow.  The structure of the paired cilia Danielle found aligns well with those of primary cilia in other animals, further supporting that these are sensory cilia that allow the sponges to detect their environment.

The cilia line the osculum, the chimney-like opening of the sponge.  If that osculum is removed, the sponge also is not able to initiate a sneeze response.  This led Danielle and co-authors to determine that the osculum can be thought of as a sensory organ, and not just a giant chimney.

Figure 4 from Ludeman et al. 2014

The “sneeze” response is shown by an increase in canal diameter followed by a rapid decrease (the black lines in the graphs). Various drugs that affect the cilia also affected that inflation/contraction. Source: Ludeman et al. (2014).

Why does this matter to us, and how does it apply to evolutionary theory?  Sponges are one of the earliest branches off of the animal tree of life (the Metazoa).  While they are animals, their distant relation to us and to all other animals (collectively called the Eumetazoa) means they diverged from whatever last common ancestor the Metazoa shared and evolved into something quite different and independent of what other animals have evolved into.  This isn’t unique–every animal phylum is very different from every other.  What is unique is their placement at the base of our collective “family tree.”  If a sponge shares a feature that we also have, it’s likely that the proto-animal–the last common ancestor that all animals shared–had that feature as well.  It brings us a little bit closer toward understanding how we evolved from single-celled organisms to the multicellular, fantastically complex and coordinated animals we are today.

Still think sponges are boring?
(Hint: they are, but only in one way that word is defined!).

Citation:

Ludeman, D.A., N. Farrar, A. Riesgo, J. Paps, and S.P. Leys (2014).  Evolutionary origins of sensation in metazoans: evidence for a new sensory organ in sponges.  BMC Evolutionary Biology, 14(3).  doi:10.1186/1471-2148-14-3.

To learn more about sponges and research on the origin of animal body plans, check out the Leys lab website.

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