Re: If sponges have no eyes, ears, and nerves, then how can it react to stimuli

Thank you for your question. Sponges (Phylum Porifera) are fascinating animals (although often not much to look at!)

As you point out, sponges have no individual cells or organs that are specialised to detect environmental stimuli (like smells, movement, darkness) or to transmit information around the sponge body.Sponges do, however, show a number of simple behavioural responses. These include closing the external body openings (which consist of a number of ostia, through which water enters, and a single osculum or small number of oscula through which it leaves, the sponge), constriction of internal canals, and reversing the flow of water through the body (i.e. flowing frow osculum to ostia). These responses are generally triggered by the presence of sediment , which may result, for example, in the closing of deeper internal canals and reversing water flow if the sediment gets inside some of the feeding chambers, or by direct physical stimulation (e.g. stroked by a finger or attacked by a fish), which results in closure of external openings. The most common response shown by sponges is to stop pumping water through their bodies.

It is the normal sponge cells, rather than special sensory ones that detect these stimuli, and the sponge's responses are probably coordinated by simple mechanical coordination between the ordinary body cells. Some chemical signals could also be involved - all eukaryotic (non-bacterial) cells have mechanisms that detect and respond to individual cell damage, and these could be detected by neighbouring cells. The transmission of signals by sponge cells is much less efficient and slower than transmission by nervous systems. For example, recent study of the glass sponge Rhabdocalyptus showed that both mechanical and electrical stimulation resulted in the complete cessation of pumping within 20-50 seconds, it was calculated that the stimulation was conducted at between 0.17 and 0.3 cms per second - much slower than nervous transmission, but much faster than simple chemical diffusion.

I hope this is of interest.
Trevor.

References:

Bursca, R. C. and Brusca, G. J. (1990) Invertebrates. Sinauer Associates.

There are a limited number of web sites with information about sponges. A good simple introductory page is available on the Ocean Animals pages: here, or try the University of California Museum of Paleontology's Introduction to Porifera (http://www.ucmp.berkeley.edu/porifera/porifera.html) or Phylum Porifera at the Universit of Michigan (http:// www.oit.itd.umich.edu/projects/adw2k/porifera.html) for more detailed information.