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Density impacts and Hydra

Posted by PLOS_CompBiol on 26 Feb 2008 at 15:16 GMT

Originally submitted as a Reader Response on 3rd January 2008

Seymour and Doncaster suggest that Hydra's sessile habit would have strong density impacts on juvenile recruitment. I'm not sure this is a realistic expectation. The Hydra polyp does attach to the substratum via its adhesive basal disk, and is thus sessile in this state. However the polyp is also highly mobile. It can detach from the substratum readily and moves by floating, creeping, and even somersaulting. Because of this mobility, the polyp should be able to escape density impacts readily. In the wild, Hydra polyps are commonly found in sparse distributions on rocks and floating material (leaves and twigs), in support of the idea that density does not necessarily impact them. Also, when Hydra reproduces sexually, the embryo (which is surrounded by a hard cuticle) drops from the parent polyp and may end up some distance from the site at which the parent polyp is attached, potentially escaping a dense population in which the parent polyp is located.

Submitted by: Rob Steele
Occupation: Associate Professor
University of California, Irvine

RE: Density impacts and Hydra

PLOS_CompBiol replied to PLOS_CompBiol on 26 Feb 2008 at 15:19 GMT

Originally submitted as a Reader Response on 8th January 2008

Our model applies particularly, but not exclusively, to organisms that are sessile in the adult stage. Although Hydra polyps can relocate, it does not follow that they “should be able to escape density impacts” (Steele 2008). The major predictions of our model require only a closed system that fills by self replication to a recruitment-regulated carrying capacity of adults, K. Invasion of new mutations then requires that they raise K above its current value, whatever that is. Density can impact strongly even on a sparse distribution of Hydra polyps on rocks and floating material, if a low K limits opportunities for recruitment to the adult population. Any mobility of polyps in such an inhospitable environment may reflect movement in search of a suitable place to produce viable offspring. Although model predictions are unaffected by adult mobility per se, we appreciate the need for more experimental data on recruitment processes for organisms such as Hydra.

Submitted by: C. Patrick Doncaster
Occupation: Reader
School of Biological Sciences, University of Southampton, Southampton, United Kingdom

Additional authors: Robert M. Seymour