Deep water organisms (Very Deep)

Osviux

Hello, I would like to ask why are organism which live in deep waters are much bigger than those that are living on land? For example in deep water you can find very big spiders. Shouldnt organisms be smaller because of the presure in big depths?

Ryan_m_b

I'm not sure if it is fair to suggest that organisms living in the deep sea are bigger than those on land. There are many constraints on the size at which species can evolve to, larger sizes may not be an advantage at all as they require more food, produce more waste heat and are more susceptible to changes in the food chain.

Getting back the the deep see there are not many organisms that are that large, food is scarce at the bottom of the sea. Options include predation, particularly on the extremophile bacteria and plankton living near hydrothermal vents, marine snow and the rare occurrence of a large organism (e.g. whale) falling from higher waters. Whilst we're on the latter I remember seeing a wildlife documentary once where thousands of species had spent months breaking down a whale corpse all the way to the bone.

Regarding deep sea biology many species are still quite small in size i.e. measured in tens of centimetres. IMO The pressure may not have too much of an effect on size, remember that the creatures will be mainly water themselves, the density of water in them could be far greater than it is in us humans so as to match that of the environment pressure. High pressure is a relative thing.

Andy Resnick

This book is *the best*:

http://www.press.uchicago.edu/books/nouvian/index.html

There are large organisms, but most appear to be very small and >95% water.

Osviux

I think i mixed up gravity with pressure ;/

mishrashubham

Where did you read this? Any references?

I think you meant sea spiders.

Like Ryan said, I don't think pressure (or gravity) would affect body size. Far more important factors are availability of resources and susceptibility to predation.
A very interesting observation is the island rule. While not exactly a rule since it
has been shown that many families do not comply with it, I have found a correlation between this trend and deep sea creatures in this study.
http://craigmcclain.com/Papers/McCla...IOGEO_2006.pdf