6ebbba77_Marine_habitats__principles__Living_in_the_sea
[Factor] Body weight [Living on the land] On the land, body weight can be decisive. Elephants can't grow bigger because their bones would not be strong enough (counts for humans too). Most animals can't fly. Birds can, but most waste much energy doing so. [Living in the sea] 
In the sea, all organisms are nearly weightless. That is why whales can become so enormous. Many organisms 'fly', often only as larvae. Larvae can easily be spread over vast distances.
Marine habitats, principles
Living in the sea