Finding the alien at home

If you're designing alien creatures for your stories there's no better place to start than on good old planet Earth.

If you're going to set your action on a planet that humans can exist on that means it's going to contain creatures facing similar challenges to Earth's species.  And if we employ the principle of convergent evolution - i.e. the same challenges force the same sets of solutions to the challenge - we end up with creatures we might recognize on Earth.

Does that mean our aliens don't look alien?  Not at all.  Take a look at any film of deep sea creatures. In that place of perpetual darkness beyond the reach of the sun there are myriad strange and glo wing  forms.  There are creatures with translucent bodies and bioluminescence, pulsing with waves of green and blue colour as they swim.  There are fish with bioluminescent fishing rods on their heads to lure in prey.

We used to think nothing could exist without sunlight to power its body, but then we discovered black smokers on the ocean floor and their colonies of creatures.  Tube worms, shrimps, and a host of other creatures get along just fine at atmospheric pressures that would crush us, getting energy from the heat and minerals ejected by the smokers from the Earth's core.

Take a look at individual animals' senses.  There are animals who can sense the  temperature of the earth so accurately they can keep the temperature of a nest constant.  Snakes can effectively 'see' with heat sensors that can detect infra-red emissions.  Plants and animals can predict the weather.

Green tree frogs croak when rain is approaching.  Some plants only open their flowers in fine weather, and some track the sun across the sky. Infrasound and ultrasound extend hearing way beyond the human range.

In addition to the senses we use there are creatures who can sense the Earth's magnetic field, plants that can tell the time, creatures that use the sun as a compass.  All animals can sense the seasons, vital if they're to breed at the right time of year.

The amount of variation in Earth species is stunning and staggering, and anyone wanting to create an alien species would have enough examples to last several lifetimes.