The number, type, size and shape of organisms is determined by environmental factors. Survival of different species can depend on the amount of light, pollutants, predators and even hiding places.
You might try to explore the different creatures that live in your area and how they survive - it may be burrowing wetland creatures, aquatic invertebrates or healthy bugs of a creek or tracking sand snails that can help your students to dive deeper.
It can be absolutely fascinating to learn how different creatures and plants can adapt to live in their own environment. These adaptations help living things to survive, avoid predators, obtain food and reproduce.
A few examples include the spikes of a sea urchin, creatures which change sex, turtles laying their many eggs and the camouflage of the leafy seadragon.
There are many resources which can help with more information - see Recommended Marine Education Resources.