The life in the Red Sea is much more than fish and corals. In the sea there are many more different types of animals than on land. A few are mentioned in the book, but there are many more!
In general invertebrates are animals with no distinct features that we intuitively relate to animals such as head or eyes, and many of them are not able to move – or do so very slowly –or using other was than legs. You are therefore very likely to ignore a lot of invertebrates that you are passing by.
There are however a few, where you are not In doubt – if you see them – such as the snails, the large clams, octopus or squid.
The Giant clams such as the Tridacna maxima are really amazing. They have beautiful colors due to the algae that live in their mantle. And the Tridacna gigas can grow to 120 cm and weigh more than 200 kg! However, you will probably mostly see giant clams in the 20-50 cm range.
You may encounter the squids along the reef, e.g. hovering around some of the buoy lines. And in places of large coral gravel, that is typically the place to look for octopus. These are fascinating animals. They have big eyes, and are very intelligent. They are known for being able to open a lid on a jar in aquariums, and all the squid/octopus are able to change their colors very rapidly. They use it for camouflage but also to communicate with each other!
The Reef octopus, cuttlefish and squid can change color very rapidly!
If you see see something that you think look like flowers. Then there is a good chance that it is an animal. One of the many invertebrates. However, it can be difficult to tell which. There are worms, polychaetes, that have long feathery tentacles, but so does some hydroids as the feather hydroids mentioned in the book (and they are more related to jellyfish in the Cnidaria group), or sea anemones, which are also a colorful group of animals in the Cnidaria).
Some of the feathery animals are in the sand, some are under overhangs, whereas the Christmas tree worm live in the corals.
Crinoids also look like flowers, however, you will often find it dark black/red, sitting on top of a small point where it can put its arms out in the flowing water some meters down (from 5-10 meters and deeper).