Skeleton Shrimp - Caprella sp.

Find the skeleton shrimps in this picture! For me it was also the first dive where I could see them and photograph them. They are very small! 1-3 cm (1/2 to 1/4 of an inch) so it is easy to overlook them because they are also very well mimetized in hydroids like Aglaophenia cupressina for example.
In spite of their name they are not real shrimps, but instead they are amphipods of the family Caprellidae.
The males are larger than the females and they use to live in groups from dozens to hundreds on top of sponges, hydroids, gorgonians, algae and mooring lines.
Their bodies are long and thin jointed. They bear two sets of antennae, large grasping claws, with saclike gills below. Instead of legs they have backward facing hooks for clinging which extend from the last three body segments. The grasping claws are used to grab planktonic food. The females have brooding puches on their abdomen and bear live young that cling to their mothers for several weeks.
(info extracted from page 234 of Reef Creature identification - Tropical Pacific, by Paul Humann and Ned Deloach)

There are many species of Caprella, so they are difficult to ID at the sp level:
https://en.wikipedia.org/wiki/Caprella