The Chathamiidae (named after the Chatham Islands in New Zeland) are a small family of caddisflies, only 2 genera and five species. All Chathamiids inhabit tidepools in their larval stage, the only insect found thus far that has colonized these habitats. They can be found on the Southern coasts of Austrailia, the coasts of New Zealand, and the Chatham and Kermadec Islands.
For many reasons, aquatic insects have failed to colonize the oceans. It is more than just a matter of osmotic potential, the high salt content of sea water. It is also a matter of competitive exclusion. Ocean invertebrate niches are well filled, there are very few openings for invasion by insects. However, at least 5 species have found a place in this environment. (Note: There is also a genus of marine water striders that can be found quite far from land, but these are not considered fully aquatic in the sense that Chathamiids are.)
The life cycle of marine caddisflies is quite amazing in its own right. In the species Philanisus plebeius, larvae begin their lives as eggs oviposited within the body cavity of the Australian cushion star Patiriella exigua. After more than 30 days the larvae hatch and exit their hosts through the stomach and out the mouth. They then spend 7 larval instars feeding within the tide pools on red calciferous algae in full sunlight. There are several odd things about this. One, the normal number of larval instars, that is, the normal number of times that a Trichopteran larvae molts before pupation is 5. Two, these are the only caddisflies known to feed in tide pools on calcified algae, and three, they feed in full sunlight, while most caddisfly larvae are rather secretive and tend to feed in shaded regions.
Marine caddisflies construct portable cases of tough silk, tubes to which they attach various algal fragments, supposedly for camouflage. Larvae pupate on the red alga Corallina officianalis. Adults fly on the open coast through out the summer. The females have long pointed ovipositors that are able to penetrate the cushion stars that serve as hosts for the larvae.
Unfortunately, no one has supplied any photographs of marine caddisflies, larvae or adults, to the internet. If anyone finds a photograph of the larvae, especially their case, I would love to see it.
Riek EF. 1976. The marine caddisfly family Chathamiidae (Trichoptera). Australian Journal of Entomology, 15: 405-419. (PDF; with drawings and keys)
Ward J. 1994. The New Zealand marine caddisflies (Trichoptera). Weta, 17: 18-20. (PDF)
Winterbourn M, Anderson N. 1980. The life history of Philanisus plebeius Walker (Trichoptera: Chathamiidae), a caddisfly whose eggs were found in a starfish. Ecological Entomology, 5: 293-304.