Some Intertidal non-crustacean arthropods

By Nicole Webster

The intertidal is a transitional zone, from the water to the air, but also from salt to fresh water. A lot has been said about the ecological consequences of this transition, perhaps idealized by salmon’s migration and how they transfer nutrients from the marine to terrestrial/freshwater ecosystems (Cross-boundary subsidy). I’m not going to say anything in depth about this, and if you are interested, I recommend speaking to Caroline Fox (uvic website), who is in and out of Bamfield semi-regularly and is completing her PhD on this topic.

This post is nothing so ecological or philosophical. This summer I came across a couple arthropods (non-crustacean) that are not commonly mentioned and wanted to expand your view of the intertidal.

This is Diaulota densissma (I believe).


This is a rove beetle (Diaulota densissma) in a petri dish, with possibly a larva. They are not easy to photograph. I could have sedated one for better ID and imaging, but didn’t. Credit: N Webster

I encountered these beetles on the barnacle rocks that I heave to the lab to feed my greedy snails. Quite a few of them I found regularly swimming desperately on the water in my seatable and crawling around on the dry rocks. At first I thought they were unfortunates brought in from a true terrestrial habitat, but there were too many, and too consistent. It was suggested to me to check out Kozloff’s Seashore life of the Northern Pacific coast. It is not a field guide, nor is it like his Key to Marine Invertebrates. It is really a nice introductory book about the seashore, looking from high to low intertidal, from sandy to rocky shores, and describing the types of organisms found there.


This is the beetle on a barnacle rock. Credit: N Webster

Indeed my rove beetle is in the book. It is a small intertidal predator that eats mainly amphipods and other tasty arthropods at low tide. According to Ahn (1996) these beetles live even in the low intertidal, and can spend most of their day submerged. They don’t have gills, they just find an air pocket (barnacle test, under a rock) and wait. Watching them swim, they obviously have a highly hydrophobic coating, probably with lots of hairs to trap air bubbles.


This is Neomolgus littoralis. It is an intertidal mite, known as the red velvet mite.

Red velvet mite on some Ulva, in the high intertidal on Edward King. They are ~3mm in length. Credit: N Webster

Red velvet mite on some Ulva, in the high intertidal on Edward King. They are ~3mm in length. Credit: N Webster

These mites are found in the high intertidal and eat kelp flies (another intertidal insect). They are very easy to spot, as they are shockingly red.



  • Kozloff, E.N. 1993. Seashore life of the Pacific Northwest: an illustrated guide to northern California, Oregon, Washington, and British Columbia. 370 p.
  • Kozloff, E.N. 1974. Keys to the marine invertebrates of Puget Sound, the San Juan Archipelago, and adjacent regions. Seattle, Washington, University of Washington Press, 226 p.
  • Kee-Jeong Ahn. 1996. A Review of Diaulota Casey (Coleoptera: Staphylinidae: Aleocharinae), with Description of a New Species and Known Larvae.The Coleopterists Bulletin Vol. 50:3, pp. 270-290


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