Thursday, June 25, 2015

Dotted line with eyes

I think I could spend a lifetime on one small area of our intertidal shelf, looking, looking more closely, and looking again, and never run out of something new to discover, And for each basic body plan, Ma Nature, who loves to tinker, has devised an almost infinite round of changes.

The latest critter I have found looks, at first glance, like an ironed amphipod. He has long, pointed antennae, a good collection of legs and other assorted limbs, a segmented body, prominent eyes. But he lies flat and swims in a straight line, which no self-respecting amphipod would ever do.

Tanaid, about 1/8 inch long

The last two collections of fuzzy eelgrass and clamshells from the tide flats carried dozens of these tiny beasties, all about the same size. Without a lens, they look like short dotted lines, with a darker dot at the front. ...

Side view. The antennae have a reddish spot halfway along their length.

I checked out all my references. I spent hours Googling. (Why, when asked for planktonic crustaceans, does Google give me umpteen photos of walruses?) I checked E-Fauna, for BC; there are 389 species of amphipods in their list, 75 species of isopods, very few with photos. But this is neither amphipod nor isopod, but something in between.

ASnailsOdyssey didn't have anything on these. They weren't in Beachwatchers. I finally found an image that led me to RealMonstrosities, and the Tanaids.  (But these aren't monstrosities at all; I think they're cute.)

Face view; breaking the surface of the water, and heading into the shadow of the camera. She didn't like the light.

Once I had a name for them, I did find some information.

There are more than 700 species of this family, several thousand of the larger order. (From

In some areas their population size has frequently been measured at more than 10,000 individuals per square metre and, on occasion, over 100,000! In the abyssal plain they are often the most abundant crustacean and their numbers almost rival that of polychaetes. (From RealMonstrosities)

Most are very small, part of the almost invisible base of the food chain that starts with tiny swimmers and ends with us.

The abundance of tanaids is strong evidence of their ecological importance. Despite this, they have been neglected in most ecological surveys (Baldinger & Gable, 1996). This ecological ignorance of the Tanaidacea is caused by the immense difficulties associated with identifying these animals. (From

So I'm not sure of the species of these beasties. Nor of their lifestyle. Some families are tube-dwellers, others are free-living, some live in burrows. Some are filter feeders; others are predatory.

Some species are hermaphroditic. Others start out life as sexless neuters, then turn into females. After their final molt, they change into males, stop eating, reproduce, and die.

The males of many species have very large pincers, sometimes as long as the rest of the body. Here's a photo, taken on the west coast of Vancouver Island.

Dropped in my tank, the little dotted lines swam happily away to burrow into the fuzz on the eelgrass. A day later, after the hermits had eaten their fill of fuzz, there was no sign of them. A couple turned up in the filter when I cleaned it a week later; a temporary shelter for them, free from hungry hermits and dancing shrimp.

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