Between micro and macro

It's embarrassing when someone uses a word that I should have, should definitely have, known, but that draws a blank. Snail caught me with the word, "meiofauna". She wrote, "If you can find a good guide to meiofauna, ..." The "fauna" part was easy, but, "meio-"? I had to look it up.

This is how Wikipedia describes them:

Meiofauna are small benthic invertebrates that live in both marine and fresh water environments. The term Meiofauna loosely defines a group of organisms by their size, larger than microfauna but smaller than macrofauna, rather than a taxonomic grouping. One environment for meiofauna is between grains of damp sand (see Mystacocarida).
In practice these are metazoan animals that can pass unharmed through a 0.5 – 1 mm mesh but will be retained by a 30 – 45 μm mesh, but the exact dimensions will vary from researcher to researcher. Whether an organism passes through a 1 mm mesh also depends upon whether it is alive or dead.

And I always have to look up micrometres (μm), too. Was that 100 times, or 1,000? A millimetre (mm) is 1000 micrometres (μm). So a member of the meiofauna is from 45 μm to 500 μm long. Quite a range.

With that out of the way, I Googled "meiofauna" and found a few - too few - pages. I'll have to continue the search, but already I'm fascinated.

Some of my meiofauna. Taken by removing the eyepiece from the phase microscope and pointing the camera down the tube.

The marine meiofauna, defined as animals of microscopic size living in marine sediments, is one the earth’s richest and most diverse community extending from the shore to the deep sea. The marine meiofauna still contains numerous undescribed species and higher taxa. Special morphological adaptations evolved, especially in meiofauna living in the intertidal zone which is under a strong abiotic regime. Certain higher taxa evolved exclusively in the marine interstitial system. Evolutionary constraints caused elaborated life-cycles, migration patterns, special reproductive behaviours and structural adaptations. The interstitial system is also habitat for larvae and juveniles of certain macrofaunal species.
From Marbef.org (Marine Biodiversity and Ecosystem Functioning)

The majority of these organisms live in the top inch of the sand or mud. I sampled the top half inch. With Snail's help, I have more or less identified diatoms, (like those in the photo above, and more), ostracods (the jittery #8 from Sunday's post), hydroids, copepods, and worms. Hundreds of worms.

Most contaminants reside in sediments, and meiofauna are intimately associated with sediments over their entire life-cycle, thus they are increasingly being used as pollution sentinels. Because meiofauna have short lifecycles, the effects of a contaminant on the entire life-history can be assessed within a relatively short time. (Marbef)

Nematodes (worms) are relatively insensitive to man-made pollutants. I found in my samples several small, fat worms, a few polychaetes, identifiable by their red colour and their hysterical swimming style, and hundreds of long, sleek worms, from almost invisible at high magnification, to monsters a millimetre long or more. 

Mostly diatoms in various sizes and stages, probably a hydroid, and something making a chain. Taken with the low-power lens.

And while I was watching the diatoms, a worm snaked by, knocking away the sand grains in his path . This is his mid-section. At bottom left, one of the critters I was looking at.

My grandson has come for his microscope, leaving me in its place a less powerful direct light one that the camera can't cope with. Back to the drawing board ...

Stalked animals. I think the upper one is a hydroid.

The diatoms next, probably tomorrow.