Micro-rock-pooling: an Intertidal Insect

There aren’t many insect species that live in the intertidal zone. The two you’re most likely to encounter are the springtail called Anurida maritima that scoots across the surface of rock pools on the upper shore and seaweed flies Coelopa frigida that breed in vast numbers in the rotting heaps of seaweed that pile up on the strandline. Even fewer insects complete their life cycle actually submerged in salt water but one is this little chironomid midge called Clunio marinus, that I found amongst the fronds of Cladophora seaweed in rockpools on the beach at Whitburn near Sunderland yesterday.

This specimen was about 5mm. long and living in a tube constructed from fine sand grains amongst the fronds of the seaweed.

It fed by hauling itself through the branches of the seaweed using two short leg-like appendages just behind the head.

The midge larva seemed to be nibbling away at much smaller organisms that encrusting the fronds, using a pair of pointed jaws. As far as I could tell it didn’t eat any of the seaweed – just the organisms that encrusted it.


Chironomid midges are extremely common in fresh water, breeding in vast numbers on ponds and even in small bodies of water like waterbutts in gardens. This species, though, as evolved to survive the rigours of life in salt water rockpools on the middle and upper seashore, where temperature and salinity levels can be extremely variable.

You can find a web site devoted to chironomid midge biology here

Snail Eggs

Some of our project students have been working on the feeding habits of garden snails Helix aspersa and one of our postgrads, Chantelle Kerr, drew my attention to the fact that some of their snails had been laying eggs in the tanks where we were keeping them. I was hoping to see some signs of the developing embryos when I took a look at these under the microscope, but the eggs were disappointingly cloudy. But then, on closer examination, the 'cloudiness' turned out to be something rather interesting. Take a look at the eggs on the microscope slide above and you can just about see clusters of white specks inside them (double click for a larger image), especially in the two at the back.

These turned out to be vast numbers of calcium carbonate crystals, embedded in the outer gelatinous egg capsule. The purple background colour is the result of using a colour filter to improve the contrast - not the real colour of the egg interior, which is colourless. 

 

At higher magnification you can see that some crystals are simple cubes, while others are aggregated together. What are they for? Well, a quick search of the web reveals  that the embryonic snails use this store of calcium to produce their first shell - the parent snail provides them with a  supply of building materials for a shell when it lays the egg. You can read more about the chemistry of snail shells by visiting the excellent Snail's Tales blog.

Microscopic Living Do-nut

Here's another interesting organism that lives in the temporary pools of water, known as phytotelmata, that accumulate in cavities in the surface roots of beech trees. It's a testate rhizopod - an amoeba that lives in a shell that it secretes - and is called Arcella. It's about a tenth of a millimetre in diameter. The shell in old specimens tends to be brown but this one is nicely translucent, revealing the exquisite sculptured pattern that decorates its surface.

Here's the flip-side, showing the hole in the underside of the hollow do-nut shaped shell and...

... here, with a shift in focus, you can see the amoeba inside, with one arm or pseudopodium protruding out of the hole. The dark structure that you can see in the cytoplasm is a vacuole filled with carbon dioxide - the organism produces one of these to increase its buoyancy when it needs to float up to the surface.

Here you can see the whole organism inside the shell in side view, poised above the hole. The circular structure in the cytoplasm, top left at about 10 o'clock, is a nucleus.... the control centre of the cell.

There are always two nuclei in the cytoplasm of each Arcella.... so which one is in control? Who knows?

And finally, here it is on the move, extending a pseudopodium that it uses for locomotion and food capture.

This is probably Arcella discoides. If you'd like to see some wonderful illustrations of more testate rhizopods take a look at this page, where you'll need to double-click on the images to enlarge them.

Teeming Millions - Paramecium Population Explosion

Well, teeming thousands anyway. These little organisms are Paramecium, single-cells protists that swim with incredible speed using ciliary hairs on their surface, that beat in rhythm. Here they're magnified one hundred times. This single drop of  water on a microscope slide probably contained about five hundred...

... and this little pool of water, trapped in a cavity formed by the coalesced roots of a beech tree, must have contained millions, feeding in bacteria and other microscopic organisms that were in turn feeding on the rotting leaves trapped in the water. The pool is fed by rivulets of water that trickle down the trunk when it rains. Temporary pools of water trapped in plants like this are known as phytotelmata. The best-known examples are the pools of water trapped by the leaves of bromeliads (urn plants) that grow as epiphytes in the rainforest tree canopy. They host all sorts of exotic animals - tree frogs, land crabs, dragonflies - but this beech tree-root equivalent hosted nothing larger than rat-tailed maggots - the larval stages of drone flies. But while the species diversity in the beech-tree pool might have been low, the sheer abundance of the Paramecium was staggering. 

Here they are magnified two hundred times. The circles that you can see in some of them are contractile vacuoles, that constantly expell water from the cell cytoplasm.

  

At 400 times magnification you can see the fine cilia (top right) that are arranged in rows over the surface of the cell - you can just make out their dark parallel lines and you can also see algae that the Paramecium has ingested, inside the cell.

Static images don't really do justice to the helter-skelter movement of these little protists, but the video clip below gives some impression of what is going on in those little temporary pools of water trapped by the tree roots.

 

Micro-rock-pooling

This is probably the low point of the year for exploring life in seaside rock pools but if you search amongst the fronds of seaweeds like the red Ceramium and green Cladophora, that can still be found on sheltered parts of the shore, and take them home for microscopic examination you can still find a wealth of marine life in miniature. Here, beautifully camouflaged in a flecked green exoskeleton, is a sea slater Idotea sp.

The multi-facetted compound eye of Idotea is exceptionally beautiful.

In amongst the weed and hanging from the surface film in the rockpool you'll almost certainly find large numbers of juvenile gastropod molluscs, each only a couple of millimetres long - this one is almost certainly a Littorina (winkle) species. Notice the single dark eye at the base of each antenna.

Acarine mites are incredibly diverse animals that live in almost every habitat imaginable (click here for more information on them). Scores of these little 8-legged animals, each only about a millimetre long, were scurrying around amongst the seaweed fronds. Not much is known about the ecology of marine mites, which mostly belong to a single family - the Halacaridae. Notice the long, hooked claws that stop them from being washed out of their seaweed shelter, and the piercing mouthparts at the head end. There's a short video clip below.

You can find more on freshwater marine mites here.

Nature's Stained Glass Windows

Looking like a stained glass window, this is the remains of the seed pod of one of last summer's garden flowers, Lunaria annua (commonly known as honesty, because the dry seed pods resemble silver coins), magnified around two hundred times and viewed using polarised light.

When honesty seed pods ripen they are flattened and composed of three components. Imagine three large 'coins' joined to each other all around their rims, with the central 'coin' attached to the plant via a stalk. Swelling seeds are attached to the rim of both faces of the central 'coin' via their own slender stalks, visible when sunlight shines through the whole structure.

When the pod dries out and ripens the tensions in the drying, contracting cells of the walls of the 'coins' tear them apart around their rims, so the two outer 'coins' detach and flutter away in the breeze, followed by the winged seeds, leaving the central 'coin' attached to the dead plant and surviving deep into winter.

These are the cells of that central, surviving 'coin' magnified about one hundred times and using ordinary white light....

      

... and these are the dazzling interference colours generated when polarised light is used.

          

At two hundred times magnification it's clear that the 'coin' is formed from two layers of cells, orientated at different angles, so that the tensions they develop when they dry will twist and distort the 'coin' and help to rip apart the sutures with the outer 'coins'.

       

 At four hundred times magnification you can clearly see the pores through the thick cell walls which were the plasmodesmata - the channels of communication between the cytoplasmic contents of one cell and the next, while the whole structure was still alive and the seeds were still developing.

The dry, dead cells form intricate patterns...

... whose colours change as they are rotated in the plane of the polarised light.

The Colour Purple

This rather beautiful flower is Tibouchina urvilleana and the purple of its petals is due to the presence of anthocyanin pigments, which are dissolved in the cytoplasm of the petal cells.

If you magnify the petal surface about 200 times you can see the way in which the petal cells fit together, like pieces of a jigsaw puzzle. This piece of petal is mounted in water but if it's transferred to a concentrated solution of sugar ...

  ... the water in the cell begins to flow out through the semi-permeable cell membrane by the process of osmosis, with water travelling out from the less concentrated solution in the cell to the more concentrated solution surrounding it. Within a few minutes spaces become visible between the cytoplasm and the cell wall, where the cytoplasm shrinks ....

... and within a few more minutes the cytoplasmic contents of the cell have shrunk even further, so the purple anthocyanin pigment becomes even more concentrated in the remaining cytoplasm.

  While I was looking at the petals I noticed something unusual around their edge  - a fringe of microscopic hairs, invisible to the naked eye.

 Each hair is tipped with a  glandular head.



... that looks as through it may contain oils.

What are these hairs for? Secreting aromatic compounds that attract insect pollinators, perhaps?

Sticky Jack

This is a cross section of the stem of the plant commonly known as goosegrass or sticky Jack and more scientifically as Galium aparine. Sticky Jack is a very common weed that scrambles up through other vegetation using its covering of hooked hairs on the stem and leaves and which sticks to clothing with these when kids throw handfuls of the stuff at each other.

This image was produced using fluorescence microscopy, staining the cells with compounds that bind to the cell walls and fluoresce. The blue cells have walls made of cellulose and their blue fluorescence is due to the calcofluor that they've been stained with, which fluoresces blue in ultraviolet light. Calcofluor has been used as a 'blue whitener' in washing powders - it binds to the cellulose in cotton fabrics and fluoresces faintly blue in the UV component of sunlight. The yellow staining is due to another fluorescent dye (fluorochrome) called auramine O, which binds to cutin in the outer cuticle of the plant, and to dead, lignified cell walls that give the stem its strength - and it fluoresces yellow. The cuticle in this cross section is the thin yellow line covering the outer surface of the section. The yellow circle in the centre is composed of dead, lignified cells - not particularly well developed in goosegrass because it scrambles over surrounding vegetation rather then investing resources in producing a stout lignified stem of its own.

Strings Attached

Deciding on the prime time to pick a bean pod is s tricky business. Leave it too late and the pod will become tough and stringy - and the reason for that is because as it grows the pod begins to prepare to shed its seeds. Members of the pea family - the leguminosae - carry their seeds in pods that naturally become brittle when they dry and ripen, when tensions developed along the suture between the two pod halves and in the pod wall eventually become so great that the pod splits open violently, hurling out the seeds. Plant breeders have worked hard to breed this trait out of legume crops, but species like runner bean still produce long strands of woody, lignified cells in their pod walls as they ripen. In this fluorescence micrograph, showing a cross section of the upper suture of a developing pod, the bright yellow arcs of cells at the top are the 'strings' that you need to strip out of the pod before you eat and cook it if you've left it too long before harvest. The yellow cells just creeping into the picture at bottom left belong to the parchment layer that develops in the pod wall. Together, these thock-walled cells develop the tensions in the pod as it dries that will eventually split it open along its longitudinal sutures and release the seeds.

Twister

To appreciate the true beauty of mosses you really need to explore them with a hand lens or low power microscope. This is Tortula muralis, wall screw-moss and to find out how it acquired that colloquial name you need to take a close look at the spore capsules.

Wall screw moss grows in the mortar-filled cracks in walls, where it produces spore capsules that are carried aloft on stalks that are a couple of centimetres long at maturity. These are capsules in the very early stages of development, before their stalks lengthen, but if you take a really close look at a mature spore capsule....

 ...it looks like this. Notice how the capsule's stalk (seta) has twisted helically. If you take a close look at the capsule (double click on the image for an enlarged version) you can see that most of it is sheathed in a membranous covering - the calyptra. Gently pulling this off with a pair of forceps reveals....

 ... a lidded capsule underneath and if you pull the lid (operculum) away....

... it reveals a screw thread-like arrangement (the peristome) underneath, that gives the moss its common name. These threads twist up tightly in moist air but untwist in a dry atmosphere, allowing the minute spores to be shaken out when the seta trembles in the wind. In this image you can see that the operculum that has been removed has become temporarily stuck to the base of the capsule - normally it will just fall away.