In fisheries, “by-catch” refers to those fishes which are caught unintentionally, alongside the species which the fishing vessel sets out to harvest. In looking for protists, the by-catch comprises all manner of small animals. Some of them are depicted here.
Rotifers are also called “wheel-animalcules”, in reference to the ciliated rings that they bear on their heads. The beating of these cilia brings food into the mouth of the rotifer. Upon consumption they are processed by the muscular pharynx (mastax) which contains hard structures called trophi; these can be seen in some of the images above.
In some rotifers the lorica (hard covering) is modified into various shapes, including flaps and “wings”.
Unusually-shaped rotifer with long cilia that emerge from lobed tentacles around the rim of its mouth. When the lobes retract, the tentacles still protrude and still keep waving. It also keeps an egg close by along its stalk.
Watch it capture its prey with insouciance:
Insect larvae tend to be larger than the other organisms we might encounter in these pages.
Small animals that may be related to the platyhelminth flatworms. Their bodies are covered in a spiny cuticle, and yet they also have cilia (apparently passing through pores in the cuticle) covering the ventral surface, which they use for locomotion. At the caudal (tail) end of the body are furca, or “toes”, which have adhesive tubes.
These are among the most abundant animals in the planet by number or biomass, being nearly ubiquitous in soils. They are also called “roundworms”, for their bodies are round in cross section and do not have any segmentation. They also lack circular musculature in the body wall. The body surface is covered in a layer of cuticle, which is periodically shed (ecdysis), one of the developmental characters that link them to their relatives in the Ecdysozoa, which includes (surprisingly) the insects and arthropods. One nematode, Caenorhabditis elegans, or simply C. elegans, is a well-known laboratory model organism.
The flatworms were formerly thought to be evolutionarily primitive, because they lack a true body cavity, or coelom. This is now thought to have been a secondary loss of the character, and they are related to groups which are thought to be quite derived. The epidermis of the body is ciliated, and the cilia are used for locomotion. To aid movement, specialized cells secrete quantities of mucus.
Worm-expert Chris Laumer says that this looks like a Stenostomum, a catenulid flatworm.
This structure suddenly cracked open, propagating a shock-wave through the water that I witnessed a millimeter away (it’s a long distance under a microscope).
According to Chris: “The egg-on-a-stick is very likely a neoophoran flatworm’s embryo. Whose, I couldn’t say, though the shape can actually be species-specific. You can see the nearly yolkless embryo itself on the one side, and the yolk cells themselves opposite. That kind of leathery appearance is really typical.”
Segmented worms with bristles on every segment, but without parapodia, which are a feature of polychaetes.
Fungi growing on an insect carcass, which was also being fed on by ciliates.
Cyanobacteria are an important part of freshwater habitats, both as photosynthesizers and as substrates for the attachment of various organisms including rotifers, ciliates, and diatoms. They were traditionally classified as algae (the “blue-green algae”) and hence thought to be simple plants, but they are in fact large bacteria. They lack nuclei and other organelles, though they do have a system of internal membranes, the thylakoids, studded with the machinery for photosynthesis.
In the course of evolution a cyanobacterium is thought to have been engulfed by an early eukaryotic cell, and harnessed for its ability to photosynthesize. The enslaved cyanobacterium then became the ancestor to modern chloroplasts, which also have thylakoids and a bacteria-like but highly reduced genome. This is known as the endosymbiotic hypothesis.
Here are shown the heterocysts of some filamentous cyanobacteria (the yellow cells among the blue-green cells of the filament). Heterocysts are specialized cells where nitrogen fixation takes place. The enzyme nitrogenase is responsible for converting atmospheric nitrogen gas (N2) to ammonia, but it is inactivated by oxygen, making it necessary to have such a specialized structure solely for this physiological process.
Cyanobacteria are also able to move by gliding:
Bacteria are also capable of movement. Some species swim by beating their flagella, while others have gliding motility (like the cyanobacteria above), which depends on adhesion to the substrate.
Calculations based on diffusion rates and the viscosity of water at such small scales show that it is actually less economical for bacteria to intercept food by swimming than it is to simply let the food molecules diffuse towards them, though they would benefit by swimming up a concentration gradient.