18.1   The three major groups of zooplankton in Lake Annecy are Rotifers, (pictured above being measured at around 300 - 400 microns), which is an entirely separate class of animal, and two types of Copepod, Calanoid and Cyclops.

18.2   The most distinctive feature of Rotifers is a ciliated structure, called the corona, on the head (at the bottom of the picture above, facing to the right). 

18.3    The coronal cilia create a current that sweeps food into the mouth. The mouth opens into a chewing  pharynx (called the mastax). The pharynx has a powerful muscular wall and contains tiny, calcified, jaw-like structures called trophi, which are the only fossilizable parts of a rotifer. In suspension feeders, the trophi are covered in grinding ridges, while in more actively carnivorous species, they may be shaped like  forceps to help bite into prey.  Behind the mastax lies an oesophagus, which opens into a stomach where most of the digestion and absorption occurs. The stomach opens into a short intestine that terminates in a cloaca on the posterior dorsal surface of the animal. Up to seven salivary glands are present in some species, emptying to the mouth in front of the oesophagus, while the stomach is associated with two gastric glands that producedigestive enzymes.  A pair of protonephridia open into a bladder that drains into the cloaca. These organs expel water from the body, helping to maintain osmotic balance. And all that in a tiny creature less than half a millimetre long.

18.4   Rotifers are dioecious (meaning there are both male and female rotifers) but can reproduce both sexually or parthenogenetically,  where the unfertilized ovum develops directly into a new individual  (from the Greek partheno + genesis meaning "virgin birth" i.e. immaculate conception).   The females are always larger, and in some species ten times larger, than the males. In parthenogenetic species, males may be present only at certain times of the year, or absent altogether. The female reproductive system consists of one or two ovaries, each with a vitellarium gland that supplies the eggs with yolk. Together, each ovary and vitellarium form a single structure in the anterior part of the animal, opening through an oviduct into the cloaca.

18.5  Males are sexually fertile at birth. They have a single testicle and sperm duct, associated with a pair of glandular structures. The sperm duct opens into a gonopore at the posterior end of the animal, which is usually modified to form a penis.  However, the do have one other minor drawback apart from being much smaller than their female companions - they do not usually have a functional digestive system and are therefore short-lived.  Not much fun being a male Rotifer then!

18.6   Daphnia and Copedpods belong to the phylum arthropod (from Greek arthro-, joint + podos, foot) an invertebrate animal having an external skeleton, a segmented body, and jointed paired appendages, along with the insects, arachnids, myriapods, and crustaceans.  Daphnia are part of the order of small crustaceans, commonly called Cladocera or water fleas. Around 620 species have been recognized so far, with many more undescibed. They are ubiquitous in inland aquatic habitats, but rare in oceans. Most are 0.2–6.0 mm long, with a down-turned head with a single median compound eye, and a carapace covering the thorax and abdomen. Most species show cyclical parthenogenesis (see in Rotifers above) where asexual reproduction is occasionally supplemented by sexual reproduction,  which produces resting eggs that allow the species to survive harsh conditions and disperse to distant habitats.

18.8    Daphnia are typically filter feeders ingesting unicellular algae and various organic detritus including protists and bacteria.  Beating of the legs produces a constant current through the carapace bringing such material into the digestive tract. Food particles are formed into a food bolus which moves down the digestive tract until voided through the anus. The second and third pair of legs are used in filter feeding, ensuring large unabsorbable particles are kept out, while the other sets of legs create the stream of water rushing into the organism.

18.9     Daphnia move using Diel Verticle Migration (DVM) i.e. they migrate daily from the upper water layers, where they spend the night, to the deep and dark layers, where they spend the day. This behaviour reduces exposure of diurnal visual predators (such as many fish) by finding refuge in the dark near the bottom and then feeding undisturbed during the night in the food-rich upper water layers.   An adult female may produce a clutch of eggs every 3 to 4 days until her death. In the laboratory, females may live for more than 2 months.

18.10   Daphnia is one of the first organisms with a well-known ecology to be DNA sequenced, which is a milestone in ecological genetics.  A major question in evolutionary biology is how species have adapted to different environments and how the underlying changes in morphology, physiology and behaviour relate to modifications in corresponding genes. The publication of the first crustacean genome sequence, that of Daphnia pulex [1,2], is part of an effort by the Daphnia Genome Consortium to establish Daphnia as a model system for evolutionary environmental genomics. Daphnia is  one of the best-studied subjects in ecology.

18.13   Copepods vary considerably, but can typically be 1 to 2 mm long, with a teardrop-shaped body and large antennae. Although, like other crustaceans, they have an armoured exoskeleton, they are so small that in most species, this thin armour, and the entire body, is almost totally transparent. Some polar copepods reach 1 cm. Most copepods have a single median compound eye, usually bright red and in the centre of the transparent head. Like other crustaceans, copepods possess two pairs of antennae, the first is often long and conspicuous.

18.14   Planktonic copepods are important to global ecology and the carbon cycle. They are usually the dominant members of the zooplankton, and are major food organisms for small fish and other crustaceans such as krill in the ocean and daphnia in fresh water. Some scientists say they form the largest animal biomass on earth, competing for this title with Antarctic Krill.

18.15   The matter of copepods in the water supply, however, has raised a problem for some Jewish people who observe Kashrut.   Copepods, being crustaceans, are not kosher, and are not small enough to be ignored as nonfood microscopic organisms (since some specimens can be seen with the naked eye). The discovery of Copepods in the New York water supply in the summer of 2004 in particular caused significant debate in rabbinical circles and led some observant Jews to buy water filters.

18.16   Copepods are a holoplankton species, meaning they stay planktonic for all of their lifecycle. During mating, the male copepod grips the female with his first pair of antennae, which is sometimes modified for this purpose. The male then produces an adhesive package of sperm and transfers it to the female's genital opening with his thoracic limbs. Eggs are sometimes laid directly into the water, but many species enclose them within a sac attached to the female's body until they hatch. In some pond-dwelling species, the eggs have a tough shell and can lie dormant for extended periods if the pond dries.

18.17   Because of their smaller size and relatively faster growth rates, and because they are more evenly distributed throughout more of the world's oceans, Copepods almost certainly contribute far more to the secondary productivity of the world's oceans, and to the global ocean carbon sink than krill, and perhaps more than all other groups of organisms put together. The surface layers of the oceans are currently believed to be the world's largest carbon sink, absorbing about 2 billion tons of carbon a year, the equivalent to perhaps a third of human carbon emissions, thus reducing their impact. Here’s looking at you, Copepods!

18.19   Cyclops is one of the most common genera of freshwater copepods, comprising over 400 species, and is one of the two main species of copepod in Lake Anency. Together with other similar-sized non-copepod fresh-water crustaceans, especially cladocera, they are commonly called water fleas. The name Cyclops comes from the Cyclops of Greek mythology which shares the quality of having a single large eye, which may be either red or black in Cyclops.

18.20   Cyclops has a cosmopolitan distribution in fresh water, but is less frequent in brackish water. It lives along the plant-covered banks of stagnant and slow-flowing bodies of water, where it feeds on small fragments of plant material, animals or carrion. It swims with characteristic jerky movements. Cyclops has the capacity to survive unsuitable conditions by forming a cloak of slime. Average lifespan is about 3 months.

18.21    Calanoida is the other main type of copepod in Lake Annecy. There are around 40 families with about 1800 species of both marine and freshwater copepods. Calanoid copepods are dominant in the plankton in many parts of the world's oceans, making up 55%–95% of plankton samples. They are therefore important in many food webs, taking in energy from phytoplankton and algae and 'repackaging' it for consumption by higher trophic level predators.

18.18  As with Daphnia mentioned above, many planktonic copepods move with DVM, feeding near the surface at night, then sinking (by changing oils into more dense fats) into deeper water during the day to avoid visual predators. Their moulted exoskeletons, faecal pellets, and respiration at depth all bring carbon to the deep sea which is how they are able to act as the world's most important natural carbon sink. 

Continue Reading   Chapter Nineteen