Invertebrates

Invertebrates

Invertebrates are found in abundance in any form of water, and the pond or lake is no different in its diversity. For ease of reference and clarity, they will be discussed under their individual groups, citing the Latin Phyla, followed by a brief description of the more common members to be found in freshwater bodies.

Protozoa

The members of this group are the most primitive single celled animals of which about 80,000 have been identified and named. Most of them will require a microscope to enable more detailed study. Within this group it is sometimes difficult to define which is animal or vegetable. The Protozoa, although small, play a large role in the community's health and structure. Some form a large part of the plankton keeping check on minute plants and providing food for larger animals.

To assist in some form of classification, Protozoa has been split into its four main groups and a main example taken from each.

1.Flagellata

Within this group all members bear a flagella, the movement of which drives it through the water. The best known of this species is Euglena. It is often found in great abundance in ponds, in some cases in such great numbers as to cause a green scum or colouring in the water.

2.Rhizopoda

Maybe the best known of all is the Amoeba, often referred to in biology lessons as a prime example of a single celled animal. Although the body of the amoeba consists of only two layers of protoplasm, it is quite an efficient predator; any food material that may be encountered is quickly surrounded by pseudopodia and a thin film of water and becomes engulfed by the body.

Although the Amoeba is the best known example of the Rhizopoda group there are members which have shells made of chitin such as Arcella and Difflugia. A very common member quite unlike the others in structure is Actinophrys sol, an exquisite creature very much like a sea urchin in structure.

3.Ciliates

Ciliates are characterised by the possession of cilia, which they either use to move through the water, or to create currents to move the suspended food to them. An example of a mobile member of this group is Paramecium, unlike the Amoeba, it maintains a constant shape and possesses an outer layer of skin. The whole of the outer layer is covered in cilia. The movement of these enables it to be propelled through the water, and by a reversal of this action it is capable of moving backwards.

A static form of Ciliates is Vorticella. These can be found in dense masses looking very much like a fungal growth. The body is rather like an inverted bell around the rim of which are found the cilia; these are used to create the current, which carries food to them.

 

 

 

 

4.Sporozoa

These are the internal parasites, which can have very complex life histories. The most common form to be found in ponds or lakes would be the fish parasite Glugea. These cause white nodules upon the fish's body, and in due course release large numbers of spores, which are able to infect other fish.

Porifera

These are the sponges, a relative of the sea sponge. They represent a link between the Protozoa and the many celled creatures Metazoa. There is believed to be around 150 species of freshwater sponges worldwide, and they are a lot more common than is realised.
They are far more complex than is realised at first glance. One way of thinking of them would be as colonies of protozoa; even though they are made up of many thousands of cells, these have not developed into special organs and each retain some of its independence. I will not delve too deeply into a sponge's structure interesting though it is.

Upon close examination of the external structure of a sponge it will be seen to be covered in pores, some large and some small. Water is drawn into the body of the sponge through the smaller pores and expressed through the larger ones. By this means food particles and oxygen are drawn into the body, and waste matter and carbon dioxide are removed.

A common example would be Spongilla lacustris. These can be found growing in fairly clear water as too much suspended matter will choke them. Also, the water contain enough silicon for the creature to extract enough to support its body. When present they will be found encrusting plants and tree rootlets or even wooden posts submerged in the water.

Coelenterata

A group which includes well-known sea animals such as sea anemones and jellyfish - something, you may well not expect to find within a pond, but they exist. The most common is the Hydra. These multi-headed creatures may extend up to an inch in length, having a hollow tube like body from which a crown of tentacles extends. When disturbed they have the ability to reduce to just a small round blob, quite difficult to identify with the naked eye.

They only feed on live creatures, such as small crustaceans or worms, but they also take very young fry if they are available. They hunt by waving the tentacles in the water; any prey which touches one is immediately injected by a cell called a nematoblast, a harpoon-like structure which paralyses the prey. Once trapped it is encircled by the tentacles and conveyed to the mouth.  Although normally they breed by sexual reproduction they do have the ability to multiply from pieces which are broken from the body, which is how they came by the Greek name Hydra.

The second member of this group is the medusa or jellyfish; an example would be Craspedacusta sowerbii. These have an interesting life cycle, which is in two stages a fixed hydra-like creature and a free-swimming medusa. The medusa arises on the side of the fixed form, and forms a jelly-like appearance. These become detached and swim away. This is a reproductive stage, sperms being formed in some and eggs-cells in others. Once fertilised the egg cells develop into small free swimming organisms, which later settle on a solid object and turn into the hydroid form so completing the cycle.

Platyhelminthes

The flatworms are the lowest form of worms with unsegmented bodies. In fact their bodies are flattened to such an extent that they posess no body cavity, and their organs are embedded in to the compact middle section. They are hermaphroditic in nature, having both male and female sex organs, and are capable of producing either eggs or sperm; but the eggs of one individual must be fertilised by the sperms of another.

Tapeworms (Cestoda) and Flukes (Trematoda) are better known members of the Flatworm family but here we will look at the Planarians (Turbellaria). These are the largest of the Tubellarians and live a free existence and are not parasitic on other creatures.

Planarians are mainly a nocturnal creature. Their bodies are covered in very tiny cilia with which by vibration they are able to move. They are normally unable to swim freely, so they produce slime from glands just under the surface of the body which enables them to slide over any surface. They are often found on leaves or overturned stones and resemble small pieces of jelly. The mouth is not situated at the head but is found in the centre of the underside of the body. They feed on almost any small creatures and will also feed on dead animal matter. They are quite capable of lasting for a long time without food but they will reduce in size as they digest their own body tissue.

Flatworms are the simplest animals to have a central nervous system, a very primitive brain is found in the head region from which strands of nerve-cells course through the body. So they can be influenced by external stimulations such as heat, light, food and water currents.

Nematoda.

Roundworms are found both on land and in water, many being parasitic of man or other animals, but a lot less is known of those found in water. Roundworms have long unsegmented bodies which are normally colourless, these may be pointed at both ends and covered in a tough cuticle. Sexes are normally separate.

Many are parasitic for at least part of their lives, both in animals and plants. Also quite a number of free-living nematodes can be found within the pond. An example would be Dorylamius stagnalis. These may reach a length of 5mm and can be found in the silt especially where there are plants.

Rotifera.

The Rotifera are again minute creatures but more advanced than the protozoa even though some may be smaller in size. They are multi-cellular and normally consist of a cylindrical body shape, which tapers to what is called the foot. The top section of the head is surrounded in cilia which are used both for locomotion in mobile species and also to supply oxygen and food to the body.

Most rotifers have a clear body so it is quite easy to study the passage of food through their body, and identify most of the internal organs. Most specimens caught are female. Males have no digestive organs and are normally smaller. I will not attempt to suggest any particular species as a reference, and would suggest a search under the heading of Rotifera will bring good results, as the genera is quite large and name variations are quite common.

Polyzoa.

Freshwater Polyzoa are not as many in number as their saltwater cousins, but they have the same sort delicate structure. They form colonies around the waters edge and can also be found growing on floating debris. The most common structure is that of a sac, which forms the body, from which a crown of tentacles protrudes, which can be withdrawn into the body at will. A fine example for observation is Cristatella, with a fine crest of tentacles and not easily frightened into retracting them. These tentacles are waved around and create a swirl of water in which their food becomes engulfed and is quickly directed to the stomach. The diet consists of diatoms, infusoria, rotifers and other similar minute creatures, and they have three means of reproduction, these being sexual reproduction, budding and statoblasts, a form of internal budding.

Annelida.

We now come to the worms, creatures that any one should recognise from the garden. These are the true worms in having segmented bodies unlike the flatworms and round worms above. The Annelids are divided into two groups: Chaetopoda being worms which have bristles and Hirudinea which make up the leech group. These groups again can be further sub-divided, but we shall not delve too deeply into the family structures. Most freshwater worms are relatively small and often go unnoticed by the casual observer. Let us look at Naididae as an example. This is one of the largest families of freshwater worms, they are small rarely exceeding 25mm (1 inch) in length some only being as little as 3mm (1/8th inch ) long. They rarely lay eggs, budding being the commonest form of reproduction.

The leeches which, with their blood sucking activity, are not often considered as desirable creature to have within a pond, are in fact common in the freshwater habitat. But the good news is that not all leeches are large blood sucking creatures that will attach themselves to those who step into the water. True they require blood to survive, but not necessarily human blood. Snails, worms, fish even frogs can be the chosen victim. There are two main groups of leech; Rhynchobdellae, leeches with a proboscis but no jaws, and Gnathobdellae, leeches with jaws. The horse leech (Haemopis sanguisuga) is a good example. It is usually found living under stones at the edge of a pond and even leaves the water at times to enter damp earth. They grow to about 60mm (2 1/2 inches) but can extend themselves to as much as 150mm ( 6 inches). Their colour can vary with location and they have five pairs of eyes. They mainly feed on snails, worms and tadpoles but will take small fish and frogs. Now for the surprise: they swallow their prey whole as they are not a blood sucker. Despite the name and reputation, it is quite harmless to mammals as it would be incapable of
piercing the skin.

But not all leeches are quite so harmless to mammals. Those with a proboscis pierce the skin of their victim whilst those with jaws bite, leaving a Y shaped wound. All the blood suckers secrete from there salivary glands a anti-coagulant to ensure that the blood flows freely whilst they feed.

Arthropods

This group found within the pond are often associated with crabs, lobsters and shrimps. In fact this group includes spiders mites and insects. Its the largest and most successful group of animals on the face of the earth. Arthropodia means joint limbed animals, and they make up three-quarters of the total animals living in fresh water. Obviously with such a group that is so diverse we can only touch upon them within this page, but general characteristics apply in the majority. The commonest feature found in arthropods is the fact that they have paired jointed legs. They have external skeletons, which is a toughened layer called chiton: this serves to carry all the internal organs. This tough outer layer is what makes them so successful, but has a disadvantage. They are unable to grow with such a rigid structure and so must moult as they grow. Aquatic arthropods have devised many different ways to breathe, some having gills to extract oxygen from the water, others absorbing air through there skins, or even air tubes which are thrust through the surface to obtain their life giving air. So let's now take a look at a few examples from each group within this phylum.

1.Crustacea

The branchipods, which are also known as the gill footed crustaceans, are what could be classed as the largest group within the freshwater environment. The first example we shall look at is the Fairy Shrimp, a little creature about 25mm (1 inch) in length, with eleven pairs of limbs along its thorax with which it both propels itself and circulates water over its body to supply it with oxygen.
They are very active creatures, swimming on their backs, feeding on microscopic algae which are captured by their legs and passed to their mouths. They are transparent and the eggs are carried by the female in a brood pouch. They can be found in any body of water no matter how small and their eggs are able to withstand desiccation in fact some species require such a period before hatching.

Next we have the Cladocera, what I consider one of the most important invertebrates to be found within a pond. Perhaps the most commonly known is the Daphnia ( water-flea). There are many different species, so let's look at Daphnia magna, perhaps the largest of the group growing up to 5mm in length. They can be found in some waters in incredible numbers. Taking a close look you would notice that apart from the head and its antennae, the rest of the body is encased in what appears to be a two shells but is in fact a single covering folded over the back. This shell is transparent and you are able to see all the internal organs working within the animal. The heart can be seen and also the blood being pumped around the body. The blood is of particular interest as with most cladercerns as they are able in oxygen poor water to increase the amount of haemoglobin to assist in respiration. This can make them stand out as brown patches swimming with their jerky movement which earned them the name of water-flea. Minute floating algae is the main diet along with bacteria, and during periods of bright sunlight they can multiply rapidly as the algae increase, but they are sensitive to light and will be found in the water column at the level most suited to them at different times of the day.

Next we jump to the Decapoda. These are the crayfish and the crabs. Both may be found in freshwater but we shall stay with the crayfish. They are very much like small lobsters in structure, the males being larger than the female of the species. Crayfish need well oxygenated water and do better in hard waters where limestone and chalk are found. They are rarely seen during the daylight hours as they are a secretive animal. Crayfish are carnivorous animals eating any small creatures they can catch such as insect lavae, water snail or tadpoles. They will even turn cannibal if food is in short supply. They are unable to tolerate impure water so they are a good indicator as to water condition.

2.Insecta

Insects are the most commonly seen creatures in the pond environment. Midges, mosquitoes, dragonflies and gnats we have all seen them but these are the adult stages that we see flying around the pond which rarely enter the water. Its the immature young that are at home beneath the surface. There are so many different kinds of insect that it would fill a book if we attempted to discuss them all here, so we shall mention only two - the dragonflies and the water beetle.

The Dragonflies are perhaps one of the best known of all pond insects, if not by their name then certainly their bright colour. There are two kinds of dragonfly, the hawker or darters as they are called, and the damselflies. Both are voracious predators, eating large numbers of other insects, even members of there own family. They are totally harmless to man or livestock as neither can they bite or sting, but they do have powerful jaws with which they dismember and devour their prey. They have large compound eyes so they have very good vision but even with that they are not the best of hunters.

 

The dragonfly nymphs are found in most still waters. They are fragile creatures with three large leaf-like gills at the tail end. They are sluggish creatures found hanging on plant stems or crawling on the bottom waiting to capture their prey. They have strong jaws which they are able to thrust forward to capture their prey so that they can devour them at there leisure. When the time comes for them to emerge from the water to change into their adult form the nymph climbs from the water often up a plant stem and waits to dry. As it does so the skin splits down the back and the adult climbs free until it has completely emerged. The wings are small at this time so blood is pumped to them and the body until they expand. Once this has completed the adult is able to fly and may live for as much as a month.

Although some beetles spend their whole life in water, they are in the minority. Some just use water as a stage of their development like the dragonfly. The beetle's wings are hidden when not in flight under their forewings, which have become hard and horny. They serve no purpose in flying and are used solely for protection. The true wing lies below this and is normally quite long and membranous, but is folded away when not in use.

 

Dytiscus is a true water beetle, and a close examination of its body will show this. It is streamlined for easy progress through the water, and the legs especially the rear are covered in hair to increase water contact when it swims. It is not common for a beetle to be able to obtain all its oxygen from the water and so they rise to the surface to take air. This they do through air -holes which are situated under the wing cases. They are also able to slightly lift the wing cases and trap air in fine hair so as to increase the time they are able to remain below the surface. Beetles can be fiercely carnivorous, taking any creature that passes by even those larger than themselves. The larvae which also live under water are normally lighter than water so that they float to the surface to breathe. They are even more ferocious than the adults and eat anything that passes their way.

3.Arachnids

These are the spiders and mites. Both are found in freshwater with the mites being the more common of the two. Spiders are common at the water's edge, but true water spiders are often overlooked. They are similar in appearance to the normal spider, but these live totally in water but still breath air. When the spider is first placed in water, air becomes trapped in the fine hairs which cover its body. This is enough to last the spider quite a while, but not good enough to allow it to live beneath the surface. To do this the spider spins a platform beneath the surface then travels to the surface to collect air that is trapped beneath the platform. the spider does this many times and in the end an air bell is created. It does not need to be able to insert its whole body - just the abdomen will be enough - and from this supply it is able to live if need be for months below the surface. Although this may not seem very efficient, oxygen is able to diffuse into the bell from the water and carbon dioxide is able to diffuse out. From this diving bell the spider is able to lay lines in the water to trap live or dead animals which it will carry back to the bell to consume.

I hope from these few brief paragraphs suffice to introduce the reader to the exciting world of invertebrates that lies below the surface of a pond. It has creatures of great beauty and also savages that would compare to the most ferocious lion. They are mostly small but without them our beloved ponds would not be the places we have come to enjoy, so next time you visit take a closer look you may well be surprised.