The craspedote, a freshwater jellyfish

They scare, fascinate: jellyfish populate the seas and oceans, and leave neither holidaymakers nor naturalists unmoved. And when we meet a small species in our lakes, ponds, canals, after a moment of surprise, we can only go to discover Craspedacusta sowerby, present in freshwater of all continents.

A small jellyfish in our water bodies

In 1880, London scientists discovered a jellyfish in a pond of exotic plants. Translucent, it is clearly visible to the naked eye with its 2 cm diameter. Named Craspedacusta sowerbyi, or freshwater Craspedote, it belongs to the Cnidaria, a cousin of the jellyfish of the seas and oceans, but also of anemones and corals. It is the only freshwater jellyfish that meets in its class of Hydrozoans only Hydras, polyps always fixed.

Since its first description in London, the Craspedote has been observed on all continents, in 1929 in the Garonne pond and then in all the rivers of France.

Fragile and elegant

The encounter is surprising: bending over the waters of a lake, a canal, an old gravel pit, or even a pond, one sees small translucent, parachute-shaped organisms the size of a 2 euro coin floating. The freshwater Craspedote is a typical Hydrozoan: its umbrella is lined with more or less long tentacles, up to 400, a velum closes the parachute on its lower face. Under the umbel, crossing the velum, a tube, the manubrium, springs out, equipped with an external orifice and connected to 4 radial channels in intern. All the anatomy of the medusa is visible by transparency.

It floats in the middle of its preys

The freshwater Craspedote can move by vigorous contractions of its umbrella, but lets itself float according to the movements of the water, among the zooplankton. It is thus within reach of its preys. Copepods, daphnids, all the small animals of the plankton will be captured by its tentacles, carried to the opening of the manubrium, the mouth, to be swallowed. Once the digestion is finished, the waste will be rejected by the same orifice, serving as anus this time.

In Craspedotes the tentacles are provided with cnidocytes, stinging cells provided with harpoons with paralyzing effects. Only on small preys, the man does not seem to have to worry about its presence, its epidermis cannot be crossed.

It appears and disappears suddenly

The Freshwater Craspedote can go unnoticed one year, as well as proliferate in abundant populations the following year. Then disappear again. Before being a floating jellyfish that can be observed with the naked eye, it takes the form of a tiny polyp of a few millimeters attached to the bottom, to vegetation, to rocks. Its small tentacles allow it to feed on animal micoplankton. This polyp will reproduce by asexual way, by budding, release of crawling "larvae" moving and forming a new polyp, and production of medusoid buds. Some will become medusas leading a sexual reproduction. The fertilized eggs will pass through the planula stage, a small floating ciliated larva that will settle and become a polyp.

Present on all continents and therefore considered as an invasive species, the freshwater Craspedote would not represent a threat to ecosystems. Its feeding in zooplankton remains limited, its presence remains discreet. The impact of climate change on the evolution of the populations of this species may however modify the current data in the coming years.

The koi carp

The Koi carp is a pond fish particularly appreciated by gardeners because of its large size, its colorful appearance and its proximity to humans.

The Koi carp, its origin

The Koi carp is, like the common carp of our rivers, a cold water fish particularly appreciated by gardeners with an outdoor pond. Originally, the Koi carp was Chinese. It is a descendant of the Magoi carp which was quite close to our European carps since they were essentially grey.

Introduced in Japan around the year 200, they became around the year 1000 ornamental carps raised exclusively by the Japanese nobility. Over the centuries, the Koi carp has been adorned with beautiful contrasting colors which makes it look today like very large goldfish that can reach 60 cm long.

Koi carp were introduced in Europe after the Second World War. These animals are distinguished from other species by their temperament and their peaceful and routine behavior, very close to humans. As long as the feeding is at regular time, the Koi carp comes easily to the contact of the hand that dispenses its food.

According to the habits, the gardener can thus perfectly caress his fish! The other great advantage of the Koi carp is that it is essentially herbivorous. It cohabits perfectly with all varieties of goldfish which is an important aesthetic argument since the pond can be adorned with a beautiful ballet of small and large fish.

There are several types of Koi, among which the scaly carp which meets the Japanese standard, the mirror carp of European origin, with large scales on the dorsal side and the attachment of the fins, the leather carp of German origin without scales or the carp with sail fins of American origin. In terms of color, if the red and white of the classic Kohaku carp dominate, many variants are possible such as the Sanshoku (red, white and some black scales), the Tancho (circular red spot on the head), the Shiro Itsuri (bicolor black and white), the Ki Utsuri (bicolor orange and black), the Shiro Muji (integrally white), the Ki-Goi (monocolored bright yellow), the Goshiki (5 colors: black, red, white and different shades of blue). ..

Did you know that ?

Koi carp is an imposing fish. For reasons of space, there are usually 10 to 12 carps of less than 15 cm for one cubic meter of water and only 1 to 2 of more than 45 cm for the same volume of water.

The Ide Melanote, a little known fish

Here is an animal that is not very common in our ponds and yet deserves to be better known! The Ide Melanote is a fish that is not afraid of the cold and that, by its mode of feeding, helps to regulate the ecosystem of the pond.

Presentation

The Melanote Ide (Leuciscus idus) is a member of the Cyprinidae family. This fish looks like a roach. Of gregarious nature, it likes to live in schools.

Its body is tapered and its color is mostly silver but can vary from blue-gray to a very dark green depending on the age and the period. The belly is whitish and the sides are always lighter and silvery. The fins are gray except for the pelvic and anal fins which are pink or red. Its head is narrow, with a mouth directed upwards.

The maximum adult size is 75 cm in length for a weight of 4 kg.

Habitat and feeding

This fish is found in the wild in rivers, ponds and lakes in Central and Eastern Europe, as far as the Ural plain. It is also very common in Great Britain. In France, it can be found in the Rhine, the Moselle or the Somme, more rarely in the Loire and the Sarthe. The Ide is not a difficult fish, it can live as well near the bottom as at a few centimeters of the surface and tolerates also brackish waters.

The Ide is a good ally in the pond because it feeds mainly on insects, small crustaceans, plant debris and algae, which allows to limit maintenance operations. Be careful though, because in very large ponds, where it can grow to its maximum size, it could feed on small fish.

Reproduction

The reproduction period of the Melanotus Ide is in spring, between April and June, depending on the weather conditions. A female can lay between 42,000 and 260,000 eggs in a shallow area with a gravelly bottom where the eggs can cling. Many will be destroyed before hatching or absorbed by predators in the first few hours of their life, thus limiting the number of individuals. The Ide can live more than 20 years.

Behavior in the tank

The Ide is a gregarious fish, it will be necessary to introduce several subjects of this same species (at least 5).

A pond of at least 25 m² is necessary for its lively movements under the surface. It is a hunter and spends its time cleaning the pond of insects that are on the surface or that have fallen into the water. It also appreciates aquatic insects, mosquito larvae and can also be very useful to rid the pond of green algae and other plant invasions.

The stickleback, a nesting fish

A superb fish the size of a small finger slips into the vegetation: blue eye, red belly, slate blue back. Exotic, installed in the living room aquarium? Not at all, the stickleback lives in our rivers, our ponds, busy building a comfortable nest for its young.

A small bony freshwater fish

The three-spined stickleback is a bony fish of the Gasterosteidae family. It lives in rivers with calm currents, but also in ponds, puddles and isolated waterholes, all over France. Common, ubiquitous, it walks its long body from 4 to 10 cm, with a greenish back, a silver belly, in the abundant vegetation, on muddy or sandy bottoms. On its sides, no scales, but bony plates; on its back a row of spines, three in Gasterosteus aculeatus; under its belly not fins but pelvic spines. And especially remarkable spring colors. In the male, only, which from March prepares for the mating rituals.

A colorful male

The arrival of spring announces for the male stickleback a new outfit, with bright colors. His throat, his flanks, will become orange-red, his back slate blue. And his eyes of dull gray become azure blue. The female doesn't change anything in her wardrobe, it is she who will choose the most attractive male, with the most attractive parade. The one who will fertilize the eggs. Prepared by dozens in the females, they are so numerous and so big that she grows spectacularly, up to 25% of her weight in addition. She will entrust them to the paternal care, particularly serious.

A safe and cozy nest

Not content with adorning himself with beautiful colors, the little male stickleback works for his offspring. He chooses a territory, in shallow fresh water, with dense vegetation. On the bottom, he builds a nest of intertwined plants, bound by a mucus of his own making. Until July he will watch for females. And will seduce them of a dance, parade of presentation of its body of male and its sure and cosy nest. The conquered female will then lay her eggs in the nest. And the male will chase her away immediately, to take care of his young. He watches over them, ventilates them by creating an oxygen-carrying current. 8 to 10 days later, when the young are born, he will still be there, ensuring their protection until they leave, carried away by the current to other territories.

Dreaded carnivores and sought-after prey

The young sticklebacks will disperse, forming small scattered groups from several nests. Carnivorous, they will feed on zooplankton, crustaceans, insects, larvae and adults, worms, fry... And will even practice cannibalism, devouring each other. Sometimes cited as harmful for their voracity, sticklebacks are themselves hunted. They seem to be formidable with their row of hard and sharp dorsal spines rising up during an attack, not to mention their lateral pelvic spines always ready to hurt. But some predators have been able to adapt: the kingfisher knocks it out before swallowing it, the spines at rest are always folded. The heron will prefer to crush it with its powerful beak. The egret gobbles it head on...

The stickleback is a common fish of our fresh waters, not well known despite its amazing habits. Often raised in aquariums, it offers a fascinating show, spring colors, nest building, ventilation of the young. But under the waters of our rivers, ponds, sticklebacks are in regression and are classified on the list of protected species in "minor concern".

The argyroneta, a diving spider

Discreet spider, the Argyronetus lives, hunts, feeds, reproduces, under water. Often unknown inhabitant of our ponds, it leads an original life, totally adapted to the aquatic environment.

An aquatic spider

It lives underwater in the middle of fish, frogs and dragonfly larvae. The Argyronet, Argyroneta aquatica, is the only spider known to lead its complete life cycle in an underwater habitat. Without gills, with its aerial respiratory system, it has managed to invest the third dimension of the aquatic world. And among spiders, it reigns as the uncontested master. Of small size, 8 to 15 mm, it knew how to adapt with ingenuity to an environment full of resources. The only one of its kind, of the family Dictynidae, it can be found in the whole Palearctic area.

It lives underwater but breathes oxygen from the air

The Argyronete has not developed an aquatic respiratory system, its adaptation is simple, ingenious and efficient. No need for tanks or scuba. It goes to the surface, captures air from its back legs which comes to be fixed solidly on its fine hydrophobic gray hairs and a bubble is then formed around its abdomen, conferring its silver aspect. This bubble supplies air to its respiratory trachea. This form of air storage having its limits, she has developed a custom-made habitat.

A self-breathing air bell

The Argyronet is the architect of the pond. It weaves its habitat, an inverted bell attached to the plants and impermeable to water. Its appearance has earned it its name, "Argyroneta" meaning "silver web". The properties of this cloth are amazing: it filters gases. The oxygen in the water diffuses into the bell filled with air by the Argyroneta, the carbon dioxide is rejected into the water. The web breathes. But these exchanges do not allow to keep a balance between the different gases, and regularly the Argyronetus must go back to the surface, recover air in the hairs of its abdomen and release it under its bell.

An accomplished swimmer

The powerful front legs of the Argyronetus allow it to unhook itself from the plants and swim with ease. The air bubble, depending on its size, more or less large in males or females, would encourage floating and would influence the agility and speed of each individual. The network of tracheas, extremely developed as in many insects, allows to diffuse the air in all the body, but would also play a hydrostatic role supporting the stability in water.

A hunter on the lookout

A predator, the Argyronetus hunts on the lookout, chasing its prey at full speed, scouring the waters in search of insects, small crustaceans, and even fish. It captures them with its powerful front legs, then neutralizes them definitively with its venom. She cannot consume them underwater, and must therefore bring them back under her bell, or to the surface. Encounters of the Argyronetus with humans, with very distinct habitats, are rare, but cause painful bites, although the venom is harmless.

Males and females, aquatic encounters

The male in the Argyronete is distinguished by its larger size, it joins the female in her bell where mating takes place. The eggs will be laid at the top of the lodge, surrounded by white silk, late spring and summer, the young are born and remain a long time in their shelter. The successive moults will take place in the air, under a bell or on the surface, the young will remain close by or will disperse along aerial threads to explore and colonize new aquatic spaces

Two cousins that walk on water

Large and powerful wetland spiders, Dolomedes live near ponds and calm waters. Powerful, the adults can capture large preys, like damselflies. But also small fish, their fishing technique is unstoppable: they vibrate the water with their front legs and wait for their meal. They make brief appearances underwater, clinging to plants, but generally remain on the surface, leaving the Argyronetus the exclusivity of the aquatic depths.

A pond, calm waters, plants to anchor its web, a little polluted water, some preys: the Argyronet can establish itself, live its adult life for two years, settle its young. And thus perpetuate its species with its so particular way of life. An unsuspected world to preserve.

Introducing fish into a pond

What could be more pleasant than watching the bright and colorful movements of fish in a pond? They help to control the growth of algae and mosquitoes and contribute to the balance of the ecosystem. Let's see when and how to introduce them into a pond so that they live happily.

A question of volume

Before thinking of introducing your fish, it is necessary to evaluate the volume of the pond in order not to overcrowd it and to install adapted species.

Another parameter to take into account is the depth, because in regions where it freezes in winter, the layer of ice can be thick, which does not pose a major problem for certain species as long as the depth is sufficient so that the fish are not caught in the ice. A minimum depth of 50 cm is required for small species in areas where freezing temperatures are prevalent.

When should I introduce fish into my pond?

The ideal time to introduce fish is during the summer months after the last frost. Spring and summer are good times to bring fish into the pond.

If you have just built your pond, wait at least 1 month after filling it before introducing your fish so that the water has acquired a good biological balance.

Planting aquatic plants beforehand is a plus because they oxygenate and purify the water, the fish will be able to hide there, lay eggs and enjoy their beneficial shade.

How do I get fish for my pond?

• Do not take fish from the natural biotope (lake, river, pond...) to introduce them into your pond.
• When buying fish, always check the sanitary conditions of the point of sale. The aquariums must be clean and no sick fish must be present at the risk of contaminating the others.
• Choose lively fish with well-developed fins and no suspicious blotches.
• The gills should not be red or excessively expanded.
• The fish should swim briskly between two waters. Avoid fish that are apathetic, stay on the surface or too deep (except for bottom cleaning species).

Which fish for my pond?

Depending on the volume of the pond and the climate, various species can be introduced, but make sure that they are compatible with each other because some fish are carnivorous and will not hesitate to attack others.

Let's start with the most common species and probably the easiest to raise at first: the goldfish.

The goldfish

Goldfish are gregarious animals that enjoy the company of others of their kind. Introduce at least 3 fish at a time to your pond. The classic species (Carassius auratus) has the advantage of being able to withstand the cold and adapt to the size of the pond by stopping its growth. A goldfish can live 25 to 30 years and reach 40 cm in length in large ponds. Whatever the size of the pond, provide at least 0.3 m³ of water per fish. Beware of ornamental varieties such as sail tails or fish with bulging eyes (Lorgnette de Ciel, Uranoscope, telescope...) which are much more fragile and require much more heat than the typical species.

The melanot fish

Melanotas are easy to live with and do not fear the cold. Silver in color, they move briskly creating reflections under the surface of the water.

The Ide is a gregarious fish that needs others of its kind to be happy, introduce 5 ides at a time to the pond.

The Ide can live more than 20 years and reach 75 cm in length. Allow 1 m³ of water per fish and a depth of at least 80 cm of water.

Koi carp

The legendary Koi carp need the same volume of water as the Ide per fish. Revered in Japan, these colorful carp can reach 80 cm in length and weigh 8 kg.

This fish can withstand cold weather and live up to 50 years without worry, but the pond must be deep enough (at least 1.20 m).

Sturgeon

The sturgeon is to be reserved for very large ponds because it can reach more than 3 meters in length for a weight of 300 kg. A total size of 25 m² and a depth of at least 1.20 m is required to accommodate this fish, which can withstand the cold very well in good conditions.

How do I introduce fish into my pond?

Introduce one species at a time, in small groups if the species is gregarious.

If there are already fish in the pond, make sure to choose a compatible species and introduce the new fish in small numbers.

Do not empty the bag of fish directly into the pond. Put it in the pond water for about half an hour to allow the temperature to adjust and the fish to get used to their new environment.

Then open the bag and add some water to the pond.

Wait another hour or so before releasing your fish.