Giant Fishes of the Rivers

How much longer will we have them?

The vertebrate class of Pisces contains members of two major groups of fishes: the cartilaginous species and the bony fishes. The most massive (if not the longest as well) of all fish are the Whale Shark (Rhincodon typus) and the Basking Shark (Cetorhinus maximus). These two species are peaceful giants, feeding on plankton that they filter out of the ocean water with their comb-like gill rakers. However, in this blog I want to write about giant freshwater and not marine fish. Yes, river fish, for several of them are bigger than the marine bony species (with one exception: the oarfish Regalecus glesne, of which during the 1967 “Walther Herwig” South Atlantic Expedition I caught a glimpse of one 7.30 m long specimen. This midwater species is said to reach a maximal length of 13 m).

What prompted me to write this blog was the sad news that the giant Yangtze River paddlefish Psephurus gladius was officially declared extinct. It thus follows the Yangtze freshwater dolphin Lipotes vexillifer, affectionately called ‘the baiji’ in Chinese but declared extinct in 2006. In 1970 there were still 25 tons of Yangtze paddlefish caught, but the last individual was probably killed in 2007. Specimens of at least 5 m in length existed. Another Chinese giant with a length of approx. 4 m is the Chinese sturgeon Acipenser sinensis, but it may also soon be gone forever for its population decreased by 98% from 1973-2010. As of this year (2021) all commercial fishing in the Yangtze will be banned for 10 years, but as welcome a measure as this is, it cannot bring back what is lost forever.

Probably the largest with a maximum length of around 7 m of all freshwater giants is the Beluga sturgeon Huso huso of the Caspian Sea and the Sea of Azov. Because of the value of its roe (the famous “Caspian caviar”) it is farmed, but the eggs of its also farmed but only 5.5 m long relative, the Huso dauricus, are considered even tastier. In northern Europe adults of the 3 – 4 m long sturgeon Acipenser sturio used to enter rivers to reach their breeding grounds upstream, but one of the few streams where these sturgeons now still breed is the Gironde in France. Recolonisation efforts are in progress elsewhere, but so far with minimal success. A huge surprise for our professor and us students of the Zoology Class at the University of Kiel in 1968 was when an almost 2 m long catfish of the species Silurus glanis was delivered. It had been caught the day before in the small North German freshwater stream known as the Trave. The East Asian Mekong River Pangasianodon gigas catfish and South American species of river catfish are said to reach maximum lengths of 4 m and can become a danger to bathers. Of the other South American freshwater giants the Arapaima gigas with a reported maximum length of 3.4 m and the electric eel Electrophorus electricus with a length of 2.5 m need to be mentioned.

Africa’s largest freshwater fish is the Nile perch, but it hardly reaches 2 m. On the other hand there are some quite big bony fishes in the North American Mississippi-Missouri river system. There is the North American paddlefish Polyodon spathula and the fierce-looking alligator gar that can both reach a length of 3 m. Although not in imminent danger of becoming extinct, these two species are considered vulnerable and an analysis of what led to the extinction of the Yangtze River paddlefish may help save the North American species and other freshwater fish giants too. It is believed that the biggest threats to the vulnerable freshwater fish giants (but even smaller species as well) are dams that block the fish’s access to upstream spawning grounds, and furthermore overfishing, the use of illegal methods (like electro-fishing, nets with unacceptably small mesh widths, and explosives). Pollution of the river water by chemicals like insecticides, by effluents and other wastes from urban areas, and by fertilizers used in agriculture must not be forgotten. But what is certainly interesting is that rivers compared with the oceanic habitat contained more giant bony fish species than the sea. Any suggestions why?  

lication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

© Dr V.B. Meyer-Rochow and, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

Star-Nose Mole and Desman

They are weird, but cute is something different

In this blog I want to introduce two rather extraordinary species of mammals, of which I had the luck to observe at least one: the Pyrenean desman (Galemys pyrenaicus). In 1981 when I was working in the CNRS Laboratoire souterrain de Moulis in France, I inquired what that long aquarium which I saw in one of the rooms was used for. And I was told of the research on the rare aquatic mole-relative, known as the desman, by a former priest, who had earlier been on a mission in India, fallen in love with a nun, left the priesthood, married her and had returned to France, where he was now carrying out observations on the biology of the enigmatic “desman des Pyrénées”. I met Monsieur Richard, was invited to his home to meet his wife and assisted him in his work on the desman on one evening.

What a strange animal, was my initial thought, when I saw this hyperactive mammal of less than fist-size for the first time! It was bobbing up and down on the water surface like a cork, stuffing a little worm it was holding in its short front paws into its tiny mouth, only to frantically diving about 30 cm to the gravelled bottom of the aquarium again to stick its long and incredibly mobile and prehensile nose between stones and pebbles to detect prey, e.g. insect larvae or small worms that it could then grab and take to the surface to eat or to climb (with the aid of some sharp claws) onto a small stone platform to rest or to scratch and dry its wet fur. I stopped the time the peculiar animal stayed under water and marvelled at (for its body size) huge webbed hind feet. The animal had small eyes, but exceedingly long whiskers (called vibrissae) around its nose and a very long and thin, scale-covered tail, reminiscent of that of a rat (to which the desman is not at all related as the rat is a rodent and the desman belongs to the moles and thus insectivores). It seemed to have a buoyancy problem and easily floated to the surface, which made its dives seem very laborious, requiring a lot of energy; something that must also have been a problem with the very cold mountain streams it preferred as its habitat. Already not exactly common 40 years ago, it has apparently become much rarer still today.

If this aquatic mole-relative wasn’t weird enough, then lets look at another species of mole, one I’ve never seen and know only from reading about its incredible sensory adaptations: the star-nosed mole (Condylura cristata), sometimes also referred to as ‘star-muzzled mole’. This species, like the more common Talpa europaea is virtually blind and spends all its life undergroud in an environment of moist soil in the northeastern region of North America. What makes this species so unique are the grotesque 22 fleshy appendages around the nose, which contain sense organs that respond to the minutest vibrations and serve as what has been termed a “tactile eye”.  In spite of its location on the nose, the appendages do not contain olfactory receptors and do not function like tentacles to grab prey, for there are no muscle fibres in them. Arranged in two groups of 11 on either side, the fleshy appendages, each with its own directional sensitivity, represent specialized mechanoreceptors that contain superfast conducting myelinated axons. Neuronal responses to the faintest stimulation occur with a latency of an average of 11.6 milliseconds. To decide if something is edible requires 25 msec for this mole, but in humans, by comparison, the process has been reported to take 600 msec.

According to Vanderbilt University’s Drs. K. Catania and J.H. Kaas, a little more than half of the brain of the star-nosed mole is dedicated to analyse signals that arrive from the star-like appendages. Each hemisphere of the cerebral cortex possesses clearly visible 11 stripes representing the 11 appendages of the nose-star’s opposite (contralateral) side. Now do these strange two mole species that this blog has been about have something in common? Yes, they do, for the star-nosed mole also loves water and swims well and, as Kenneth Catania in the year 2000 has written  “Mechanosensory organs of moles, shrew-moles, and desmans: a survey of the family Talpidae with comments on the function and evolution of Eimer’s organ”, they also share the organ responsible for the amazing tactile sensitivity: named after Swiss born Theodor Eimer, who first described the organ in 1871.

© Dr V.B. Meyer-Rochow and, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

To Breed some Animal Species in Captivity

Why is that so Hard or Impossible?

I suppose I have mentioned a few times before that I love fish (and not only to eat) and that one of my hobbies as a teenager was to keep and breed aquarium fish. At that time the big challenge was the Amazon Blue Discus Symphysodon aequifasciatus, because food, temperature, aquarium plants, lighting, the pH and the hardness of the water  – they all played critical roles. When captive breeding eventually succeeded, the price of this formidable species came down significantly. Another blue species (and there seems to be something with the colour of “blue”) is the marine Blue Tang, which has also been difficult to breed (like other coral reef species), but recently has become possible. But why do people want to breed fishes in captivity anyway? One of the main reasons would be the commercial aspect (some species are valuable as a food item and some in the aquarium trade as pets). But there is also the aspect of safeguarding that a species in the wild does not become overfished and ultimately extinct. A third reason would be vanity, namely to be the first person to succeed in finding a way to successfully breed a species that others had found impossible to breed.

But what actually makes it so difficult to breed a species, whether it’s a fish, a bird, a mammal or indeed any animal, in captivity? Looking at the two marine organisms that I had been associated with first, namely eels and rock lobsters (also known as spiny lobsters or ‘langouste’), it is mainly the fact that the larvae of these species spend a long time as marine plankton in the ocean and need a special kind of food that is difficult to prepare and to administer in captivity. Although one Japanese lab has succeeded to rear the Japanese eel Anguilla japonica from egg to a young individual, the effort in terms of costs and equipment is prohibitive and not yet a solution to reduce the collecting of eel fry and glass eels when they arrive from the ocean to enter freshwater and to curtail their sale to eel farmers who then fatten them up to turn them into young and sellable eels. A similar story is that of rock/spiny lobsters: for 20 years Japanese researchers tried to copy the entire life cycle from egg to marketable size in the lab of Panulirus japonicus and the best they could achieve was that about 1 out of a thousand larvae that hatched from the eggs in the lab and were then looked after for many years, would in the end survive to maturity. Commercially that would have been a flop and the research was terminated (at least in the lab to which I had sent a student to work in).

Not only are some aquatic species, and in case of the marine ones, hard or impossible to breed because of their long larval and planktonic phase (and also their size when thinking of tuna fish, the Blue Marlin or giant squid), but there are also the specific water quality requirements especially in case of the freshwater forms. However, some birds, mammals and other animals are equally well known to be difficult to breed in captivity and the cheetah and the panda are good examples. For the panda the extremely short oestrus period of 1-3 days/year is one problem, its choosiness regarding partner compatibility and its specific food requirements are another. Food for captive cheetahs is not a problem, but the very low sperm count in males is; the genetic relatedness of all extant cheetah individuals and the high mortality of the cubs are further notorious difficulties.

When it comes to birds (and apart from chickens and pigeons I never kept any), I heard that some parrots are not easy to breed and that especially their chicks require a great deal of care. I also don’t believe that it can be easy (or can be done at all) to breed swifts, house martins and swallows in captivity.  Or, thinking of cuckoos that need a host species’ nest with eggs in it, so that they can add their own egg to those of the hosts. Adult albatrosses cannot even be kept in a cage, let alone ‘be persuaded’ to construct a nest and breed in it. In fact keeping and breeding sea birds in captivity would be a challenge best left alone and forgone in favour of protecting these birds’ wild breeding places. There are, of course, many, more species of animals that are inclined not to breed in captivity, but I realize that for some, sadly, captive breeding may be the only way they will survive as a species so that our children and their children can see them ‘for real’ and not just in the computer or books.

© Dr V.B. Meyer-Rochow and, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.