Why are pufferfish poisonous?
It's well known that pufferfish are poisonous - but why are they? Rupert Collins enlightens us.
Most puffers harbour a neurotoxin called tetrodotoxin (TTX) which if ingested causes paralysis and death among humans. It is concentrated in the liver, ovaries and skin of the fish, but the flesh of the Fugu puffer (Takifugu rubripes) is an expensive delicacy in Japan.
The toxin is accumulated through the food chain. Marine bacteria initially create the TTX which builds up through grazing and scavenging invertebrates such as starfish and snails.
Ancestors of today’s pufferfish evolved resistance, enabling them to feed on these poisonous sources.
Once immunity was acquired, they stored and utilised the poison as a defence against predators.
Puffers raised in captivity and fed no TTX-containing foods are safe to eat.
This article was first published in the December 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.
Are captive-bred fish released into the wild?
Do any captive-bred fish that are endangered in their natural habitat ever make it back to the wild? Rupert Collins has the answer.
So far, the conservation status of only 10% of more than 31,000 fishes has been assessed, but 1,273 species have been classified as vulnerable or endangered. Currently 13 species are considered extinct in the wild but still maintained in captivity.
Captive breeding programmes, such as those of the Butterfly splitfin (Ameca splendens) undertaken at London Zoo have kept the lineages alive.
Releasing into the wild depends on whether the habitat is fit enough for them and whether sufficient funds actually exist for the necessary long-term monitoring of the species’ genetic diversity and environment.
When fishes are prized for sport or food, however, wild populations are frequently augmented with hatchery fish, yet studies have demonstrated that captive-bred stocks of Rainbow trout are poorly adapted to the wild and less likely to survive and reproduce.
Higher population densities in captivity have also been shown to make fish more aggressive.
Captive breeding for conservation should be a last resort — or a temporary safeguard.
There are huge consequences though for indiscriminately releasing any aquarium fish into the wild and many can become pests, like the Common plec (Pterygoplichthys pardalis), now invasive in more than a dozen countries. New deadly diseases can also be spread, decimating native creatures and ecosystems. A fish should never be released into the wild after being in an aquarium.
This article was first published in the December 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.
Why do seahorses swim standing up?
The fact that seahorses swim in a vertical position is well known, but why do they do it? Rupert Collins has the answer.
Hippocampus spp. are believed to have evolved from their horizontal ancestors, the pipefishes, 25-28 million years ago in Australasia. They have since spread to most temperate and tropical seas.
The familiar vertical posture and prehensile tail is understood to have been adopted in response to increased seagrass habitat available at that time.
This could also be interpreted as better behavioural camouflage from predators and/or to help them become more efficient predators themselves.
Seahorses are capable of rapid colour changes, especially when interacting. They lead a largely sedentary lifestyle as ambush predators of zooplankton, but limited locomotion is provided exclusively by the dorsal and pectoral fins, as the tail and pelvic fins have been greatly reduced.
Densities of H. guttulatus studied in the wild have been shown as low at one seahorse per 14m2, although no territorial defence behaviour has been observed in the genus.
Individual seahorses are reported to have home ranges as small as 4.4m2 for the Australian H. breviceps, with the female’s territory larger than the male’s.
Most seahorses live only a few years.
This item was first published in the December 2009 issue of Practical Fishkeeping. It may not be reproduced without written permission.
Why do plecs have curved pupils?
Loricariids and some other bottom dwelling fish have pupils that appear to be round in shape. Rupert Collins explains.
Loricariids seem to be able to control the shape of their pupil with a flap-like extension to the iris called the dorsal iris operculum. When dilated the pupil appears round, while when constricted it appears curved like a crescent.
This adaptation seems confined to bottom-dwelling fishes such as plecs, flounders, skates and rays. The exact function is not fully known, but the flap might help break up the distinctive shape of the eye and aid camouflage.
Alternatively, the flap may protect the retina from strong light.
Observations of visual ganglion cell patterns in the loricariid retina indicate plecs see best directly behind and in front of them. When moving plecs between aquariums, it is perhaps best to approach them with a hand or net from above.
This item was first published in the December 2009 issue of Practical Fishkeeping. It may not be reproduced without written permission.
Why don't polar fish freeze?
If saltwater at the poles is below freezing, how do the fish avoid becoming frozen?
Although saltwater doesn’t turn to ice immediately when below zero, ice crystals can still form and if these develop inside fish cells, then they can cause significant damage which may result in death. To prevent ice crystals harming them, polar fishes have evolved antifreezes that prevent them from freezing in saltwater with a temperature that is below freezing.
At both the North Pole and South Pole different groups of fish have evolved similar antifreeze proteins, called antifreeze glycoproteins or AFGPs. The proteins act as biological antifreezes and bind to tiny ice crystals and prevent them from growing larger and damaging cells.
At the Arctic, members of the cod family Gadidae are famous for these, while at the South Pole Notothenioids have evolved similar proteins. Although the two groups of fish are unrelated, evolution has seen them separately evolve proteins that perform identical roles and are structurally very similar.
Antifreeze proteins are being used by some ice cream manufacturers as an additive to prevent ice crystals forming in ice creams and frozen yoghurts and keep them soft and creamy.
This item was first published in the November 2009 issue of Practical Fishkeeping. It may not be reproduced without written permission.
Can moonlight increase my chances of breeding fish?
Do some fish spawn according to the phases of the moon?
At least four orders of fish including Salmoniformes (salmon and trout), Perciformes (perches, cichlids and damselfishes), Atheriniformes (silversides) and Tetraodontiformes (puffers and triggers) use the moon to time their reproduction – something known as lunar spawning synchronisation.
The moon goes through several phases over a 29 day period and fish use the different phases to allow them to time their spawnings. Although it occurs in some freshwater species, it is especially common in marine species, as the phase of the moon is directly linked to the tides, which can bring food and aid the dispersal of eggs and fry.
There are thought to be several reasons why fish adopt this strategy when spawning. For those species that provide brood care, such as cichlids, it’s easier for them to prevent predators eating their brood if they can see them. So they often lay their eggs so that the moon is bright when the eggs hatch, allowing them to see the brood and predators.
Many substrate spawning cichlids are believed to use this strategy, like Tilapia zillii pictured above, but at least one mouthbrooder, Cyprichromis leptosoma, has also been recorded using synchronised lunar spawnings.
Many fish species use the phase of the moon to allow them to know when to spawn and ensures that their eggs and sperm are ready at the right moment. Synchronised spawnings are also common in corals, which spawn en masse on specific nights.
Simply sticking a moonlight bulb on your tank won’t do much, but placing the tank somewhere near natural moonlight could have an effect.
This item was first published in the November 2009 issue of Practical Fishkeeping. It may not be reproduced without written permission.
What is electrofishing?
How is electrofishing carried out, and is it safe for the fish?
Electrofishing is a technique commonly used by scientists and fisheries biologists to survey rivers and assess what fish are present. When done properly, it stuns fish for a short period allowing them to be removed carefully with a net, examined and then returned alive with no permanent harm done.
The device used, called an electrofisher, is often mounted in a backpack and uses two electrodes – an anode and a cathode – to deliver a high voltage charge into the water. People doing electrofishing wear rubber waders to prevent zapping themselves.
When the electrofisher is triggered near a fish it is affected by the electricity and undergoes an uncontrolled muscular spasm called a convulsion which causes the fish to swim towards the ring-shaped, pole-mounted anode. This trait is known as galvanotaxis.
Electrofishing is a two man job, so one person discharges the electrofisher while another holds a net near the anode to catch the stricken fish. The scientists can then collect the fish, weigh or measure them and then return them to the river unscathed.
This item was first published in the November 2009 issue of Practical Fishkeeping. It may not be reproduced without written permission.
Do Aloe vera and Tea tree oil really cure fish ailments?
Are these traditional herbal medicines useful in fish health? William Wildgoose explains.
Both of these herbal extracts have been used as traditional medicines in humans for many years but it is only recently that they have been studied scientifically to investigate their medical properties.
Tea tree oil for example, is a complex mixture of over 90 different compounds, of which a few are known to have antifungal and antibacterial properties.
Unfortunately, although these are natural products, they are toxic if swallowed and irritant to the skin at certain concentrations.
There has been very little published research about their use in fish, which can be harmed by small amounts of any toxic substance in their environment. There have been some promising results from studies on the use of herbal medicines in fish, but much more has to be done.
All medicines in the UK are strictly controlled by the Veterinary Medicines Directorate. The scientific proof that is required for any medicine involves extensive testing on many species, with various diseases and under different conditions to ensure that medicines are safe, of good quality and efficacious. In other words, a medicine shouldn’t kill the patient and it should treat the disease it claims to be effective against.
Although there is often a strong temptation to make up your own remedies based on other hobbyists’ experience with basic ingredients, manufacturers offer the safest option for using these medicines.
This item was first published in the November 2009 issue of Practical Fishkeeping. It may not be reproduced without written permission.
Can fish be vaccinated against whitespot?
Whitespot is one of the most common aquarium problems, so is there anything that can be produced in the way of a vaccine to prevent fish from getting it?
Yes, new research about to be published has provided evidence to suggest that fish can be vaccinated to reduce the likelihood of infection by the whitespot parasite, Ichthyophthirius multifiliis.
Whitespot is a tiny parasite called a protozoan and it damages fish by busting into their skin and gills. The fish reacts by attacking the parasite using its immune system and creates a little capsule around it – which is the white spot you see on affected fish.
It’s normally treated by adding chemicals to the water, but while these can work, some fish are sensitive to them, it can be expensive (if you have a lot of water to treat) and they could harm or stress the fish.
One of the most popular treatments for whitespot, malachite green, has already been banned on fish farms as it causes cancer in humans, but it’s still legal for use with aquariums. The search is already well under way for alternatives.
Fish develop immunity to whitespot following an infection, so scientists have been conducting experiments based on this knowledge to immunise fish to see whether vaccines can be developed.
Unlike other vaccines, which typically tend to work best when injected, the whitespot “vaccine” works by bathing fish in water containing live parasites at a specific stage in their life cycle when the parasites are known as ‘theronts’.
Fish exposed to the theront treatment have much higher levels of anti-whitespot antibodies in their blood, which helps them fend off further infections. Work is still underway with the treatment, so it’s not likely that we’ll be seeing fish vaccinated against whitespot on sale in the shops for quite some time.
This item was first published in the November 2009 issue of Practical Fishkeeping. It may not be reproduced without written permission.
How widespread are fish venoms?
It was once believed that venom production had evolved in around 200 different fish species, however, new research has suggested that they’re much more widespread, with 1500-2000 species presumed venomous.
Venoms are a popular target for scientists looking for potential molecules or proteins to study for use in the production of drugs, something known as bioprospecting. Most studies have focused on snake venoms, which have led to the development of a number of drugs to treat strokes and cancers.
The scientists who made the discovery produced an evolutionary family tree known as a phylogeny which allowed them to predict which groups were most likely to have venom, which means bioprospectors can examine the fish to see if they could lead to useful new drugs.
This item was first published in the November 2009 issue of Practical Fishkeeping. It may not be reproduced without written permission.
Why are brackets sometimes used in scientific names?
When you read fish books you'll often see brackets around the names of the author and a date. What are these for and what can they tell you about a fish? Matt Clarke explains.
The correct way to note a scientific name is to write the genus and species names in italics and then the name of the author (or authors) who described it, as well as the date of the description. For example, Hyphessobrycon frankei, Zarske and Gery, 1997.
This tells us that the fish is a species called frankei in the Hyphessobrycon genus and was described in 1997 by Zarske and Gery. Simple really.
The brackets, or parentheses, are important. They’re not added just to make the name look neater in books and magazines, as many people believe.
Parentheses around the author’s name and date of description show that the species was originally described under a different name, so synonyms exist. If there are no parentheses, the fish has only had one scientific name and the author correctly placed it in the right genus when first described.
Take the Celestial pearl danio. It’s current name is Danio margaritatus (Roberts, 2007). This tells us it has another scientific name and was originally placed in the wrong genus by the original author.
Most never use author’s names, but for scientists and specialists these can add information that the name alone cannot.
This item was first published in the October 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.
Are fish cold-blooded?
Are fish really cold-blooded animals, or is it more complicated than that?
Like reptiles and amphibians, fish are cold-blooded poikilothermous vertebrates —meaning they get their body temperature from the surrounding water.
Therefore, as body temperature is directly linked to water temperature, and changes in body temperature have an effect on how the body works, these can be stressful or deadly.
At higher water temperatures water holds less dissolved oxygen, so when water warms it affects fish respiration and they have to move their gills more rapidly to extract the oxygen they need.
Temperature also affects metabolism and metabolic processes occur quicker in warmer water. This also adds to the amount of oxygen fish require.
Some fish, such as tuna and certain types of shark, are more warm-blooded and able to warm their red muscle tissues to 26-32°C/79-90°F when the ambient water temperature is between 6-30/43-86°F.
This item was first published in the October 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.
Why do some scientific names have cf. in them?
Lots of fishkeepers mistakenly think that cf. in a scientific name means "colour form" but it has a rather different meaning, as Matt Clarke explains.
The abbreviation cf. comes from the Latin word conferre, which means “compare to” or “confer.” It’s not short for colour form, as some mistakenly believe.
The use of cf. in a scientific name (for example Schistura cf. balteata - the loach pictured above) means that the person using the name is saying the fish should be compared to a given species, as it might not be exactly the same species.
It’s a way of applying a provisional name to a species and is most frequently used when new fish are discovered that look slightly different to the form normally encountered. It indicates that the fish might be a variant of the same species, but could also turn out to be something completely different.
Another term species affinis is also used to perform a similar role and is sometimes seen in scientific names using the abbreviation aff (or ‘sp. aff.’). For example, Danio sp. aff. rerio.
Both are examples of ‘open nomenclature.’ Another example of this is the use of a question mark, ie. Devario anomalus? (a Devario, but species uncertain) or Devario? anomalus? (to indicate both an uncertain genus and an uncertain species).
This item was first published in the October 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.
What does fish urine consist of?
Fish urine is often said to be mainly ammonia, but as Matt Clarke explains, that's not really 100% accurate...
Most people assume that fish urine is made up mainly of ammonia, but fish excrete only 2-25% of their nitrogen waste via their urine.
Most is excreted from their gills as ammonia.
Carp expel about 56% of their nitrogen from gills as ammonia. The rest of their waste nitrogen is excreted in urea and simple nitrogen compounds, also via the gills.
Urine contains other stuff, including the nitrogenous organic acids creatine and creatinine, plus probably amino acids and a little urea.
This item was first published in the October 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.
How do swimbladders work and do all fish have them?
Swimbladders allow fish to travel up and down in the water, but how do they work? Rupert Collins explains.
Fish occupy a three-dimensional space, so movement up and down the water column needs to be controlled with minimal energy expenditure. A swim (or gas) bladder allows neutral buoyancy, so going up or down requires less effort.
This hydrostatic organ comprises an impermeable gas-filled sac which originated historically from the oesophageal lining.
The swimbladder may have initally evolved in aquatic ancestors as an accessory air breathing organ, as seen in 'primitive' fishes such as the pirarucu (Arapaima spp.). This then evolved into the lungs seen in humans and swimbladder of modern fishes.
In some fishes, the gas inside the swimbladder is obtained by surface gulping, while in others the swimbladder is filled under pressure with gases, mainly oxygen, from the blood via active diffusion and actions of the gas gland.
Of the some 425 extant families of fishes, swimbladders have been absent or substantially reduced in at least some members of 79 families, indicating multiple evolutionary losses.
Fishes having lost swimbladders are generally either substrate dwelling, where negative buoyancy is an advantage, as in loaches, or found in deep sea habitats where pressure is too great to maintain a gas bubble. Many fast movers such as tuna have lost the swimbladder as the organ is not able to adjust quickly enough to rapid vertical movements.
The swimbladder structure is used secondarily for hearing and sound generation. Many bottom dwelling and sedentary catfishes have lost the hydrostatic function but retained the organ due to its auditory advantages. The Pictus and Shark catfishes (Pimelodus pictus and Hexanematichthys seemanni) appear to be able to amplify warning croaks using their swimbladder.
This article was first published in the Christmas 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.
Why do elephantnoses produce electricity?
Elephant nose mormyrids produce little zaps of electricity, but why do they do this and what allows them to produce electricity in the first place?
Elephant noses, or mormyrids, are weakly electric, so while electric eels produce a powerful 500 volt zap to stun prey, mormyrids produce less than a volt and use it for communication in the murky waters in which they live.
The zap, known as an electric organ discharge or EOD, comes from a special muscular organ in their tail and the length, frequency and size is specific to the species producing it.
The fish can detect the EODs of other mormyrids allowing them to locate members of the same species to spawn with, and allows them to prevent hybridising with other species even when they can’t see who they’re spawning with.
Recent research on populations of the elephant-nosed species Campylomormyrus numenius has found that different fish produce slightly different EODs.
These were once considered to be age-related differences, as the fish varied little in their appearance, however, DNA studies have shown that the fish actually differ genetically.
This suggests that C. numenius contains so-called cryptic species that are as yet undescribed, which means more species will be named later.
This item was first published in the September 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.
How do you sex a stingray?
Stingrays are relatively simple to breed in captivity, but in order be successful you\'ll obviously need a pair. So how do you sex a stingray?
At sexual maturity, male rays have external sexual organs called claspers which are visible near the base of the tail.
Effectively, these are two penis-like organs that lie within modified pelvic fins and are used to internally fertilise the female. Claspers are often “rolled up” which makes them harder to spot.
They’re also smaller in younger specimens, so they can be overlooked more easily. Females typically grow to larger sizes than males and have a greater disc diameter.
This item was first published in the September 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.
Does penis size matter to female livebearers?
If you’re a female mosquito fish, yes, size definitely matters.
Male mosquito fish are livebearers and scientists have shown in experiments that females prefer males with larger genitalia.
This preference has led to the males evolving much larger private parts than many other livebearers.
However, having enormous genitals make the males more vulnerable to predators as they make swimming more cumbersome.
This means that there’s an “evolutionary trade-off” for the male mosquito fish in avoiding predators and having smaller genitals, or attracting ladies with a bigger a larger gonopodium but placing themselves at greater risk of predation.
This item was first published in the September 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.
What is in Discus mucus?
Discus and some other cichlids produce mucus secretions to feed their fry. But why do they do it and what\'s in their mucus that helps the fry to grow?
Discus and several other South American cichlids, such as Uaru, produce mucus secretions that are used to feed broods of fry.
These milky secretions are generated during spawning but it was not until relatively recently that scientists learnt what was in them.
The mucus secretions are produced by cells in the epidermis layers of the skin of both male and female Discus from the genus Symphysodon.
Recent studies have revealed that 18 different proteins are present in the mucus secretions of Discus whether they’re breeding or not, but the fish produce an additional 17 proteins when they have fry to raise.
The proteins in the mucus include fructose biphosphate aldolase, nucleoside diphosphate kinase, and heat shock proteins, which are used in energy provision, producing and repairing cells, handling stress and defense during the brooding period. Antioxidants are also present.
However, perhaps the key thing is the presence of a sugar-binding protein which is believed to provide protection against bacterial infections to both the parents and the fry.
This item was first published in the September 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.
How can glass catfish be transparent?
This is a question for which there is no satisfactory answer, because we are only beginning to understand the physical and anatomical basis of transparency in living tissue.
Although the physical and anatomical bases for some transparent tissue (e.g. the cornea and lens in the eye) are better understood than others, the situation in the eye is unique in the sense that the tissues are highly modified for transparency and these modifications (e.g. complete absence of a circulatory system) are not applicable to muscular tissue.
Furthermore, many of the primary modifications for transparency are ultrastructural and can only be seen with an electron microscope. For biological tissue to become transparent, the primary mechanism is to reduce the amount of light being scattered as it passes through it: the less light scattered by the tissue, the more light will be transmitted through it and the more it becomes transparent.
While we do not yet fully understand how biological tissues (particularly muscles) can be transparent, there are several possible mechanisms which might contribute. The first is that transparent fishes such as glass catfishes and glassfish have very thin bodies. The flatter the body, the less the potential scattering of light (and hence the easier it is to make the tissue transparent).
Another possible mechanism is the ordered packing of small molecules within the cytoplasm of the cells to reduce the scattering of light.
Lastly, theoretical models also predict that the many subcellular components of transparent tissues (e.g. mitochondria, ribosomes) should be small and highly dispersed.
As a recent review paper on biological transparency states, this field of study has more questions than answers, and we await future studies to fully understand this phenomenon.
This item was first published in the September 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.