The crucian is a cyprinid, like most of our coarse fish. Its scientific name is Carassius carassius, linking it to Carassius auratus, the goldfish, with which it readily inter-breeds. It also forms hybrids with the common carp, Cyprinus carpio.
So, there are two Carassius species in our waters. Abroad, there is a third species or sub-species (scientists aren't agreed on this yet) called the gibel carp (Carassius gibelio or Carassius auratus gibelio). This fish has not been identified in the UK. Suspected UK gibel carp have proved to be crucian × goldfish hybrids.
It is possible that there are more Carassius species in Asia.
Perhaps the best way to confirm that your stock of crucians is true is either to have a fin clip or scale DNA tested or to dissect a casualty and examine the key internal features, which I'll explain later. Most of us, though, want to identify the fish in the landing net and it is likely to be a true crucian if the following apply:
Crucians vary in shape. Some are very flat and have very high backs, like the well-known Yateley fish. Others are slightly rounder and are lower-backed, like the Marsh Farm fish. There seem to be "strains" of crucian in the same way as there are well-marked "strains" of common carp and, of course, goldfish.
Crucians also differ in colour. Young fish are sometimes more silvery than older ones. Fish in small ponds over-shadowed by trees are often darker than those in more open waters. In peaty waters, crucians can be almost black.
Usually, though, the backs of crucians are olive-green or browny; the flanks are brassy, with the yellowness intensifying towards the belly and onto the gill covers and head. Quite often the lower fins are orange, sometimes darkening towards the tip, sometimes they are browner. The graduations of colour are much more subtle than those of the ornamental goldfish. Confusingly, the crucian is much more golden than a "brown goldfish", which is an ornamental goldfish that has not "turned" gold.
If you can't check on all these details by the waterside, then take a good photograph so that you can do so later. Usually a "trophy shot" is not much good for checking all the features of the fish - thumbs and flash flare mask scale details, dorsal fins are flat, tails are not extended, etc.
These pictures may help you to be more certain about the identity of the fish you've just caught. Check them against the full list above. Characteristics of young or stunted fish are sometimes less conclusive than in older or bigger ones, remember.
Check mouth for absence of barbules
Check dorsal fin for convexity
Check caudal fin for near-straightness
Check anal fin for ray count
Count the scales. You need to use both counts to eliminate almost all hybrids
Common carp and goldfish both vary a great deal in body shape. It's not surprising, then, that their relative, the crucian, is the same. Shape variation in crucians is caused by a number of factors, such as environment, the presence or absence of predators and genetic make-up. So, these two fish are both true crucians, though very different in form.
As far as I know, there has been very little selective breeding of crucians and there would seem to be plenty of room for experimentation along those lines. Our own crucians spring from just 17 fish introduced in 1972 and have their own characteristic shape because of the in-breeding. Some, however, do show slightly higher backs than others and breeding from those individuals would almost certainly result in a deeper strain of crucian. So far I haven't got round to doing this, but the challenge is there!
As well as what are called "meristics", i.e. the features described above, there are internal characteristics by which scientists confirm the identity of a crucian. You can do this, too. Sometimes you'll find a dead crucian on the bank, the remains of an otter's meal or a corpse discarded by a disturbed heron. If you have any doubts about the "trueness" of your crucian, here is your chance to prove it by means of some amateur dissection. These are the things to look for, and they are what make a crucian a crucian, so it's worth having a go.
The colour of the peritoneum and the gill-raker count are relatively easy to discover: if they are right, then you have a true crucian. The pharyngeals are more difficult, especially in a small fish.
Please remember that I'm not a trained scientist and that an expert would no doubt advise better ways of doing what follows. Nevertheless, you should be able to observe the main features even from amateur dissection, as long as you are careful and methodical. I use a very sharp blade from a model-making kit to do the cutting.
To get at the gill rakers, remove the gill cover on the left side of the head (looking forward). Inside, you'll find the soft red gill filaments and white, bony gill rakers, all attached to the gill arch. Sever the arch at the very top and bottom and remove from the body. Be careful not to damage the arch and rakers as you make the cuts - that can be quite tricky. There are two sets of gill rakers on the same arch. Count the top set. It's sometimes easier to do this when the gill arch has dried. The rakers are very fine and there's a danger you'll count "one" when in fact there are two or three stuck together. A hand lens helps. I use a very sharp craft tool point to tease out individual gill rakers.
I've never tried to extract the pharyngeals, because the absence of pigmentation in the peritoneum and the number of gill-rakers are sufficient evidence in themselves and easier to get at. If you want to have a go, though, you'll find them just behind the gill slits. Go in from the side or from the belly. When the pharyngeal bones have been located, remove them carefully by cutting away any attachment tissue.
The peritoneum is the membrane surrounding the organs, holding everything together in a bag, if you like. The colour of this body lining, or lack of colour in the crucian's case, is easily detected when you decapitate the corpse, which you can do last of all when you've finished your other investigations.
The scales of a crucian are quite distinctive. Here's a case where a picture is much more useful than a description, so one is shown above. It's quite handy to know what they look like if you find a pile of scales on the bank after an otter kill and you want to check if the victim was a crucian.
The hi-tec identification test, of course, is genetic. The Environment Agency or CEFAS should be able to give you up-to-date information about the availability and cost of DNA testing.
The BRFC have now established two record lists for crucians (and other difficult-to-identify species), one where the DNA has been confirmed and the other where it has not. At the time of writing the qualifying weight for the DNA-verified list is 4lbs 8oz.
I am not sure how this is going to work in practice but the technology is there.
Shape, colour and scale counts should tell you which is the crucian (on the right) and which is the hybrid (on the left).