Abstracts from Scientific Papers

Scientific papers are usually introduced by an "abstract", that is a summary of the experiment and its conclusions. Those that follow may be of interest. Some of them can be accessed free of charge on the Internet. The language is often "science-speak" but it's not usually so difficult that the ordinary reader can't grasp the essentials.

(1) This paper deals with the ability of the crucian to change shape in the presence of predators

Phenotypic plasticity and predator effects on morphology and physiology of crucian carp in nature and in the laboratory

By J. Holopainen, J. Aho, M. Vornanen, H. Huuskonen

Crucian carp Carassius carassius show great phenotypic plasticity in individual morphology and physiology, and strong variation in population density in different fish communities. Small fish with shallow bodies and large heads are typical in overcrowded monospecific fish communities in small ponds, whereas deep-bodied, large fish are found in larger, multispecies lakes. crucian carp are especially vulnerable to predation by piscivorous fish and their greater relative body depth in multispecies fish communities has been proposed to be an induced defence against size-limited predation, and hence to be an adaptive feature. Data are presented here on the two divergent body forms in field populations in eastern Finland, together with results of laboratory experiments on predator effects on morphology and physiology (growth, respiration, heart rate). The deep body can be achieved in a few months by introducing a low population density of shallow-bodied fish into a food-rich environment with no piscivores. In the laboratory, both the presence of piscivores (chemical cues) and enhanced food availability increased the relative depth of crucian carp, but only to a modest extent when compared to natural variation. It is concluded that the deep-body form of crucian carp in the low density populations of multispecies fish communities is the normal condition. Reproduction in monospecific ponds results in high intraspecific competition, low growth rate and a stunted morphology. According to pilot tests, the mechanism behind the predator effect in the laboratory might be a behavioural reaction to chemical cues (alarm substances/predator odour) causing changes in energy allocation: predator-exposed crucian carp adopt a 'hiding' mode with decreased activity (less swimming, lower respiration and heart rate) and with higher overall growth. Whether, and to what extent, this predator-induced mechanism works in nature is unclear.

That last sentence is interesting. Anglers' observations could be of value here. Do you fish for crucians in a pond or lake where there are pike and/or big perch. If so, what are the habits of the crucians - how do they behave? Can you only catch them near "features" like in-growing willows or alders, or close to lily beds, for example? Do they feed only at night?

(2) This paper argues that the deeper-bodied crucian is stronger and more agile in its avoidance of predators

Predator-induced morphology enhances escape locomotion in crucian carp. Full Paper here.

By Paolo Domenici, Håkan Turesson, Jakob Brodersen, and Christer Brönmark

Fishes show a remarkable diversity of shapes which have been associated with their swimming abilities and anti-predator adaptations. The crucian carp (Carassius carassius) provides an extreme example of phenotypic plasticity in body shape which makes it a unique model organism for evaluating the relationship between body form and function in fishes. In crucian carp, a deep body is induced by the presence of pike (Esox lucius), and this results in lower vulnerability to gape-limited predators, such as pike itself. Here, we demonstrate that deep-bodied crucian carp attain higher speed, acceleration and turning rate during antipredator responses than shallow-bodied crucian carp. Therefore, a predator-induced morphology in crucian carp enhances their escape locomotor performance. The deep-bodied carp also show higher percentage of muscle mass. Therefore, their superior performance in escape swimming may be due to a combination of higher muscle power and higher thrust.

(3) Here the authors suggest that there are disadvantages attached to being deep-bodied - if you're a crucian, that is!

Density-dependent costs of an inducible morphological defense in crucian carp

By Lars B. Pettersson, Christer Brönmark

...Recently, a predator-induced change in body morphology was discovered in a fish, the crucian carp, Carassius carassius (Bronmark and Miner 1992). crucian carp is a widely distributed cyprinid that lives in ponds and lakes in Europe and central Asia (Maitland and Campbell 1992). In the absence of piscivores, crucian carp form dense, stunted populations (often > 30,000 individuals/ha), whereas populations coexisting with piscivores consist of few, (1-250 individuals/ha) large individuals (Bronmark et al. 1995). This suggests that crucian carp is very vulnerable to predation, which has been confirmed by experimental manipulations of predation pressure (Tonn et al. 1989, Bronmark and Miner 1992). When northern pike, Esox lucius, were introduced to small ponds in southern Sweden, crucian carp populations decreased markedly and surviving individuals became deeper bodied (Bronmark and Miner 1992). Laboratory experiments showed that body shape of crucian carp changes in response to waterborne chemical cues from piscivorous predators (Bronmark and Miner 1992, Bronmark and Pettersson 1994). The deeper body benefits crucian carp by decreasing predation efficiency of gape-limited piscivores, e.g., northern pike (Nilsson et al. 1995). However, theoretical estimates suggest that a deeper body increases total drag, thus increasing swimming costs (Bronmark and Miner 1992).

Few studies have investigated costs of inducible morphological defenses under field conditions (but see Lively 1986c, Harvell 1992). Whereas laboratory studies are important for detailed studies of, e.g., cost mechanisms, field experiments are essential for showing selective forces under natural conditions. crucian carp is well suited for field studies because fish that have experienced induced body-shape changes still differ from non-induced control fish after 180 d in the absence of piscivores (Bronmark and Pettersson 1994). While the benefits of inducible defenses are manifested in the presence of predators, their costs should be most apparent during intense intraspecific competition when predators are absent. We hypothesized that shallow-bodied crucian carp should have a competitive advantage over deep-bodied fish as they avoid costs associated with the induced defense. Therefore, in this study, we investigated effects of intraspecific competition on shallow-bodied and deep-bodied crucian carp in a field situation with no predators present.

The authors conclude that it may be that deep-bodied crucians revert to being longer and lower in profile when predators are removed and intraspecific competition (ie from other crucians) increases.

(4) This brilliant paper discusses crucian response to predation and its resistance to adverse environmental conditions

Tales of two fish: the dichotomous biology of crucian carp (Carassius carassius (L.)) in northern Europe.

By Holopainen, I. J., Tonn, W. M. & Paszkowski, C. A. 1997

Crucian carp, a common Eurasian cyprinid fish, shows striking dichotomies in several aspects of its physiology and ecology, at both the individual and population levels. These dichotomies consistently reflect the communities and ecosystems in which they occur, contrasting crucian carp that occur in "monocultures" (single-species fish assemblages) in ponds with those occurring in multi-species assemblages, primarily in lakes. Dichotomies also occur in the physiological state of individuals between summer and winter. All these dichotomies, involving differences in morphology and population structure, population dynamics and life history, reflect in an integrative way the adaptive strengths and limitations of a unique species. In numerous northern ponds, dense monocultures of crucian carp are sealed in by thick ice and snow to live more than six months in dark, anoxic waters at near freezing temperatures. In summer, these populations experience temperatures of up to 30°C during a relatively brief period of vigorous growth and repeated bouts of reproduction in almost continuous daylight. crucians in lakes experience a more benign abiotic environment and, with densities that can be orders of magnitude lower than those in ponds, are likely unaffected by intraspecific interactions. However, co-occurring species present a challenging biotic environment. crucian carp is exceptionally vulnerable to predation and populations persist via three kinds of refugia. Aided by extreme physiological adaptations, crucian carp can be the sole piscine inhabitant of seasonally harsh but productive small ponds, the refugium habitat, where they form dense monocultures of stunted individuals. The structural complexity offered by dense beds of macrophytes in productive larger lakes, the other common habitat (habitat refugium) of crucian carp, ensures survival of a few offspring, even in the presence of piscivores. The risk of predation still remains high for crucians until a certain length (and/or body depth), the size refugium, is attained. Crucian carp's unique anoxia tolerance, accompanied by many exceptional structural and functional features, and the species' suitability for laboratory studies, has recently raised crucian carp to a status of a physiological model species like its relative, the goldfish. Moreover, the dichotomy found in the structure of natural populations has made crucian carp an attractive model for ecological studies in competititon and predation. crucian carp's resistance to adverse environmental conditions further suggests promising economical use in aquaculture to produce hybrids endowed with higher survival capacity than common carp.

That last sentence is ominous. Experiments have a nasty habit of escaping into the wild.

(5) This is the ground-breaking DNA research carried out at Hull University in 2004/5

A molecular approach to detect hybridisation between crucian carp (Carassius carassius) and non-indigenous carp species (Carassius spp. and Cyprinus carpio). Full paper here.

By B. Hånfling, P. Bolton, M. Harley and G. R. Carvalho

  1. Releases of non-native fish into the wild is an increasing problem posing considerable ecological and genetic threats through direct competition and hybridisation.
  2. We employed six microsatellite markers to identify first generation hybrids and backcrosses between native crucian carp (Carassius carassius) and introduced goldfish (C. auratus) and common carp (Cyprinus carpio) in the U.K. We also investigated the genetic characteristics of the taxonomically controversial gibel carp (Carassius spp.) from sites across Europe.
  3. Natural hybridisation between goldfish and crucian carp occurs frequently, although hybrids between all other species pairs were observed. Only 62% of British crucian carp populations (n = 21) consisted exclusively of pure crucian carp. In some populations hybrids were so frequent, that no pure crucian carp were caught, indicating a high competitive ability of hybrids.
  4. Most hybrids belonged to the F1 generation but backcrossing was evident at a low frequency in goldfish x crucian carp hybrids and goldfish x common carp hybrids. Furthermore, some local populations had high frequencies of backcrosses, raising the opportunity for introgression.
  5. Gibel carp from Germany and Italy belonged to two triploid clonal lineages that were genetically closely related to goldfish, whereas all individuals identified from British populations proved to be crucian carp x goldfish hybrids.
  6. Our study suggests that the release of closely related exotic cyprinids not only poses a threat to the genetic integrity and associated local adaptations of native species, but may also contribute to shifts in community structure through competitive interactions.

Although some of the waters from where these fish originated would have been commercial pools where numbers are artificially high and where stocking has been indiscriminate, this paper underlines the dangers facing our crucians. Informed and considered stocking is essential, as I've tried to make clear in the crucian fishery stocking and management section.

(6) Here the authors studied low- and high-density populations of crucians in ponds where they are the only species

Density-Dependent Effects and the Regulation of crucian Carp Populations in Single-Species Ponds

By William M. Tonn, Ismo J. Holopainen, and Cynthia A. Paszkowski

Crucian carp (Carassius carassius) is often the only fish species present in small ponds of northern Europe. Such populations are typically at high densities and consist primarily of small, relatively slender, and short-lived fish; populations in multispecies assemblages in lakes display opposite traits. Field and laboratory experiments have implicated predation as an important mechanism structuring crucian populations in multispecies assemblages.

To investigate the role(s) of density dependence in structuring pond populations, we manipulated densities of crucians stocked into four sections of a subdivided natural pond in Finland. Mortality of stocked fish during the 3-mo experiment was low (0-11%) and unrelated to density. Growth rates of crucians were both size and density dependent. Larger fish did not noticeably grow in either high- or low-density sections, but growth of smaller size classes was 22% greater in low-density sections. Lower densities and smaller sizes of zooplankton, especially of inshore cladocerans that are an important food resource for smaller crucians, indicated that resource limitation caused by exploitation competition was an important density-dependent process restricting growth of small fish in high-density sections.

Crucians rely on reserves of glycogen, stored in the liver and muscle, to fuel the over winter anaerobic metabolism that allows them to maintain populations in ponds where winter hypoxia eliminates other fishes. Liver size was significantly larger and glycogen levels were 82% higher in our low-density pond sections than in high-density sections. At higher, but not lower, densities, relative liver size and glycogen levels decreased with body size; combined with the lack of growth of large fish, this indicates that density-dependent energy limitations contribute to the short life-spans and rarity of large fish that characterize pond populations.

An increase in relative body depth can be induced in crucians by the presence of piscivorous fish, reducing vulnerability to predation. It has been proposed that increased body depth incurs a substantial energy cost during swimming and should be restricted to specific size classes in populations sympatric with predators. However, in our experiment, crucians of all sizes became significantly deeper bodied, which contributed to higher condition factors in low-density vs. high-density sections. This morphological change, occurring in the absence of piscivores, suggests that energy benefits (accumulation of overwintering reserves) can override any proposed costs for this sluggish, but metabolically unique, fish.

Crucian carp successfully reproduced in all sections during the experiment, but growth and recruitment of 0 + fish (young of year) were strongly and negatively related to the numbers of older fish present. The observed density-dependent recruitment was likely caused by starvation-induced mortality and cannibalism; reduced growth observed in 0 + fish would also be expected to cause higher first-winter mortality, reducing further recruitment at higher densities. Density dependence has a variety of regulatory effects on pond populations of crucian carp that contribute to their presence and resilience.

Crucian-only ponds and lakes aren't as common here as in Europe but this paper underlines my advice that in a nursery pond it is important to net and crop regularly to ensure growth and survival.

(7) This paper stresses the almost unique ability of the crucian to survive without oxygen and hints at the relevance this may one day have for human health. (This is a press release and not the abstract)

Seasonal changes in glycogen content and Na-K-ATPase activity in the brain of the crucian carp

by Matti Vornanen and Vesa Paajanen of the University of Joensuu in Joensuu, Finland

Remarkable physiology allows crucian carp to survive months without oxygen

Goldfish cousins evolve remarkable physiology to avoid predators

BETHESDA, MD (August 25, 2006) -Cooling water temperature during the fall prompts the crucian carp to store vast amounts of glycogen in its brain to keep the brain functioning and healthy from February to April, when there is no oxygen left in the ponds, a new study finds.

The study from Finland found that the amount of glycogen in the brain was at its peak in February, when the pond becomes nearly depleted of oxygen (anoxic). Glycogen, an energy supply that the carp brain uses to survive anoxia, was 15 times higher in February, compared to brain glycogen content in July, when oxygen in the pond is at its peak.

At the same time, the carp brain's sodium-potassium pump activity, a measure of energy demand, decreased 10-fold to its low point between February and April, said the study's lead author, Vesa Paajanen. Taken together, these findings indicate the carp extends the amount of time it can survive without oxygen in frigid water by 150-fold. Further, the study found that it was the dropping water temperature that sets these physiological changes into motion.

"This is the first study to show that sodium pump activity is controlled by water temperature, not by the amount of oxygen available in the water" Paajanen said. The findings help explain how the carp pulls off the remarkable physiological feat that allows its brain to survive for months in a nearly anoxic state.

There is currently no direct tie between these finding and humans. However, physiologists only recently realized the human brain contains glycogen, so who knows? Maybe this line of research will one day be important for humans to survive anoxia, Paajanen said. The study appears in the American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. The American Physiological Society published the study.

Safe neighborhood

The carp developed this remarkable physiological adaptation as a way to avoid troublesome neighbors: predators. But the predator-free ponds where they live are inhospitable and require the fish to survive several months in only a few feet of water covered by several feet of ice and snow. The oxygen content of Finnish ponds drops dramatically when the ice and snow cover them, preventing diffusion of oxygen from the air and cutting off aquatic plants from the light needed for photosynthesis, Paajanen said. The ponds in Finland gradually lose oxygen following the summer, and from February to April the ponds have virtually no oxygen.

Members of the carp family are known for their ability to adapt to anoxic conditions. The crucian carp's cousin, the goldfish, was the pet fish of choice before the advent of equipment to aerate tanks. But the crucian carp is a standout even within the carp family at surviving without oxygen.

When crucian carp live in larger bodies of water such as lakes, they make another remarkable adaptation to avoid the mouths of predators: They shift shape to make themselves shorter and fatter, making it much more difficult for predators to get their jaws around them. In ponds, the fish elongate, but they move much less during the winter and don't eat at all. They may remain immobile for up to two minutes when removed from the cold water, Paajanen said.

Matter of supply-demand

To measure energy demand, Paajanen and Vornanen looked at sodium-potassium pump activity. The sodium pump is the body's chief way of keeping cell function in balance in the face of extreme conditions. The pump is necessary to transmit information among cells, including the neurons in the brain. The quieter the pumps, the less active and energy-consuming the brain is. However, if the pumps shut down, the cells die.

Glycogen, the form in which the body stores carbohydrates, is the energy supply that vital organs such as the heart and brain use to survive anoxia. "Glycogen is commonly found in the liver and in the muscles, but glycogen stores were generally believed to be minimal in the brain," Paajanen said. In an earlier study, Paajanen and Vornanen found the carp stored extra glycogen in the heart during winter. In the current study, they hypothesized that the crucian carp brain has vast stores of glycogen, too, which it draws on when oxygen runs out.

Water temperature, not oxygen, acts as trip

The researchers spent 12 months gathering 20-30 carp each month from a fish trap they set in a nearby pond. They began the experiment in May 2002 and finished in June 2003, tracking water temperature and examining the brains of the fish shortly after removing them from the pond.

They found that as the water got colder in October and November, the carp began to consume less energy (sodium pump slows) and build up their glycogen (carbohydrate) stores, even though the water still had plenty of oxygen.

The study fund that glycogen and the sodium pump play equally important roles in carrying the fish through the anoxic period. The glycogen level that supports brain function for 16 hours in the anoxic winter when energy demand is low, would support the fish for only eight minutes in the summer with the same amount of oxygen. That is because energy demand rises as the water temperature rises, Paajanen noted.

PR - There is plenty of information on the Web about this remarkable ability of the crucian. It is a remarkable fish in many ways.

(8) This paper (2011) describes keys to identify Carassius species and hybrids from examination of head bones found in animal faeces and archaeological sites. It also examines how the size of the prey can be calculated from the size of the bones.

Diagnostic features and biometry of head bones for identifying Carassius species in faecal and archaeological remains

By L. Masson1, D. Almeida, A. S. Tarkan, B. Onsoy, R. Miranda, M. J. Godard and G. H. Copp


The identification of fish species from head bone remains is employed in various sciences, including archaeology, paleontology and field ecology, with the estimation of fish size from biometric relationships being useful in the assessment of predation pressure exerted by increasing numbers of piscivorous species (e.g. Eurasian otter Lutra lutra and great cormorant Phalacrocorax carbo). This is particularly relevant for crucian carp, Carassius carassius, which is in decline in Europe due to changes in land use and to increasing numbers of non-native Carassius species (i.e. goldfish C. auratus, gibel carp C. gibelio), which hybridize with C. carassius. However, diagnostic keys and biometric relationships are lacking for C. carassius and its most common hybrids, crosses with C. auratus. The present paper addresses this gap in knowledge, providing diagnostic keys and biometric relationships for the head bones of all Carassius species found in Europe as well as for C. carassius x C. auratus hybrids. All bone size to body length relationships were statistically significant. Similarly, all bone size to body weight relationships were significant for C. carassius, C. auratus, and C. gibelio, but none were significant for C. carassius x C. auratus hybrids. Diagnostic structures were found to distinguish easily between the Carassius species and hybrids, which will assist in determining the identity and sizes of prey found in faecal and archaeological remains...


...In conclusion, the present study contributes directly to conservation initiatives through the assessment of predator/prey interactions between BAP species. It is also useful to paleoecological research, aiding in the reconstruction of past fish communities, with sediment dating techniques permitting the determination of whether a species is native (i.e. present prior to first known introductions) or introduced, such as remains a topic of debate amongst some persons as regards C. carassius in England.