Foreword

Size variability develops in size-matched fish or in post-hatched fry. Studies on trout fry led Brown (1957) to conclude that the presence of larger fish has a suppressive effect on smaller companions, through the direct impact of aggression on growth rather than through competition for food.  Consequently, growth of stunted fish is accelerated when the "jumpers tail", the fastest growing cohort of the tank-mates, is removed. Brown (1957) called this phenomenon a size hierarchy effect. Growth variation is typical for many species of fish and crustaceans grown communally.

Scientific Dogmas

When in 1986 I became involved in the study of the grading problem, I encountered a number of dogmas circulating within eel farmers and fish biologists.  

The first dogma
In 1983, U. Wiberg showed the absence of sex (heteromorphic) chromosomes in the European eel (Anguilla anguilla L.) and suggested an environmental (metagamic) sex determination in the eel.
Much earlier the same suggestion was inferred from the highly skewed sex ratios observed in natural habitats. Quite surprisingly and contrary to the clear indications of non-genetic sex-determining mechanism in the European eel, the common belief was that sex in eels is determined by the genetic mechanism, similar to one in mammals, birds and some fishes (e. g. salmonids, common and grass carp, tilapias).

After acquainting miself with L. Bertin, F.-W. Tesch and other reviews on eel biology I became convinced that environmental sex determination in eels is quite plausible hypothesis. Discussing this issue with one of the professors at the Department of Experimental Zoology of the Hebrew University in Jerusalem, I've been accused of being a follow of a bad memory Soviet Academician Trofim Lysenko. This came as a shock to me because in the Soviet Union, from where I emigrated to Israel in 1982, I opposed the heavy heritage of Lysenkoism.

Actually, the overwhelming majority of fish species are sexually labile; even those, possessing the X-Y mammalian type of sex chromosomes, demonstrate variant modes of sex determination (Francis, 1992). I was not aware then that Western science is infected by "genetic absolutism", as I call it, and it was possible that I'll be labeled as coming from pseudoscientific background. It was not the single expression of a genetic absolutism I have encountered - other manifestations of it concerned eels' growth rate I will mention below.

The second dogma was a corollary of a concept that animal species have two sexes - males and females, and that in eels females are larger than males. Consequently, "the smaller eels in rearing tanks should be males".

The third dogma was also based on mere formal logic: since migrating silver females are much larger than those of males and since in eel culture the fastest growing cohort always consists of females, then the growth rate is a matter of sex (like in humans, cattle and chickens). Consequently, since physiological sex in fish is not determined ultimately by the genetic factors (contrary to humans, cattle, chickens and fruit fly), hormonal feminization of a growing stock could be a magic solution of the grading problem.

Actually, the grading procedure routinely used by eel farmers to promote growth of slow-growers rejects a priori the existence of genetic determinants of individual growth rate in eels. Effects of size, culling and social history on growth of cultured elvers suggest that growth in elvers may be governed by behavioral or/and physiological responses to handling and changing social environments (Wickins, 1987). Nonetheless, studies on genetic variation of slow and fast growing elvers by polyacrylamide gel electrophoresis (PAGE) or DNA fingerprinting designed for the revelation of the "selfish gene" (Degani & Gallagher, 1995) are more prestigious than "primitive and cheap" deduction of the truth.

Surprisingly some students of eel biology conclude, that males rather than females grow faster at the sizes where the gonads are not completely differentiated (Holmgren & Mosegaard, 1996).

As it can be seen, the matter becomes too ambiguous when "only" two sexes are kept in mind: who grows faster, males or females? - Probably neither...?

G. Colombo et al. (1984) revealed that testis-like gonad of the yellow eel is more primitive, and possibly reversible, than the frilled organ of Mondini - a previtellogenic ovary.  A testis-like gonad differentiate completely at the beginning of sexual maturation and the metamorphosis into the silver (migrating) male-eels (this never happens spontaneously in indoor tanks). Thus, the rearing stock comprised of eels larger then 20 cm (the minimal length at which an ovary was found) consists of females and undifferentiated (sexless) yellow eels, which can differentiate into either females or males. This means that there are no males in an indoor eel culture! That's, probably, why it is too hard for us to "catch a head" of those who are not predestined to be boys or girls by their parents from the "very beginning".

The fourth dogma
F. – W. Tesch (1977)cites a few descriptions, interpreted by him as a tendency for eels to cluster in natural habitats: a few eels may hide together under the same stone during the day (Krause, in Tesch, ibid.; Fig.79, p.210) or several eels squeeze themselves into the same tube due to “tactile tendencies” (Tesch, ibid., p. 209), irrespective of the availability of additional unoccupied tubes placed in an aquarium. Clustering of eels in culture tanks during daylight near heating and air pipes is also a common sight interpreted generally by the same argument or "as being gregarious”.

Indeed, several eels squeeze themselves into the same tube at the very beginning of being placed in an aquarium... however in the course of time this disposition changes indicating some other "tendencies" governing eel's behavior.

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Understanding eels helps to solve problems

3.   Measuring cortisol level by radioimmunoassay (RIA) in the blood of eels revealed low levels (less than 22 nmol/l) in blood of isolated eels and high variation in eels sampled from the communal tank (from less than 22 nmol/l to 344 nmol/l). A negative correlation between eel size and blood cortisol content is assumed. This implies an adverse effect of the General Adaptation Syndrome (GAS) regarding health and growth of eels.

5.   For the first time, sexual size dimorphism was clearly established in yellow European eels on the basis of the regression of weight on length. Regression analysis showed that within the size range of 24-47cm total length and 22-201 g weight, female eels are on average lighter than sexless individuals with undifferentiated gonads. The phenomenon is due to the loss of accumulated weight associated with ovarian development up to the stage of basophilic oocytes, spontaneous both in situ and in cultivo, in eels of particular size and social rank. The difference in the Condition Factor (CF) between sexless and female eels permits sexing of about 83% accuracy; this was used as a complementary method to sexing based on macro- and microscopic appearance of the gonad (Kushnirov and Degani, 1995).

6. Absence of heterochromosomes and a skewed sex ratio in natural habitats, assuming a environmental sex-determining mechanism in anguillid eels, promoted  the experimental study on sex-determining factors. It appears that the position in the social hierarchy determines the definitive sex, namely, sexless high-ranking individuals are predetermined to become females, while submissive eels differentiate into males; to verify the definetive sex, gonads differentiation was induced by HCG injection (Degani and Kushnirov, 1992; Kushnirov, 1994).

For testes development induced by HCG see Micrographs of eel gonads .

7. Feeding elvers 60 mg/kg 17ß-Estradiol is found to induce femaleness, confirming the lability of sexual development in eels. However, the method is not practical for eel culture, as growth is effected by factors other than sex, namely, by the amount of social pressure to which they are exposed.

A new concept, outlined in the present study - agonism in a compound of A. anguilla eels is both a growth- and sex-determining factor. A space-time structured artificial environment, reflecting the eel’s adaptation to benthic life and solitary social organization, enables controlling the above biological characters.

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References

Wiberg, U.H., 1983. Sex determination in the European eel (Anguilla anguilla, L.). Cytogenet. Cell Genet. 36:589-598.

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Responses

Dear David,

I found your work and ideas on environmental (behavioral) sex determination fascinating. This is not my expertise, but I found your reasoning sound. It could explain the undifferentiated nature of gonads, the size dimorphic pattern and suggestion of density as ESD factor in American eels. I had often wondered why rearing studies didnot better inform our understanding of reproductive physiology/ecology of eels. I have shared your website (and this message) with colleagues better informed on reproductive physiology.

Regards,

David H. Secor, Professor
UMCES - Chesapeake Biological Laboratory
1 William St., Solomons, MD 20688

See also

Feb 2003 TFH Column
Fish Social Behavior
Richard J. Sexton

http://rs79.vrx.net/works/columns/tfh/2003/Feb/

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 Last revisied: June 29, 2006