Classify Each Trait to Indicate Whether It is a Continuous Trait Meristic Threshold

Population Genetics

Brian Charlesworth , in Encyclopedia of Biodiversity (Second Edition), 2013

Quantitative Variation

Quantitative variation is all-pervasive. This can involve either meristic traits, such as bristle number in Drosophila, in which there is a large number of discrete categories, or continuously varying metrical traits such as body size. Variation in typical quantitative traits is known to be under the joint control of environmental effects, accidents of development, and sets of genetic variants whose individual effects are small relative to the total range of variation in the traits (Falconer and Mackay, 1996; Flint and Mackay, 2009). Statistical methods that utilize the degree of resemblance between close relatives enable the determination of the proportion of the total phenotypic variation that is contributed by additive genetic causes – the heritability, which controls the rate of response to the selection on a trait (see Selection on Quantitative Traits). Heritabilities for quantitative traits are typically between 20% and 80%, corresponding to the fact that artificial selection is highly effective in changing the mean value of almost every trait that has been examined (Falconer and Mackay, 1996).

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Human impacts 1: sea fisheries and aquaculture

Frances Dipper , in Elements of Marine Ecology (Fifth Edition), 2022

Age determination and census

In many temperate species, age can be determined by examining the periodic markings on meristic structures such as scales, otoliths and opercular bones. The best structure to use varies between species (see Table 8.1). There are many difficulties of interpretation of these markings and they tend to become less reliable as the age of a fish increases, but the method is useful and widely applied. The age groups of fish are commonly designated as follows:

Table 8.1. Usual methods for age determination in various North Atlantic fish species.

Species	Usual method	Comments
Atlantic Cod (Gadus morhua)	Scales	Only reliable to 3 years
Haddock (Melanogrammus aeglefinus)	Scales
Hake (Merluccius merluccius)	Otoliths
Plaice (Pleuronectes platessa)	Otoliths and scales
Lemon Sole (Microstomus kitt)	Scales	Otoliths are too small
Atlantic Herring (Clupea harengus)	Scales
Atlantic Mackerel (Scomber scombrus)	Otoliths	Reliable to 6 years
Wrasse (Labridae)	Opercular bones

Group 0 Fish of less than 1 complete year of life

Group I Fish between 1 and 2 years of age

Group II Fish between 2 and 3 years of age

Group III Fish between 3 and 4 years of age and so on.

Otoliths: The three parts of the membranous sac in the inner ear of ray-finned fishes each contain an ear stone or otolith. The largest of these, usually the sagitta, can often be used for age determination. Otoliths grow by the deposition of lime on the outer surface. These layers are deposited at different rates at different seasons. By cutting thin sections of the otolith, the layers can be seen under the microscope as alternating light and dark concentric rings. Each completed year of life is represented by one darker ring. Fish of less than one complete year of age, that is Group 0 fish, show only an opaque central nucleus.

Scales: Scales can be used to age many fish, though in those that lose their scales easily (such as herring and mackerel) extra care must be taken as there is risk of transference from fish to fish when sampling. The surface of the scale bears a large number of small calcareous plates or sclerites (Fig. 8.17), the number of which increases as the scale grows. Sclerites formed during the summer are larger than those formed in winter and the alternating bands of large and small sclerites indicate the number of seasons through which the fish has lived. It is sometimes difficult to distinguish the zones of summer and winter sclerites clearly. In cod, the method is satisfactory up to 3 years of age, but as the fish become older the accuracy of the scale age becomes less certain.

Figure 8.17. Scale from a gadoid fish showing growth zones. This fish is 3 years old.

Annual growth rings in the scales of some fish such as herring are not always easy to detect. The rings show up slight differences in refraction of different regions of the scale and are best viewed under the microscope using a low power objective with dark-ground illumination. In herring the rings are usually most clearly seen in scales taken from the anterior part of the trunk region. The rings indicate the interruption of growth of the scale that occurs during the winter months. Fish spawned in late summer or autumn probably fail to record their first winter as a scale ring and in these the first ring relates to the second winter.

As already described, scale markings may be used to determine growth rates in some species, because the width of each zone between the rings is closely proportional to the growth in length of the fish during the period in which that zone was formed (see Fig. 8.16). At sexual maturity the growth rate is reduced and this is indicated on the scales by a narrowing of the growth zones. The pattern of scale rings, therefore, varies according to the age at which the fish mature. This can vary with geographic location. In Atlantic Herring (C. harengus) spawning over shallow shelf areas in the southern North Sea, the first narrow zone is usually to be found between the second and fourth rings. In those oceanic populations living further out in the North Atlantic, the first narrow zone occurs after the fifth ring indicating their later maturation. In the Baltic Sea herring the first narrow zone is between the first and second ring and there is only one wide zone, indicating that most mature in their second year.

In species where scale or other meristic markings are absent, unreadable or of doubtful reliability, then indirect methods of age analysis can be used. In the Petersen method the length of the fish is used to determine the age. Samples of the fish stock are collected using techniques and equipment most likely to contain the full spectrum of lengths. The fish are then measured and the numbers of fish are plotted against the length (in suitable increments) to give a polymodal distribution. Each mode represents an age group within the sample, but this method can only be used for species that spawn seasonally within a restricted time frame. If sampling is done as soon as possible after spawning once the larval stage is complete, then the mode with the lowest length value will represent O-group fish. The next mode will represent I-group fish and so on (Fig. 8.18). The method is very good for young fish but less so for older ones where the growth rate slows down and so the modes start to overlap and merge. Accuracy can be increased by sampling and analysis at intervals throughout the year.

Figure 8.18. Typical (but hypothetical) length/frequency data for Atlantic cod (Gadus morhua).

If the Petersen method is applied to a species in which scale or other meristic markings are present, then the age assigned to each length mode can be checked against the age determined by ring counts. The Petersen method is usually applied to the analysis of cod populations because the scale markings are only reliable up to 3 years of age. The modes of the length/frequency curve indicate the length distribution of each age group. From these, average growth rates can be estimated.

Census of the age composition of Atlantic Herring shoals in the southern North Sea have shown that here the adult shoals contain fish from 3 to 11 years old, the maximum age normally reached by herring in this area. Off the Norwegian coast, where the fish mature later, the age range is from 4 to 15 years or older. Great variations occur from year to year in the number of fish entering the adult shoals and this is reflected in the relative abundance of each year class. Strong year classes, where many young survive and are recruited into the adult population, may dominate the population for many years. Knowledge of the age composition of shoals in a particular year enables a prediction to be made of the probable composition of the next year's shoals, which may be a useful guide to sensible regulation of fisheries. Additional data on the sex ratio and proportion of mature and immature fish can be determined from the same stock samples used for age and length analysis.

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Tetrapod Relationships and Evolutionary Systematics

Laurie J. Vitt , Janalee P. Caldwell , in Herpetology (Third Edition), 2009

Morphology

Three discrete classes of anatomical characters are recognized: (1) mensural or morphometric characters are measurements or numeric derivatives (e.g., ratios, regression residuals) that convey information on size and shape of a structure or anatomical complex; (2) meristic characters are those anatomical features that can be counted, such as number of dorsal scale rows or toes on the forefoot; and (3) qualitative characters describe appearance; for example, a structure's presence or absence, color, location, or shape.

1

The most common morphometric character in herpetology is snout-vent length (SVL). This measurement gives the overall body size of all amphibians, squamates, and crocodylians, and how it is measured differs only slightly from group to group depending on the orientation of the vent, transverse or longitudinal. Because of their shells, carapace length and plastron length are the standard body size measurements in turtles. Numerous other measurements are possible and have been employed to characterize differences in size and shape. Mensural characters are not confined to aspects of external morphology but are equally useful in quantifying features of internal anatomy, for example, skeletal, visceral, or muscular characters.

As in all characters, the utility of measurements depends on the care and accuracy with which they are taken. Consistency is of utmost importance, so each measurement must be defined precisely, and each act of measuring performed identically from specimen to specimen. The quality of the specimen and nature of the measurement also affect the accuracy of the measurement. Length (SVL) of the same specimen differs whether it is alive (struggling or relaxed) or preserved (shrunk by preservative; positioned properly or not); thus, a researcher may wish to avoid mixing data from such specimens. Similarly, a skeletal measurement usually will be more accurate than a visceral one because soft tissue compresses when measured or the end points often are not as sharply defined. Differences can also occur when different researchers measure the same characters on the same set of animals. Thus within a sample, variation of each character includes "natural" differences between individuals and the researcher's measurement "error." Measurement error is usually not serious and is encompassed within the natural variation if the researcher practiced a modicum of care while taking data. The use of adequate samples (usually >20 individuals) and central tendency statistics subsumes this "error" into the character's variation and further offers the opportunity to assess the differences among samples and to test the significance of the differences, as well as providing single, summary values for each character.

2

Meristic characters are discontinuous (= discrete). Each character has two or more states, and the states do not grade into one another. The premaxillary bone can have 2, 3, or 4 teeth, not 2.5 or 3.75 teeth. Meristic characters encompass any anatomical feature (external or internal) that can be counted. Researcher measurement error is possible with meristic characters. These characters are examined and summarized by basic statistical analyses.

3

Qualitative characters encompass a broad range of external and internal features, but unlike mensural or meristic characters, they are categorized in descriptive classes. Often a single word or phrase is adequate to distinguish among various discontinuous states, for example, pupil vertical or horizontal, coronoid process present or absent, carotid foramen in occipital or in quadrate, or bicolor or tricolor bands at midbody. Qualitative characters can have multiple states (>2), not just binary states. Even though these characters are not mensural or meristic, they can be made numeric, simply by the arbitrary assignment of numbers to the different states or by size comparison (e.g., 1x width versus 3x height).

The preceding characters emphasize aspects of gross anatomy, but microscopic characters may also be obtained. One of the more notable and widely used microscopic (cytological) characters is karyotype or chromosome structure. The most basic level is the description of chromosome number and size: diploid (2n) or haploid (n) number of chromosomes, and number of macro- and microchromosomes. A slightly more detailed level identifies the location of the centromere (metacentric, the centromere is in the center of the chromosome; acrocentric, the centromere is near the end; and telocentric, the centromere is at the end) and the number of chromosomes of each type or the total number (NF, nombre fundamental) of chromosome arms (segments on each side of the centromere). Special staining techniques allow the researcher to recognize specific regions (bands) on chromosomes and to more accurately match homologous pairs of chromosomes within an individual and between individuals.

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Phylum Arthropoda: Crustacea: Malacostraca

D. Christopher Rogers , ... W. Wayne Price , in Thorp and Covich's Freshwater Invertebrates (Fourth Edition), 2020

Limitations

Identification of Neotropical stygiomysids is based mainly on variations in the morphology of the carapace, eye, antennal scale, telson, and uropod, characters that exhibit little sexual dimorphism. These and other basic morphological characters included in the key are presented in Fig. 23.186. Whenever possible, mature adult specimens should be examined because many characters used in the key are from adults as opposed to juveniles and immatures. This is especially important for meristic characters such as the number and size of setae on the lateral and posterior margins of telsons and uropodal protopods. Setae may be added with successive molts as the organism matures. In addition, the length:width ratios of telsons may change with growth. Unfortunately, it may be difficult to separate immature stygiomysids from adults and even adult females from males since little sexual dimorphism exists. Some characters that separate these developmental stages are presented in the next section.

Additional problems with identification of stygiomysids result from the small number of specimens examined for some species and the limited areas collected. Many more collections are needed before the morphological variability and geographic distribution of the species in the Stygiomysida can be described adequately.

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Functioning of Ecosystems at the Land–Ocean Interface

B.M. Gillanders , ... M. Roughan , in Treatise on Estuarine and Coastal Science, 2011

7.06.4.2 Natural Marks

The above applications all involve artificially marking or tagging the animal, which is not always feasible. Researchers have therefore investigated whether natural marks can be used to identify animals. Natural marks can be broadly grouped into five key areas: morphometric marks (shape, color, or markings of body features), meristic marks (intraspecific differences in numbers of repeated tissue features such as gill rakers or fin rays), parasitic marks (presence/absence of parasites in animals from different areas or in genetics of parasites among areas), chemical marks (differences in chemical composition of animal tissue), and genetic marking. Several studies have used natural marks to distinguish individuals (Grimes et al., 1986; Connell and Jones, 1991; Wilson et al., 2006), and at least one study has used the size of individuals to determine origins of larvae (Gaines and Bertness, 1992). This latter application was possible because bay larvae were substantially larger than larvae that developed over the continental shelf such that larvae flushed from the bay could be distinguished at coastal sites (Gaines and Bertness, 1992).

These methods all require similar basic steps. First, it is necessary to obtain structures or information from organisms of known origin, such as from fish or birds collected at different locations, and ensure that differences exist between groups. Second, validation of the mark's reliability is required. Frequently, this is done using the same group of organisms as used to determine whether differences among groups occur; however, in an ideal situation additional organisms should be collected for validation and assigned to groups to determine potential error rates of assignment. Third, structures or information from organisms of unknown origin can now be used to assign fish or birds to different groups and thereby determine potential connectivity. However, it is important to remember that an assumption of these and some other tagging techniques is that all potential source populations have been characterized (Gillanders, 2005). In reality, morphometric and meristic marks are unlikely to be useful for investigating connectivity among estuaries or between estuaries and coastal areas except in unusual situations or in combination with other techniques.

Parasitic marks have been used to detect movement from estuary to adult habitat and determine relative stock composition of fish (Olson and Pratt, 1973; Moles et al., 1990). Frequencies of infection of the brain parasite, Myxobolus neurobius, were used to determine stock composition of sockeye salmon since individuals from southeast Alaska showed high infection rates of greater than 85%, and Canadian stocks showed low infection rates of less than 10% (Moles et al., 1990). In a separate study, Olson and Pratt (1973) found that the parasite acanthocephalan, Echinorhynchus lageniformis, was acquired by English sole, Pleuronectes vetulus, only while in the estuary and not while offshore. The incidence of infection in estuarine fish before emigration was similar to the incidence in young fish collected offshore after emigration, suggesting that there was little or no influx of young from potential nonestuarine habitats (Olson and Pratt, 1973). Parasites were also used in combination with nuclear and mitochondrial DNA markers to investigate connectivity across putative biogeographical barriers in the Mediterranean Sea and eastern Atlantic Ocean (Sala-Bozano et al., 2009). Similar applications have also been done on birds where connectivity of breeding and overwintering grounds has been investigated (e.g., Durrant et al., 2008).

In its simplest form, parasite assemblages have been compared among geographic areas, such as estuaries. Parasite assemblages would be expected to be more similar with increasing connectivity among areas and less similar with decreasing connectivity (Kennedy, 2001). However, Lester and Mackenzie (2009) reiterated the importance of parasite residence times and suggested using parasites that survive for longer than a year for stock identification studies, whereas parasites with life spans of less than a year may be useful for seasonal migratory studies (see also MacKenzie and Abaunza, 1998, 2005). An understanding of the length of migratory period of the host organism is therefore critical. Other criteria for use of parasites as biological tags are that the parasite should have different levels of infection in the host in different areas, the level of infection should be similar among years, the parasite should be easily detected and identified, and should not be a serious pathogen (see Williams et al. (1992), MacKenzie and Abaunza (1998, 2005) for more details). Another possibility is to investigate whether there are differences in the genetics of the parasites of individuals in different areas (see also Criscione et al., 2006; Fallon et al., 2006; Nieberding and Olivieri, 2007; Durrant et al., 2008). Parasites and hosts do not always show similar genetic structure and, in general, animal macroparasites are more highly structured than their hosts (Criscione et al., 2005, 2006). Indeed, Criscione et al. (2006) demonstrated that parasite genotypes of a nematode, Plagioporus shawi, can more accurately assign the host steelhead, Oncorhynchus mykiss, back to its source population than the host's own genotype. Such studies may help elucidate patterns of connectivity among estuaries and be useful for both fish and birds.

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Estuarine and Diadromous Fish Metapopulations

CYNTHIA M. JONES , in Marine Metapopulations, 2006

C. NATURAL TAGS WITH EMPHASIS ON OTOLITH-GEOCHEMICAL TAGS

In contrast to the well-studied nature of physically applied tags, natural tags have been used less frequently, and the underlying statistical theory for estimating migration rates and natal fidelity is almost completely undeveloped (Jones, manuscript in preparation). Historically, the use of natural tags depended on the presence of parasites, unusual marks on bones or otoliths, and differences in meristics and morphometrics that occurred infrequently, in restricted spatial areas, and had to be used opportunistically (Quinn et al., 1987). The inconsistent natural occurrence of these types of tags meant that they were difficult to use in a statistically designed field study. For such reasons, natural tags have been problematic and used less frequently in studies of fish movement and philopatry. However, recent technical development of otolith–geochemical tags to mark fish movement will increase the use of these natural tags in mark–recovery studies.

The value of otolith–geochemical tags lies in their universality. All fish incorporate the chemistry of their environmental waters into their bones, albeit to greater or lesser extent under physiological influence (Campana, 1999). When this chemistry is sufficiently unique in different habitats and when it does not have a biological overprint (uptake is not modified by the fish's physiology), it serves as a mark of an individual's environmental and nursery residence (see, for example, Campana et al., 1994; Thorrold et al., 2001; and Wells et al., 2003). These chemical tags have been used to measure the movement of anadromous fish (Limburg, 1995, 1998; Secor et al., 2001), to differentiate contingents of fish that have separate movement trajectories (Secor, 1999; Secor et al., 2001), and to track their lifetime movements (Rieman et al., 1994; Secor et al., 1995, 1998). This is an area of burgeoning study and, as a nascent field, its future value to the study of fish movement cannot be fully evaluated.

Because the use of natural tags has been infrequent and opportunistic until the recent use of geochemical tags, the development of estimation procedures for these tags has been ignored by statisticians. The use of natural tags to assess movement is problematic in comparison with those that are physically applied by the scientist in known number. Although estimation of movement based on physically applied tags provides true estimates of straying and fidelity, this estimate based on natural tags is compromised by difference in capture probabilities and survival (Jones, manuscript in preparation). Clearly, estimation procedures must be developed to use natural tags knowledgeably and this is an open area for future study.

When we compare the rates of migration and putative gene flow obtained with tags with those obtained from genetic markers, we must realize that we are measuring different things. For example, when correctly estimated, otolith geochemistry measures migration into the receiving population, but does not take account of subsequent successful gene intrusion. In contrast, as Hellberg and his colleagues (2002) point out, genetic markers measure both migration and effective population size. They note that migrants may not breed successfully or their propagules may not survive.

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Striped Bass and Other Morone Culture

Christopher C. Kohler , in Developments in Aquaculture and Fisheries Science, 1997

6.2 BIOLOGY AND ECOLOGy

6.2.1 Geographic Range

White bass are native to North America with a natural range extending from the St. Lawrence River west through the Great Lakes (excluding Lake Superior) to South Dakota, and the Mississippi and Ohio River drainages south to the Gulf of Mexico (Becker, 1983; Lee et al., 1980; Scott and Crossman, 1973). The white bass range has been greatly expanded by stocking and now includes all the Gulf and south Atlantic states, as well as New Mexico, Utah, Colorado (Jenkins, 1970),California (von Geldern, 1966), and Nevada (Trelease, 1970).

6.2.2 Morphological Characteristics

White bass (Figure 6.1) resemble other members of the genus Morone with horizontal black stripes laterally, and an overall silver color with upper sides olive gray shading to white on head and belly (Becker, 1983). The body is robust, deep, and strongly compressed laterally with the following meristic characters (from Bayless, 1968):

Fig. 6.1. Adult female white bass in spawning condition.

(Photo by V. Sanchez)

•	Lateral line	-	52 to 58
•	Scales above lateral line	-	7 to 9 (usually 8)
•	Soft anal rays	-	12 to 13
•	Soft dorsal rays	-	12 to 13
•	Teeth on tongue	-	1 patch
•	Parr marks	-	absent.

Fingerling white bass are easy to distinguish from fingerling striped bass which have parr marks (Bayless, 1968) and typically have two tooth patches (Williams, 1976). Conversely, it is sometimes difficult to distinguish the young of white bass and hybrids of striped bass, including F₁, F₂, and backcrosses (Kerby and Harrell, 1990).

6.2.3 Age and Growth

In the wild, white bass are relatively fast-growing fish that can live up to nine years of age, but usually only five to six years (Table 6.1). Young-of-the-year white bass grew about 0.1   cm/day in Lewis and Clark Lake, South Dakota (Ruelle, 1971), but likely grow considerably faster in their southern range. In general, white bass grow to a maximum of about 42.5   cm total length (TL), with weights rarely exceeding 2   kg.

Table 6.1. Calculated total length (cm) of white bass from various waters.

Body of Water	Age									References
	1	2	3	4	5	6	7	8	9
Lake Erie	11.9	20.8	27.7	31.5	33.5	34.5	35.6	----	----	Van Oosten (1941)
Lewis & Clark, SD	10.9	24.4	30.2	32.8	35.8	36.8	----	----	----	Ruelle (1971)
McConaughy, NE	11.7	25.1	32.0	35.8	39.4	41.4	----	----	----	McCarraher et al. (1971)
Oklahoma*	19.1	31.0	36.6	40.9	43.4	45.2	----	----	----	Jenkins and Elkin (1957)
Oneida, NY	13.5	26.2	31.4	33.8	35.6	37.3	39.1	40.6	44.2	Fomey and Taylor (1963)

*

Unweighted means.

(from Davin et at., 1989)

6.2.4 Food Habits

Newly hatched white bass initially feed on rotifers and similar size organisms for two to three weeks when they reach a size that allows them to feed on microcrustaceans. Copepods and cladocerans comprise the bulk of the natural diet, at least through midsummer (Ruelle, 1971; Priegel, 1970)· However, insects were found to predominate in young white bass during spring in Beaver Reservoir, Arkansas (Olmsted and Kilambi, 1971). By midsummer, young white bass will switch to a largely piscine diet, provided forage fish are available in suitable size and abundance. Alternatively, the fish may continue to consume invertebrates (Voightlander and Wissing, 1984). Adults are primarily piscivorous.

6.2.5 Natural Spawning

Female white bass in the wild usually mature at age 3 while males generally mature a year earlier (Ruelle, 1971). The spawning temperature for white bass is typically between 14.4° to 18.3   ° C (Ruelle, 1971). White bass migrate up tributaries when available and spawn in shallow waters on firm gravel or sand (Becker, 1983). They will spawn on any suitable shoreline structure in the absence of tributaries. Spawning occurs during both day and night (Chadwick et al., 1966), but spawning fish are most active crespuscularly (Voightlander and Wissing, 1984). Eggs are adhesive, demersal, and increase little in diameter when water- hardened (Rees and Harrell, 1990). No nest construction or care is provided to the eggs which stick to gravel and vegetation. Mature white bass females can each produce several hundred thousand eggs ranging in diameter between 0.61 and 0.68   mm at ovulation (Bayless, 1972). Hatching occurs in about two days, with fry being approximately 3.0   mm TL at hatch, growing in 15 days (at 17.8   °C) to 8.0   mm TL (Ruelle, 1971).

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Canalization, Cryptic Variation, and Developmental Buffering

Ian Dworkin , in Variation, 2005

INTRODUCTION

In the folklore of evolutionary biology, one of the great wedges that occurred between the advocates of the Darwinian modern synthesis and other evolutionists concerned the fundamental issue of (heritable) variation. According to Darwin (1859), the variation that natural selection acted upon was, in general, quantitative. Bateson (1894) distinguished between continuous, meristic, and discontinuous variation and on the whole thought that the latter categories of variation were the targets of evolutionary forces. These opposing views diverged further during the development of population and quantitative genetics, where a fundamental assumption for most theoretical work (and statistical models) was that a very large number of loci, each with small (additive) effects, was responsible for trait expression. From this work, several models for the maintenance of genetic variation developed, such as mutation–selection balance, balancing selection, and overdominance, among others (see Hartl and Clark, 1997; Roff, 1997; for reviews). However, Waddington (1952, 1953) suggested an alternative mechanism to explain the maintenance of some genetic variation and with it an alternative model for the evolutionary process known as "genetic assimilation." The model of genetic assimilation predicts that in the face of unusual environment conditions, phenotypes can be genetically "captured" by the process of natural selection, if strong selection occurs. Implicit to this evolutionary model was a trove of hidden (cryptic) genetic variation for the trait, which was not generally observed (without the appropriate environmental stimulus), and a buffering mechanism, referred to as canalization, which helped to "store" the genetic variation. When the buffering mechanism failed (de-canalization), the cryptic genetic variation was released for selection to act upon. In the initial formulation of the model, if selection on this novel phenotype was strong (and consistent) enough, the new trait could itself then become canalized and be produced without the environmental stimulus. However, in later derivations of the model, it has been suggested that the assimilation process may not in fact occur by the mechanism as suggested by Waddington and that selection may act to change the threshold of trait expression or rare alleles that affect phenotypic penetrance are coselected and are in fact responsible for the genetic assimilation (Stern, 1958; Bateman, 1959). This last category is known as the "Baldwin" effect. This chapter will not deal any further with the mechanisms behind genetic assimilation and instead will focus on assessing canalization and cryptic genetic variation (see Scharloo, 1991, for review of the genetic assimilation controversy).

Regardless of the concerns with the mechanistic explanation of genetic assimilation, the plausibility of the phenomena of genetic assimilation as well as the existence of cryptic genetic variation were established via some empirical experiments. Waddington (1952, 1953) demonstrated that traits that were invariant under most (normal) environmental circumstances could be sensitized so as to express phenotypic variation for these traits. The classic example of Waddington's was the use of a high-temperature "heat-shock" in Drosophila, which resulted in some flies having lost their wing cross-veins. Waddington demonstrated that the cross-veinless phenotype could be selected upon, suggesting considerable hidden (cryptic) genetic variation for this trait (Waddington, 1952, 1953). Later work demonstrated that these observations could be extended to other environmental perturbations and traits (Waddington, 1956; Bateman, 1959) as well as to genetic perturbations (Rendel, 1959). However, all of these studies (and later ones) sufficiently demonstrated that the buffering mechanism (canalization) and the cryptic genetic variation being suppressed are intertwined (although this does not imply that the cryptic genetic variants are themselves responsible for the buffering).

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Endangered Freshwater Fishes of the Yucatan Peninsula

Juan J. Schmitter-Soto , in Reference Module in Earth Systems and Environmental Sciences, 2021

The Lake Chichancanab species flock

The first hint at the very interesting evolutionary scenario in endorheic Lake Chichancanab, close to the geographic center of the Yucatan peninsula (Fig. 1), was the description of Cyprinodon beltrani Álvarez del Villar (1949), the Blackfin Pupfish. The Mexican ichthyologist noticed that the presence of an estuarine genus so far inland, in a locality with no connection to the sea or to coastal mangrove systems, was outstanding, and immediately confirmed that his specimens were quite different from the widespread coastal species in the peninsula, C. artifrons Hubbs (1936), the Yucatan Pupfish. The body is more rhombic, with a shallower caudal peduncle and smaller eyes, and there are several meristic differences. A trait that C. beltrani has in common with the putative sister species of the flock, C. artifrons, is the gut length, 2–4 times longer than in any other Cyprinodon, likely an adaptation for detritivory (Stevenson, 1992).

Three species more were described decades later, and it became clear that a sympatric speciation process producing a "remarkable" species flock had taken place in Chichancanab (Humphries and Miller, 1981). The process was clearly driven by trophic preferences. Contrary to the detritivory in C. beltrani, each of the new species exhibited diverse degrees of specialization, which were mirrored in their morphological adaptations. Cyprinodon labiosus, the Thicklip Pupfish, uses its expanded, fleshy lips, to search for crustaceans and mollusks in the sediment; C. maya, the Maya Pupfish, with its larger size and stronger jaws, is the only piscivore in the genus; and C. simus, the Boxer Pupfish, the only limnetic species in the flock, looks for plankton and neuston with its upturned jaw and larger eye (Humphries, 1981).

The fifth species described in the flock was C. verecundus Humphries (1984), the Largefin Pupfish. This one is not distinguishable by such a clearly feeding-oriented characteristic, but rather, as the common name implies, by the size of the paired fins. Contrary to the other species in the lake, it is said to prefer harder bottoms, which it "sweeps" with the fins and also with the mouth, more subterminal than in the other Cyprinodon. As for the last two species in Chichancanab, they were not discovered until this century: C. esconditus Strecker (2002), the Hidden Pupfish, which differs in morphometric details, but also in having a greater number of teeth, and C. suavium Strecker (2005), the Kissing Pupfish, with a flattened, shallow skull roof and a shorter gut.

It should be mentioned, that most of the seven species are not discernible by most molecular markers, a phenomenon explained because of the recency of their origin (Lake Chichancanab is only ca. 8000   years old: Hodell et al., 1995). Moreover, they readily hybridize in nature.

Even before all the species in the flock were described, the main threat to their survival began: in 1988, tilapia (Oreochromis mossambicus and hybrids) escaped from floating cages in Chichancanab, and by the late 1990s it was dominant in the lake, with a relative abundance greater than 20% (Schmitter-Soto and Caro, 1997). One of the effects of the invasion was the increase in parasite load for the Cyprinodon species, as well as a decrease in ostracods, important prey for several of them, the decrement likely because of sediment resuspension and destruction of bottom vegetation by the substrate-feeding tilapia (Strecker, 2006). A secondary threat is agricultural runoff into the lake, with pesticides and other pollutants (IUCN, 2021), and perhaps a more recent invasion, by the tetra Astyanax bacalarensis, probably from neighboring drainages (Lyons et al., 2020).

Horstkotte and Strecker (2005) observed that the trophic preferences that presumably sparked the speciation in the first place were changing: the formerly planktivore C. simus became a detritivore, and the piscivore C. maya shifted to snails. In fact, all seven species ingested at least 40% detritus, and pairwise niche overlap increased to 95% in some cases; feeding overlap was also wide with the introduced tilapia.

Martin and Wainwright (2011) claimed that the flock was "essentially extinct," in view of the increment in unidentifiable hybrids, the decline of all endemics (except C. beltrani) and the suspected extinction of at least C. simus (Fuselier, 1996). However, the latest Red List evaluations placed all endemic Cyprinodon species as Vulnerable, and only C. simus as Near Threatened (IUCN, 2021), because most of the population trajectories seem to have stabilized in the more recent years.

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THE NILE TILAPIA

Ambekar E. Eknath , in Conservation of Fish and Shellfish Resources, 1995

Characterization

To address the taxonomic problems and also to identify the distinctness of both natural and aquaculture populations, a variety of techniques has been used: morphometries (Pante et al., 1988; Velasco et al., in press); electrophoresis (McAndrew & Majumdar, 1983; Taniguchi et al., 1985; Macaranas et al., 1986; Galman et al., 1988; and many others); serum protein analysis (Avtalion et al., 1976); immunology and agglutination assays (Avtalion & Wojdami, 1971; Oberst et al., 1989); mitochondrial DNA restriction analysis (Seyoum, 1989); karyotype analysis (Crosetti et al., 1988); and DNA fingerprinting (Harris et al., 1991). A comprehensive description of some of these techniques and their applications is given by Kornfield (1991).

Briefly, multivariate analyses of morphometric (using truss network of landmark points on the body outline) and meristic characters can discriminate species, but not between different strains within a species (Pante et al., 1988; Velasco et al., in press). The practical utility of electrophoresis as a technique to distinguish natural populations has been demonstrated by McAndrew & Majumdar (1983). The widespread introgression of O. mossambicus genes into commercial stocks of O. niloticus in the Philippines was confirmed by electrophoresis (Taniguchi et al., 1985; Macaranas et al., 1986). The Philippine domesticated populations of Nile tilapia were genetically more diverse than the wild Egyptian ones. This is probably due to the recombinations that are characteristic of introgressed populations. Serum protein analyses have shown sex differences (Avtalion et al., 1976) only in the subpecies O. n. vulcani but not in others. Mitochondrial DNA characterisations could distinguish unambiguously the seven subspecies of O. niloticus (Seyoum, 1990). Immunological techniques and blood group studies have been successful also in discriminating species (Oberst et al., 1992) and a field kit is now under preparation through collaborative research between the Institute of Aquatic Biology (Ghana), the Zoologisches Institut und Museum, Universität Hamburg (Germany) and ICLARM.

One of the advantages of DNA sequencing over other methods is data standardization. Although only a few results are available so far, the potential is vast. Some of the applications include identification of genetic lineages, estimation of inbreeding rates in aquaculture stocks, monitoring fish transfers, and securing breeders' rights.

Clearly, the methodology for characterization of tilapia genetic resources is readily available. Through analysis of genetic distances and cladistic relationships, the identity of natural and aquacultural populations has been established. However, methods to assign conservation value to a given genetic resource are needed. This is a universal problem that should be tackled globally. Typically, methods are needed to determine whether and when a particular population or genetic resource base should be conserved. In the short term, and strictly from a breeder's point of view, screening for and estimating the extent of genetic variation and covariation between traits of economic importance, genotype x environment interactions, and identifying genetic lineages will be of great benefit for establishing sound breeding schemes.

A topic that deserves special mention here is that of the sex determination mechanism in tilapias. In aquaculture, male tilapias grow 20-40% larger than females. Therefore in almost all progressive tilapia culture systems the goal is to grow an all-male population in order to prevent early maturation and breeding (a major negative attribute of tilapias in confined conditions) and to capitalize on the faster growth of males over females. The various methods of population control in farmed tilapias have been reviewed by Mair & Little (1991). The methods include: sex reversal by androgenic hormones, intermittent harvesting, manual sexing, use of predators to control recruitment, high density stocking, cage culture, delayed sexual maturity, sterilization, hybridization, and the production of YY-broodstock. The most widely used methods include hormonal sex reversal, hybridization, and a combination of both.

One of the research areas that most tilapia biologists are engaged in is the mechanism of sex determination (Trombka & Avtalion, 1993). Briefly, four different approaches have been used: interspecific and intraspecific crosses; sex inversion of fry to females or males by hormonal treatment; chromosomal manipulations leading to ploidy and gynogenesis; and karyotyping and differential staining of the tilapia genome. On the basis of evidence gathered, several models of sex determination have been proposed (Wohlfarth & Wedekind, 1991). A major hurdle in studies on sex determination mechanisms is the lack of distinct sex-linked markers in tilapias. The general theme of most of the models is that, principally, sex determination follows a distinct gonosome strategy, albeit with autosomal and environmental influences. However, Wohlfarth & Wedekind (1991) suggested that sex ratio be treated as a quantitative trait for sex determination studies.

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