Culture and Colonization

L.K. ETZEL, E.F. LEGNER, in Handbook of Biological Control, 1999

Diapause

Diapause in the life cycle often interferes with parasitoid and predator production. For example, Eskafi and Legner (1974) showed that certain temperature and photoperiod combinations would induce diapause in adults and progeny of the eye gnat parasitoid Hexacola sp. nr. websteri (Crawford). Exposing larval parasitoids within their larval hosts to a long photophase of 16 h combined with a high temperature of 32°C caused the parasitoid prepupae to enter a diapause state, termination of which required contact of the host puparia with moisture for a few hours. However, this type of easily terminated diapause only occurred following a parental generation reared at 27°C with 14-h light. With the parasitoid parental generation reared at 32°C with 16-h light and the progeny held at 27°C with 14-h light, then >90% of the prepupal progeny entered diapause and could not be induced to terminate it by exposure to moisture. With another set of progeny from the same parents reared at 32°C with 16-h light, only 35% entered diapause. This example illustrates the great complexities involved in determining which combinations of environmental regimes in the insectary will prevent, induce, or terminate diapause.

In the case of Trichogramma spp., Zaslavski and Umarova (1990) showed that the interactive effects of environmental conditions in the maternal and filial generations governed larval diapause in the filial generation. Lowered temperature during development of the filial generation was the predominant diapause factor, but the photoperiod and temperature conditions of the maternal generation influenced the “norm” of this thermal reaction. Superimposed on these diapause reactions was an endogenous process running through the generations that changed diapause tendency and underlying reaction norms even under constant rearing conditions (Zaslavski & Umarova, 1990). Other examples of complex diapauses include the alfalfa weevil parasitoid system; Chelonus spp. parasitoids of the pink bollworm that terminate diapause at different intervals (Legner 1979c); and navel-orangeworm parasitoids, where diapause seems triggered by hormonal changes in the host at different latitudes (Legner, 1983).

Not only physical environmental conditions but also host conditions and host food can influence diapause in parasitoids. The physiological state of an alfalfa plant affects the yellow clover aphid (formerly called the spotted alfalfa aphid), Therioaphis trifolii (Monell), which then induces diapause in its aphidiid parasitoid Praon exsoletum Nees (Clausen, 1977). Similarly, the type of pollen used to rear predaceous mites can affect initiation of diapause. Amblyseius potentillae (Garman) and A. cucumeris entered diapause in a short-day photoperiodic regime when reared on ice plant pollen, but not when reared on pollen of the broad bean (Overmeer et al., 1989). After studying various combinations of parasitoid species, aphid species, plants, and environmental conditions, Polgar et al. (1995) concluded that internal factors via the host aphid, such as host aphid life cycle (holocyclic versus anholocyclic), aphid morph (oviparae, etc.), and host-plant quality, as well as environmental cues (temperature and photoperiod), can all be interconnected in the induction of dormancy or diapause in aphid parasitoids.

Laing and Corrigan (1995) showed the importance of host species in initiating diapause in Trichogramma minutum. Diapause occurred at 15°C with a photophase of 12L:12D in eggs of Lambdina fiscellaria fiscellaria, but did not occur in eggs of Anagasta kuehniella, Sitotroga cerealella, or Choristoneura fumiferana held under these same conditions.

Appropriate environmental conditions or their combinations, particularly relating to light and temperature, are often useful for manipulating diapause (Singh & Ashby, 1985). For example, Tauber et al. (1997b) showed that a biotype of Chrysoperla carnea from San Pedro, Mexico, could be reared continuously without diapause with an intragenerational increase in photoperiod or could be reared with the regular intervention of a storage period. Diapause induction in individuals destined for storage was accomplished by rearing larvae under a long-day photoperiod and transferring the prepupae to a short-day photoperiod.

Waage et al. (1985) noted that one factor to consider in rearing programs is that entomophagous insects and their hosts may have different optimal developmental temperatures. As a corollary, natural enemies may enter diapause under conditions whereby their hosts remain active. The western flower thrips, Frankliniella occidentalis (Pergande), is a year-round greenhouse pest in northern climates, whereas commercially available strains of two predaceous mites, Amblyseius cucumeris and A. barkeri (Hughes), are ineffective in the winter because of reproductive diapause induced by short-light conditions. Van Houten et al. (1995), however, were able to genetically select for effective nondiapausing strains of these predators within 10 laboratory generations.

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Moths

David L. Wagner, in Encyclopedia of Biodiversity, 2001

V.D. Diapause and Quiescence

Diapause may occur in any of the four life stages. The largest fraction of temperate moths pass through the winter months in diapause as prepupal larvae or pupae; the second largest fraction overwinter as eggs. Moths that overwinter as adults invariably need to feed over the five to seven months of winter—some of these are among the unwelcome guests in the buckets used to collect maple syrup. Reproductive diapause is the rule among moths that overwinter as adults, ovarian maturation mating and being delayed until the arrival of spring. Summer diapause and aestivation occurs in many groups that live in habitats with a pronounced dry season. Enormous aggregations involving tens of thousands of moths (usually noctuids) are known to assemble in caves or on (hill) mountain tops. Moths in these aggregations become important sources of protein for vertebrates, even for grizzly bears. The diapause longevity record belongs to the desert-inhabiting prodoxids—single larval cohorts may yield adults over a period of 3 decades.

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Order Trichoptera

Ralph W. Holzenthal, ... Blanca Ríos-Touma, in Thorp and Covich's Freshwater Invertebrates (Fourth Edition), 2015

Diapause and Aestivation

Diapause, a suspension in normal development at some stage in the life-cycle, is a commonly observed phenomenon in Trichoptera. It is a response to regularly recurring periods of difficult environmental conditions and will come to an end when an external environmental stimulus signaling an end of the adverse conditions reinitiates normal development (Wiggins, 1996a; Chapman, 1998). Photoperiod is the most important factor controlling diapause; for larval diapause, the third instar is the critical stage that is particularly receptive to photoperiod influence, while the fifth instar is the sensitive stage for adult diapause (Denis, 1978). Food quality, which influences the reserved energy stores later used as an adult, has also been observed to play an influential role in the larvae of future females in Anabolia furcata Brauer, 1857 (Majecki, 1999).

Both Agapetus bifidus Denning, 1949, and A. occidentis Denning, 1949, undergo diapause for approximately 9 months in the egg stage to overwinter (Anderson, 1976). After hatching in early spring, larvae complete a rapid development in 2–3 months. Agepetus illini Ross, 1938, which also appears as an early instar in the spring, most likely also depends on a long diapause over winter as an egg (Ross, 1944). First instar larvae of Dibusa angata Ross, 1939, appear in mid-November, after an apparent egg diapause occurring through the summer and autumn months (Resh and Houp, 1986).

When diapause occurs in the larval stage, it delays metamorphosis and adults do not emerge until the autumn. When emergence occurs, eggs are close to being fully developed and are laid a few days later. Thus, diapause can function to synchronize oviposition periods in the autumn (Novák, 1960; Novák and Sehnal, 1963, 1965). Examples of species that exhibit larval diapause include Anabolia nervosa (Curtis, 1834), Chaetopteryx villosa (Fabricius, 1798), Halesus digitatus (von Paula Schrank, 1781), and H. radiatus (Curtis, 1834). These species enter diapause in late May–June and do not metamorphose until late August into September (Denis, 1978).

If there is no larval diapause, and metamorphosis and emergence are not delayed until the autumn, an adult diapause may occur during summer. In this case, metamorphosis and emergence can happen as early as spring, but females will be immature. The adult diapause inhibits ovarian development and sexual maturity will be delayed until late summer. When diapause is ended, likely triggered by external stimulus of shorter daily photoperiods, development will resume and the eggs will be fully developed by autumn (Denis, 1978; Wiggins, 1996a). However, the level of ovary development differs between species. In Glyphotaelius pellucidus (Retzius, 1783), the ovocytes and trophocytes are very small and the first follicles are barely distinguishable, while in Limnephilus lunatus Curtis, 1834, the ovocytes and trophocytes are larger and the follicles are more distinct; the follicles of L. centralis Curtis, 1834, are even larger. The differences between the oviposition periods of these species reflect these differences in ovarian development: egg masses of L. centralis appear first in early September, while those of L. lunatus and G. pellucidus do not appear until late September or early October (Le Lannic, 1976).

Limnephilis assimilis (Banks, 1908) in Death Valley, California, is another example of a caddisfly with adult diapause. Eggs are laid in October, a shortened larval phase occurs in the winter and results in pupation in February, and adult emergence has occurred by late April. Females emerge with undeveloped ovaries, which will fully mature during the extended adult phase. In this case, the adult reproductive diapause provides a way to avoid unfavorable conditions—high summer temperatures—that can lead to temporary aquatic larval habitats disappearing during summer months (Colburn, 1988). The initial delay of the caddisfly life-cycle, caused by adult diapause, allows larvae to escape the desiccating summer conditions and develop in the cooler, wetter, and more favorable conditions of autumnal pools (Wiggins, 1996a, 2004).

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Temporary Waters

E.A. Colburn, in Encyclopedia of Ecology (Second Edition), 2008

Diapause

Diapause involves suspended development. Hormonally controlled, and initiated and terminated by specific environmental cues, diapause is the most common and most effective drought-survival mechanism. It can allow survival over years – even decades – of continuous drying.

The rapid appearance of living organisms when water fills formerly dry puddles, containers, and floodplains is not, as formerly believed, spontaneous generation, or life miraculously developed from nothing. Instead, much of the life in newly flooded areas emerges from cysts, spores, seeds, or eggs diapausing on the dry substrate.

Found from bacteria to fishes in temporary waters, diapause is common in organisms with limited dispersal. Typically, the organism is replaced by a small, highly desiccation-resistant structure that awaits rehydration in the sediment. The substrate reservoir of diapausing microbes, plants, and animals is termed a seed bank, egg bank, or propagule bank. Diapause also occurs in larval and adult stages. Reproductive diapause is seen in some insects, and certain flatworms and annelids enter diapause after encysting in mucus.

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Dormancy☆

Philip Withers, Christine E. Cooper, in Encyclopedia of Ecology (Second Edition), 2019

Diapause and Quiescence

Diapause is an ecological strategy for the avoidance of harsh conditions that involves the cessation of development of a subadult life stage. It is essentially a time-delaying tactic to synchronize further stages of the life cycle with appropriate environmental conditions. Diapause is especially common in insects but is also observed in a wide variety of other invertebrate animals (e.g., brine shrimp embryos) and vertebrate animals (e.g., annual killifish embryos), as well as many plants (e.g., buds, bulbs, rhizomes, and seeds). Some plant seeds require drying out before they can develop, ensuring that adverse dry seasons pass before the embryo starts to develop. Diapause is also a reproductive strategy in a variety of mammals for the delayed implantation and development of embryos (e.g., macropod marsupials, mustelids, and deer). Quiescence is a period of inactivity, similar to diapause, but is a facultative response to an immediate change in environmental conditions that is terminated simply by the resumption of more favorable environmental conditions, rather than a programmed and obligate response. It may be a response to harsh environmental conditions such as low or high temperature, or drought. Many invertebrates and plants (particularly their seeds) become quiescent.

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Organisms

David J. Lohman, ... Shen-Horn Yen, in Encyclopedia of Biodiversity (Third Edition), 2024

Migration

Diapause is not an option for species that cannot withstand freezing temperatures. They must live near the equator or migrate to warmer areas when winter approaches. Many birds fly between the northern and southern hemispheres to enjoy an endless summer. They learn the route from conspecifics and eventually return to their natal area, making the full round-trip every year. Lepidopteran migration is fundamentally different because generations often do not overlap and the insects thus have no way to learn the route. Moreover, it often takes multiple generations to complete a single round-trip. Thus, a newly eclosed adult must “know” which phase of the migratory route it is in, where it must go, and how to navigate there. There are many kinds of insect mass movements, and we regard one-way movement as dispersal and round-trip journeys as migration.

Perhaps the best-studied example is the Monarch butterfly (Nymphalidae: Danaus plexippus) in North America. High-latitude populations east of the Rocky Mountains migrate to montane forests in Mexico where they wait out the winter in a torpid state before breeding and dying in the southern United States on their return journey. The next generation can reach Canada. Monarchs west of the Rocky Mountains migrate to the California coast, and some populations south of the no-freeze line do not migrate (Brower, 1995). Genetic and neurobiological studies have identified genes associated with migration, as well as molecular mechanisms for the butterflies’ time-compensated sun compass and other adaptations used in orientation (Reppert and de Roode, 2018). The trans-Saharan migration of the Painted Lady (Nymphalidae: Vanessa cardui) is even more complex. The round-trip can take up to six generations, and the hordes of adults fly so high in the atmosphere that they are seldom seen. Radar is used to track their movements (Stefanescu et al., 2016).

Migration is not always in response to freezing temperatures. In southern India, several species of milkweed butterflies (Nymphalidae: Danainae) migrate east-to-west to avoid torrential monsoons (Kunte, 2005). There are numerous migratory moths, too. The Australian Bogong moth (Noctuidae: Agrotis infusa) undergoes a biannual migration to aestivate at high elevations during the hot Australian summer, often in dense aggregations (Chapman et al., 2015).

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Ecology Processes

Nelson G. Hairston, ... Masato Yamamichi, in Encyclopedia of Biodiversity (Third Edition), 2024

Introduction

Dormancy and diapause are states of greatly reduced metabolism, halted development, and resistance to stress, entered by a great variety of organisms including many but by no means all bacteria, algae, plants, invertebrates and vertebrates. Some taxa diapause or go dormant for relatively brief periods (e.g., a single harsh season), while others can remain dormant for decades or even centuries (called prolonged dormancy). Key areas of study include: (1) Why do some species possess dormancy or diapause while others do not? (2) What kinds of natural selection favor prolonged dormancy versus seasonal dormancy? (3) How, physiologically, do individuals survive during dormancy, especially those in prolonged dormancy? And (4) What are the ecological and evolutionary consequences of prolonged dormancy? This article provides an introduction to the current state of knowledge for each of these topics.

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Order Odonata

Frank Suhling, ... Jan van Tol, in Thorp and Covich's Freshwater Invertebrates (Fourth Edition), 2015

Regulated Life Cycles

These life cycles are characterized by a diapause, i.e., endogenous regulation, in one or more than one life cycle stage. The diapause serves to cope with unsuitable environmental conditions, such as cold (hibernation, winter) or drought (aestivation/siccatation, dry season). The diapause may however be facultative, i.e., absent in populations living in more favorable environments.

Obligate Diapause in the Egg Stage: Hatching from the egg occurs in spring or with onset of the rainy season, and larval development takes 2–4 months. The embryonic diapause may take place in various developmental stages. In the temperate zone, it occurs in late summer and winter. The life cycle is obligatory univoltine as in most species of Lestes and certain Sympetrum species.

Facultative Diapause as Prereproductive Adult: In these species, imagines aestivate during the dry season when habitats may dry out. Most species are univoltine. Examples are phytotelmata and tree-hole breeders in seasonal rainforests, e.g., M. caerulatus, as well as species occurring in savanna habitats (e.g., Crocothemis divisa Karsch, 1898), and species of summer dry subtropics. The variation between populations may be great in species of this type, depending on the environment in which they occur. A well-studied example is Sympetrum striolatum, which aestivates for 3–4 months as prereproductive adult in the Mediterranean. In central Europe, aestivation is facultative and shorter; and in northern Europe, it seems to be nonexistent. Also, the egg development may be direct or interrupted by a diapause.

Hibernating Adult: An obligate diapause in the prereproductive adult occurs in late summer and winter as in univoltine Sympecma Burmeister, 1839, species. Reproduction occurs in early spring and development lasts 3–4 months.

Facultative Diapause Mainly in the Larval Stage: This usually occurs in one or more later instars in winter, or in summer and winter. Occasionally there may also be an egg diapause in winter. The development is uni- to partivoltine and requires three or more years (sometimes up to 10 years at or above the Arctic Circle). This life cycle type is probably present in most temperate and boreal species but also in many lotic waters and high altitude species in the tropics.

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The Biota of Intermittent Rivers and Ephemeral Streams: Fishes

Adam Kerezsy, ... Paul H. Skelton, in Intermittent Rivers and Ephemeral Streams, 2017

In contrast, embryonic diapause within the egg is the main survival mechanism of the annual killifish Nothobranchius (Watters, 2009; Polačik and Podrabsky, 2015; Fig. 4.5.8). The essential habitat for Nothobranchius killifish is seasonal water bodies that dry in the dry season and refill in the rainy season, and with a substrate with thick black or gray vertisol clay (Watters, 2009). The life history characteristics that allow Nothobranchius fishes to succeed in such environments include small adult size, rapid growth and maturity, and daily one-on-one spawning, with eggs fertilized and deposited into the substrate regularly throughout the rainy season when there is water in the habitat (Watters, 2009). The eggs have a tough chorion able to withstand the rigors of drying pools where disturbance of the substrate is likely from terrestrial animals seeking the last available surface water. Nothobranchius ova undergo several stages of development between deposition and hatching, interspersed by phases of diapause induced by environmental stress such as anaerobic conditions (Watters, 2009). Development proceeds through the stages to a point of near hatching where a final diapause occurs. Hatching is induced by substantial flooding of the habitat after the first rains occur.

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PLANKTONIC COMMUNITIES: ZOOPLANKTON AND THEIR INTERACTIONS WITH FISH

ROBERT G. WETZEL, in Limnology (Third Edition), 2001

X. GOPEPOD POPULATION DYNAMICS

A number of cyclopoid copepods are known to enter various periods of diapause, either at the egg stage or in the copepodite stages with or without encystment, upon the sediments (see, e.g., Wierzbicka, 1966; Maier, 1994). Consequently, for reasons discussed later, the annual cycle of the copepod populations may be interrupted by a diapause that persists from one to several months. For example, the resting stage may occur in midwinter, as in Cyclops bicolor, or in summer, as is seen in Cyclops strenuus strenuus (Fig. 16-28). Nauplii develop from egg-bearing females in this species in a bimodal population during winter and spring. Then, in summer, the larger population of copepodite stage IV aestivates in the sediments for about two months in midsummer before becoming pelagic again in the autumn and developing into adults. In contrast, Cyclops strenuus abyssorum exhibits only one effective generation in each year in several lakes in the English Lake District (Smyly, 1973). In the deepest of the four lakes studied in great detail, individuals of this generation hatched from eggs laid in the spring and reached the adult stage in early winter. The adults passed the winter in the plankton and started laying eggs early in the following year. In the three other lakes, most individuals of the spring generation reached the fifth copepodite stage by midsummer and spent the next eight months aestivating in the profundal zone. The copepods left this zone in February or March to return to the pelagic zone, where they became adults and started breeding. Water temperature, oxygen concentrations, light intensity, and day length were believed to be associated with the initiation and termination of this diapause (Smyly, 1973). Co-occurring species of cyclopoid copepods are known to have their maxima and diapause periods at different times. This alternation presumably minimizes competitive interactions for the same food resources.

FIGURE 16-28. Development and resting stages of Cyclops strenuus in Bergstjern, Norway. Midsummer diapause is indicated by aestivation of copepodites IV in the sediments. (Drawn from data of Elgmork, 1959.)

Although the general pattern of diapause is similar among the cyclopoid copepods, marked variations in the timing of instar stages and their distribution in the water and the sediments have been observed (e.g., Elgmork, 1959; Smyly, 1973; George, 1976; Nilssen and Elgmork, 1977; Vijverberg, 1977; Boers and Carter, 1978; Elgmork et al., 1978; Elgmork and Langeland, 1980; Hansen and Jeppesen, 1992). The basic life-cycle pattern in limnetic cyclopoid copepods can be divided into two parts (Nilssen, 1978; Elgmork and Nilssen, 1978): (a) a period of growth, followed by (b) a period of retarded growth or diapause, the latter resulting from adverse environmental conditions (Fig. 16-29). The intensity of the lines and pathways shown in Figure 16-30 differ in different lake ecosystems. Examples include:

FIGURE 16-29. Generalized life-cycle patterns of limnetic cyclopoid copepods. COP = copepodid instar stages as indicated.

(Adapted from Nilssen, 1978.)Copyright © 1978

FIGURE 16-30. Variations in the life cycles in the planktonic Cyclops scutifer. (A) 1-yr cycle without diapause from a small humic lake; (B) 2-yr cycle without diapause in an oligotrophic lake; (C) 3-yr cycle without diapause in a subpolar lake of Norway; and (D) combined 1-2-3-yr cycle with two periods of diapause in an oligotrophic lake.

(From Elgmork, 1985.)Copyright © 1985
1.

In arctic and alpine lakes, diapause as copepodites in the sediments is less common. Adult stages frequently predominate in pelagic waters just above the sediments, perhaps to avoid fish predation. The adults feed heavily on rotifers and their own nauplii during winter.

2.

Large differences in life cycles of limnetic cyclopoid copepods occur in temperate environments. In the temperate region, a period of diapause as copepodites IV and V is the rule. In less predictable environments (eutrophic lakes and temporary ponds), many cyclopoid copepods persist during periods of reduced growth in naupliar or advanced copepodid stages in the winter; a relatively small proportion of the population enters copepodid IV–V diapause in the sediments. In more predictable and less productive lakes, during colder periods of reduced or arrested growth the tendency is for most of the population to diapause in naupliar stages or resting stages in the sediments. This response is believed to result largely as an adaptation to avoid predation by fish and predatory copepods during periods of slow growth when the population is most vulnerable (Strickler and Twomby, 1975; Nilssen, 1978; Hairston and Munns, 1984).

3.

In tropical lakes showing no great seasonal changes in environmental parameters, resting stages have not been observed.

However, adaptability is large and the same species can vary greatly in its life history among different environments. For example, the reproductive period of the planktonic copepod Cyclops scutifer can be delayed by retarded development, particularly among the nauplii (Elgmork, 1985). Differences in life cycles ranged from 1 yr to many years (Fig. 16-30). Primary causal factors are focused on differences in temperature and the prolongation was inversely related to food availability. Conversely, when temperatures are high, predation pressures low, and food availability is very high as in shallow hypereutrophic lakes, naupliar stages can be very short and the cyclopoid copepods have several (3–6) generations per year (Hansen and Jeppesen, 1992). This deviation from the normal life cycle has been observed in other eutrophic lakes and can be interpreted as a strategy to optimize the reproductive output (Maier, 1996).

A. Dormancy among Gopepoda

Copepod dormancy occurs in various ontogenetic stages as resting eggs, arrested larval development, juvenile and adult encystment, or arrested development of free-swimming nonencysted copepodids or adults (Dahms, 1995). Much study has been devoted to the study of copepod dormancy as resting eggs. Diapause eggs commonly have a thick chorion that serves as protection against digestion by predators (Hairston and Olds, 1984; Marcus and Schmidt-Gegenbach, 1986), desiccation, and bacterial degradation. Species with prolonged diapause tend to be small and tend to be found in inland waters (Hairston and Cáceres, 1996). Although diapause is widespread among the crustaceans, it is particularly prevalent among the copepods.

Diapausing eggs can remain viable in aquatic sediments for decades or longer and have a mortality of ca. 1% per year (Hairston and Olds, 1984; Hairston et al., 1995; Hairston, 1996). Diapausing eggs can serve as a major genetic reservoir for repopulating copepod communities following severe environmental disturbances. Under these conditions, years of good reproductive success can be stored in diapausing eggs in the sediments and allow species to persist during years when reproductive success is poor. The interactions between environmental variations and generation overlap produced by prolonged diapause results in the maintenance of species diversity and genetic variation.

Nonencysted dormant copepodids do not feed and they survive on stored lipids while drifting with the plankton during periods of poor environmental conditions (Fryer and Smyly, 1954; Elgmork, 1980). Encysted copepodids are also common, particularly among the Cyclopoida and Harpacticoida. Dispersal of all of these forms likely occurs only occasionally during major circulation events and is not a viable adaptive strategy (Hairston and Munns, 1984).

Dormancy functions as an energy-saving mechanism that allows individuals to survive transitional periods of harsh, unfavorable environmental conditions (Hairston, 1987; De Stasio, 1989; Hairston et al., 1990; Dahms, 1993; Fryer, 1996). Adverse conditions that have been shown to induce or alter diapause include

a.

Reduced temperatures. Production of diapausing eggs has been found to begin earlier in small lakes that cool more rapidly than in large lakes (Hairston and van Brunt, 1994). Rates of emergence from diapause are strongly temperature-dependent (Maier, 1990d).

b.

Reduced dissolved oxygen.

c.

Photoperiod, particularly long-day lengths.

d.

Desiccation, particularly in sediments of reservoirs experiencing fluctuating water levels or of temporary ponds, particularly during the summer periods (e.g., Taylor et al., 1990).

e.

Reductions in food availability. There is some evidence supporting the putative adaptive strategy that fourth-instar copepodites entering the sediment during summer would relieve grazing pressures on available food for herbivorous juvenile naupliar stages caused by the grazing of competing cladocerans (Santer and Lampert, 1995). High concentrations of flagellates are required of naupliar development among some cyclopoids. Among lakes that are more productive and have higher food availability, some copepod species do not enter summer diapause.

f.

Increased carnivorous predation by invertebrates or fish. Summer diapause in freshwater copepods is often regarded as an adaptation to avoid fish predation.

B. Copepod Community Dynamics

The life cycles of the calanoid copepods are generally somewhat longer than those of the cyclopoids. In the temperate region, most species exhibit prolonged reproductive periods with several generations per year that are indistinguishable from one another. An example of these seasonal dynamics is seen in the populations of Diaptomus in a small, shallow beaver pond of southern Ontario (Fig. 16-31). The Diaptomus hatched from resting eggs in early May and produced four complete generations, the last maturing in late autumn (Carter, 1974). During spring and summer, generation times were about 4 weeks, while the time was about 6 weeks in autumn generations. Population characteristics were similar over a 3-yr period. In one year, water levels were low, which resulted in increased mean heat content per unit volume; during this year, development time of the generation was reduced and population production increased significantly. Similar annual sequences of the generations were found among the population dynamics of a species, Eudiaptomus, in a shallow lake of southern Germany (Maier, 1990b).

FIGURE 16-31. Seasonal cycles of Diaptomus reighardi in Black Pond, Ontario. Broken lines separate the estimated limits of successive generations, G.

(From Carter, J. C. H.: Life cycles of three limnetic copepods in a beaver pond. J. Fish. Res. Bd. Canada 31: 421–434, 1974. Reprinted by permission of the Journal of the Fisheries Research Board of Canada.)Copyright © 1974

A number of studies have demonstrated that most adult cyclopoid copepods are carnivorous and that their predatory activities can play a significant role in the population dynamics of other copepod species. For example, Mesocyclops was found to be selectively predacious on copepodites of Diaptomus rather than on cladocerans (Confer, 1971). Similarly, adult Cyclops preys heavily on nauplii of Diaptomus and its own species (McQueen, 1969; Bosch and Santer, 1993). Some 30% of the naupliar recruitment has been observed to be lost by copepod predation and by cannibalism. Juvenile mortality often is very high and is density dependent and can increase conspicuously at high population levels (Elster, 1954).

The coexistence of several congeneric species of copepods in the same volume of water has been a subject of much interest. It is assumed that when coexistence does occur, the species have slightly differing tolerances and optima to existing environmental conditions. Mechanisms promoting coexistence include (a) seasonal separation, discussed earlier; (b) vertical separation in relation to stratification and the distribution of food resources; and (c) size differences in relation to available food particles utilized. All of these mechanisms were implicated as factors permitting the coexistence of three species of Diaptomus in an Ontario lake (Sandercock, 1967; see also Hammer and Sawchyn, 1968). D. minutus was separated vertically from D. sanguineus, and the latter is distinctly smaller than the former species. D. minutus and D. oregonensis also were separated to some degree by size differences and exhibited different seasonal maxima. D. sanguineus and D. oregonensis were separated vertically and had different seasonal maxima. Therefore, in this lake, differences in two of the three factors provided the mutual separation among these three congeneric species that is believed necessary to coexistence.

Two common species of predatory cyclopoid copepods, Acanthocyclops and Mesocyclops, of similar size were found to coexist in similar water strata and have similar seasonal population sequences (Maier, 1990c). Small but significant differences in embryonic and juvenile development rates, feeding rates, and selective predation by fish allowed coexistence. Considerable evidence, reviewed by Hart (1990), exists that stresses the importance of variations in embryonic and juvenile development among copepods. In the generally high predation environment of fresh waters, acceleration of naupliar development potentially reduces the vulnerability of these smaller stages to size-selective tactile predation, while larger copepodid instars are able to reduce the opposing size-selective predation by visual planktivores by virtue of their ability to move quickly and escape fish ingestion.

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