Dispersal Of Eggs Research Articles

Selective Predation of Ephippal Daphnia and the Resistance of Ephippal Eggs to Digestion

In the spring and autumn, Daphnia populations may produce resting eggs contained in pigmented envelopes called ephippia. Pumpkinseed sunfish and yellow perch preyed selectively on ephippial over similarly sized, nonephippial Daphnia galeata mendotae Birge in Fuller Pond, Connecticut. The D. galeata mendotae with darker ephippia were consumed at a greater frequency than those with less pigmented ephippia. In the laboratory pumpkinseeds and—spotted newts preyed selectively on ephippial over nonephippial Daphnia pulex Leydig under natural light intensities. Ephippium tanning and hardening probably evolved and persisted because of the increased protection given to the resting eggs after natural release of the ephippia from the maternal Daphnia. Darker ephippia retained their normal complement of two resting eggs twice as well as less pigmented ephippia did during passage through newt and perch digestive tracts. D. galeata mendotae and D. pulex ephippial eggs hatched after passage through pumpkinseed and yellowbelly sunfish, golden shiners, and newts. Seventy percent of the 27 planktivorous fish collected from Fuller Pond in October and November contained ephippia. Large numbers of ephippia were found within the digestive tracts of some of these perch and sunfish. Daphnia pulex and D. galeata mendotae ephippial eggs hatched after passage through a Black—Crowned Night Heron and laboratory rats. This suggests a possibility for Daphnia resting egg dispersal by fish—eating birds and mammals feedings on planktivores that prey selectively on ephippial Daphnia.

GENETIC DIFFERENTIATION WITHOUT ISOLATION IN THE AMERICAN EEL, ANGUILLA ROSTRATA.

Geographically widespread species commonly show genetic differences among different areas of their distribution. Differences presumably arise from some degree of isolation by ecological barriers, or mere distance, with natural selection favoring different genotypes in different areas. Isolation is assumed to be prerequisite for selection to produce statistically demonstrable genetic differentiation. In contrast, genetic uniformity is taken to indicate free gene flow among localities, or the existence of some form of balancing selection (Prakash et al., 1969). The existence of genetic differences among relatively restricted areas of prolific marine organisms with planktonic dispersal stages (Frydenberg et al., 1965; M0ller, 1968, 1969; Koehn et al., 1973) has led us to question this kind of reasoning. In the cod, for instance, passive dispersal of the planktonic eggs and larvae by ocean currents should last for weeks or months, notably in colder parts of its range. Ocean current velocities are such that dispersal for hundreds of kilometers should be common. However, expected genetic uniformity over hundreds or even thousands of kilometers is not supported by observation. There are consistent differences in the hemoglobin allele frequencies along the Norwegian coast (Frydenberg et al., 1965) as well as between inshore and offshore samples (M0ller, 1969). There are additionally, significant differences with respect to depth of the samples in both hemoglobin and blood type allele frequencies (M0ller, 1968). Microgeographically, similar observations can be made, such as the significant differences at the tetrazolium oxidase locus in the marine pelecypod Modiolus demissus between high and low areas of the tidal zone (Koehn et al., 1973). We here suggest that isolation is not prerequisite to the above described genetic differentiation, but that every locality derives its adult stock from the same zygote pool. Given selection of sufficient intensity, striking genetic differences can be produced between the sedentary adult stocks that are widely dispersed as young and belong to the same panmictic population. Also, recent literature that isoenzyme variation is responsive to local environmental conditions (Koehn and Rasmussen, 1967; O'Gower and Nicol, 1968; Johnson et al., 1969; Koehn, 1969; Prakash et al., 1969; Richmond, 1970; Marshall and Allard, 1970; Koehn et al., 1971; Smith and Koehn, 1971; Koehn and Mitton, 1972; Merritt, 1972) suggests that genetic isolation is not prerequisite for differing genetic composition throughout the range of the species. While most marine organisms with pelagic larvae breed in various geographic areas, and therefore do not offer a simple situation in which the relative importance of selection and isolation can be estimated, in the American eel, Anguilla rostrata, all divergence between localities must necessarily arise in the absence of isolation. The thorough investigations of Schmidt (1922, 1925) indicate that all spawning by the American eel takes place in a circumscribed region northeast of the West Indies. O'cean currents disperse the leptocephalus larvae to American shores from northern South America to the Arctic. The leptocephali in coastal waters metamorphose into elvers that move into estuarine or fresh waters, where they grow to adult size. These resi-

Evidence for a Marking Pheromone Deterring Repeated Oviposition in Apple Maggot Flies 1 , 2

Immediately following oviposition (but rarely following attempted boring without oviposition), apple maggot, Rhagoletis pomonella (Walsh), females were observed to circle around the fruit for about 30 seconds, dragging their fully-extended ovipositors on the fruit surface behind them. The results of field and laboratory experiments indicated that during such dragging, a pheromone deterring repeated boring is deposited on the fruit surface. Where the fruit was small (15-mm-diam cherries) and the drive to oviposit moderate (as among the field population), there was almost total deterrence for 4 days (i.e. until termination of the experiment) to any repeated boring in a fruit so marked by a single female. Where the fruit was large (55-mm-diam apples) or the drive to oviposit strong (as among laboratory flies deprived of oviposition sites), marking by only one female was insufficient to elicit strong deterrence. It is suggested that deposition of this marking pheromone is an important factor mediating uniformity in egg dispersion among available fruits, and that the amount of fruit surface area marked following a single oviposition is related to the amount of food or space required by one larva to grow to maturity. The potential utility of the pheromone in population suppression is also discussed.

Some Adaptations of Limnadia stanleyana King (Crustacea: Branchiopoda: Conchostraca) to a Temporary Freshwater Environment

Limnadia stanleyana King (Crustacea: Conchostraca) which inhabits temporary rainpools near Sydney, Australia, was studied to determine how it survives in this severe environment. Drying of the pools in which the species lives frequently kills its freeswimming stages, though eggs are drought-resistant (Bishop 1968). Eggs enter diapause in autumn and do not hatch during the cooler months of the year when the animals have a reduced chance of attaining sexual maturity before pools dry (Bishop 1967). Climate has further, more profound effects as the water level of pools is controlled by the local precipitation/evaporation ratio. Near Sydney the rainfall is irregularly distributed throughout the year and it is impossible to predict when a pool will fill or for how long it will contain water. This paper discusses some adaptations that enable the species to counteract this unpredictability of the environment. Conchostracans in a pool are frequently killed by drying before they can reproduce and the species has to withstand such population crashes if it is to survive in the pool. The following strategies are theoretical possibilities: (a) The species may recolonize the pool when this has dried and then refilled. (b) Some part of the population may be kept as a drought-resistant reserve in case the rest of the animals are killed by desiccation. (c) The species may evolve mechanisms by which it increases its chances of reproducing before the pool dries. Longhurst (1955) attributed the wide distribution of some species of Notostraca to the effectiveness of their eggs as dispersive agents. Proctor (1964) found that eggs of several branchiopods survived passage through the gut of waterfowl and were dispersed by them. Dispersal of eggs could result in recolonization of pools by L. stanleyana after drying had killed previous populations. Gillett (1955a, b) investigated a genetic mechanism that restrained some eggs of mosquitoes (genus Aedes) from hatching to form a reserve in case larvae were destroyed by premature drying of the habitat. (Aedes inhabit temporary pools and the eggs of some species are drought-resistant (Clements 1963).) Some branchiopods hatch only soon after their pool habitat fills with rain-water. This has obvious adaptive value as it maximizes the period over which post-embryonic stages can grow and reproduce thereby enabling the species to make the most effective use of the time the habitat is favourable. Hall (1959, 1961) concluded that development of eggs of Chirocephalus diaphanus Prevost (Anostraca) was inhibited in water deeper than 20 cm. Moore (1963) did not find that this was the case in Streptocephalus seali Ryder and Eubranchipus holmani (Ryder) (Anostraca). He suggested that eggs of these species

Oviposition and Egg Dispersion of the Apple Aphid with Observations on Related Mortality Factors

The apple aphid, Aphis pomi De Geer, begins oviposition in early October. Termination of egg laying is usually brought about by weather extremes. Discussed are the environmental factors which determine egg densities indirectly by reducing the number of oviparous forms prior to oviposition and directly by reduction in the number of eggs after they are laid. The typical dispersion of eggs on infested shoots is described: egg densities found in decreasing numbers from the shoot tips.

THE ENIGMA OF GEORGES BANK SPAWNING

The drift of bottles and transponding drift buoys over the Georges Bank area show that, with the exception of midsummer when the Georges eddy is most pronounced and southerly winds predominate, surface drift is offshore in the direction of the slope water band. Indications are that the currents below the surface move at a slower rate than at the surface, but in a similar direction. The effects of offshore drift, time and location of spawning, vertical distribution of eggs and larvae, and length of pelagic life on the dispersal and survival of eggs, larvae, and juveniles of commercially important foodfishes are discussed. It appears from observations on haddock and herring that under average conditions most fish eggs and larvae are carried away from Georges Bank and lost to the fishery, that only under unusual hydrographic conditions are the eggs and larvae retained in the area, and that the year class strength of the various species inhabiting Georges Bank is dependent in part upon non‐tidal drift.

Homing Instinct in Salmon

K NOWLEDGE of the migrations of fishes is of considerable economic importance and biological interest. Most fisheries depend on the congregation of certain species for feeding or spawning in definite regions; a knowledge of the movements of the fishes to and from these regions has aided the prosecution of the fishing operations and has been of value in attempts at regulation of these operations with a view to conservation. Russell (1937) states that most fish migrations consist (i) of a dispersal of eggs, larvae, or young fishes by passive drifting with the current or by active seeking of the normal habitat, then (z) of an active movement, usually against the current, to the spawning grounds, and finally (3) of a dispersal of the spent fishes, either by movement with a current or an active feeding migration. Salmon migrations are in general accord with this plan. The young fishes go down-stream to the sea after a stay in fresh water which varies in length with the species and with conditions in the stream. There follows a period of feeding and growth in the sea which also varies in length, but rarely, if ever, exceeds five or six years. The mature fishes now enter streams and proceed to spawning grounds; after spawning, the Pacific species of salmon die, while the steelhead trout and the Atlantic salmon may return to the sea, subsequently to enter a stream and spawn again. The suggestion was made many years ago that the salmon invariably returns for spawning to the stream in which it was hatched. Many biologists looked upon this view with skepticism. Jordan (igz5) opposed the idea, and Rutter (I903) said,