The population dynamics and reproductive cycle of Septifer virgatus (Bivalvia: Mytilidae) on an exposed rocky shore in Hong Kong

Exposed rocky shores in Hong Kong are dominated at mid-tidal levels by a broad band of the mytilid Septifer virgatus. A 48-month study of a population inhabiting the shores at Cape d'Aguilar has been undertaken. The species matures at a shell length of ≅ 15 mm, about one year after recruitment. Septifer virgatus is dioecious with a slight, but insignificant, female bias overall. There is, however, a significant juvenile male bias. Generally, most adult individuals remain mature year round, although spawning is limited to two periods in spring (February to March) and autumn (September to December) with subsequent recruitment into the adult population. The species lives for ≅ 4–5 years, although older individuals, possibly up to 12 years of age and with a maximum recorded shell length of 65 mm, occur as solitary individuals lower down the shore. Mortality in winter mainly affects newly recruited juveniles. A heavy mortality of adults in summer is thought to be related to high rock temperatures at midday (≅ 50°C), concurrent with low spring tides.
The life-history tactics and sexual strategy of Septifer virgatus can be correlated with seasonal changes in hydrography. There are a number of similarities with other local mytilids, notably the dioecious condition and the bimodal pattern of spawning and recruitment. Such similarities are discussed in relation to the habitats occupied by these species.     

Life-history characteristics and sexual strategy of Mytilopsis sallei (Bivalvia: Dreissenacea), introduced into Hong Kong

First recorded from Hong Kong in 1980, Mytilopsis sallei now dominates the intertidal pier community within the Government Dockyard, Victoria Harbour. Analysis of monthly samples of M. sallei has demonstrated aspects of the reproductive strategy and life-history characteristics correlated with seasonal changes in local water quality.
Mytilopsis sallei is dioecious, has a high fecundity, matures early (at 8–10 mm shell length), grows fast and the majority of the population is probably semelparous. These are characteristic traits of opportunistic r -strategists. Comparable characteristics are typical of other introduced bivalves. Maximum life span of M. sallei in the Dockyard is probably less than 20 months with, however, most of the population dying before that. Heavy recruitment, particularly during spring-summer, suffocates older individuals. The shells of dead animals similarly last for little more than one year, dissolution and crushing effecting this. A mat of byssal threads serves to bind the mass and anchor it to the substratum.
Mytilopsis sallei exhibits two periods of reproductive activity and settlement per year. These are possibly two elements of a single recruitment divided into minor (autumn-winter) and major (spring-summer) components by either high summer temperatures or low salinities. Summer and winter recruits of M. sallei , respectively, mature to produce the summer and winter juveniles of the following year. Over-stepping generations plus survival of a few individuals into their second year thus ensure continued success of M. sallei , and account for its spread and dominance in its introduced range. Such a bimodal pattern appears typical of many Hong Kong intertidal, estuarine and freshwater bivalves.     

Seasonal dynamics of a flea number (Siphonaptera) on the common shrew (Sorex araneus) in the north part of the Novgorod oblast

In the course of 5-year stationary investigations of the common shrew Sorex araneus in the north part of the Novgorod oblast, 12 flea species have been recorded on this host. Among them, Palaeopsylla soricis and Doratopsilla dasycnema are specific parasites of small insectivores, including the common shrew. Hystrichopsylla talpae is a polyxenous species, parasitizing both rodents and insectivores. Other 9 flea species are not common parasites of S. araneus and apparently have come to this host from other mammal species inhabiting the forest biotopes in the area of investigation. P. soricis and D. dasycnema have similar phenology of parasitism. These fleas appear in spring (April), are present during summer and autumn and disappear in winter. During a year, the abundance of these species shows three pikes, which correspond to three generations: spring (April), summer (June-July) and autumn (September-October) generation. The first species dominates in spring (April-May) and in the autumn-winter period, while the second species predominates in summer. These two species comprise over 90% of total number of fleas collected and determine general dynamics of the flea number on the shrews; during the year the flea number has a sinusoidal increasing from spring to autumn and minimum in winter.     

Development of a marine subtidal epibiotic community in Hong Kong: implications for deployment of artificial reefs

A 2-year study was conducted in Hong Kong to examine the effects of substratum, season and length of submersion on the development of a subtidal epibiotic community using four types of settlement panels (concrete, steel, wood and tyre). The season and length of submersion had a strong influence on the total biomass and on community structure while the type of substratum had very little impact on the total biomass or the structure of the epibiotic community. The season of submersion determined the species composition of the newly submerged surfaces. In the spring and summer, tubeworms were the most abundant. In the autumn and winter, barnacles and tunicates dominated. Community succession was not obvious in the first year of submersion as it was intermingled with strong seasonal settlement, growth and death of barnacles and tunicates. In the second year of submersion, green mussels and tunicates settled and grew to occupy most of the panel surfaces, forming an assemblage that was characteristic of climax communities in the local subtidal waters. The results suggest that the type of construction material has limited impact on the development of epibiotic communities on artificial reefs deployed in Hong Kong; the season of submersion may affect community structure in the early successional stage, but not the characteristics of the climax communities. This study indicates that the type of substratum should not be of concern when deploying artificial reefs in the subtidal waters in this region. The design of artificial reefs should focus more on other physical and economical aspects such as durability, flow dynamics, stability, cost, and effects on the ambient environment.     

Seasonal population dynamics of Paraquimpevia tenevvima (Linstow) (Nematoda) in the European eel Anguilla anguilla (Linnaeus) in three localities of southwest England

Seasonal population dynamics of Puraquzmperia tenerrima (Linstow) in the European eel Anguillu anguillu (Linnaeus) have been investigated in three localities in Devon, southwest England. The nematode is common and exhibits very similar seasonal patterns in changes of prevalence and abundance in the three localities. The population increases over winter through to late spring or early summer as the recruitment of the new generation occurs throughout this period. Reproduction of the nematode starts in spring. A sharp decrease of prevalence and abundance accompanies or follows reproduction, and infection levels are very low in autumn and early winter. The dispersion pattern of the nematode varies with the infection levels from over-dispersion in spring and early summer to almost random dispersion in autumn and winter. It is suggested that P. tenerrimu is a common specific parasite of eels, especially in small streams or rivers and that insect larvae or species of crustaceans might serve as intermediate hosts for the nematode.     

Effect of mating on survival at low temperature in females of the Japanese common grass yellow,Eurema mandarina

1. In some insects that overwinter as adults, mating occurs both before and after overwintering. Two hypotheses have been proposed to explain the adaptive significance of pre‐overwintering copulation of females. One is the bet‐hedging hypothesis, which explains pre‐overwintering copulation as a preparation for less chance of mating in the following spring. The other is the nuptial gift hypothesis, which states that secretions derived from males increase overwintering success of females. 2. In Eurema mandarina, both diapause autumn‐ and non‐diapause summer‐form male adults emerge with autumn‐form female adults in the last generation in a year. Most autumn‐form females mate with summer‐form males before winter, and re‐mate with autumn‐form males in the following spring. Because autumn‐form females have sufficient chances for mating after overwintering, the nuptial gift hypothesis has been regarded as the more probable hypothesis. 3. To test the nuptial gift hypothesis, the survival period was compared under short‐day conditions at 10 °C between mated and unmated females that had been reared on sucrose solution at 25 °C for 15–21 days. The mated females had significantly greater longevity than the unmated females, supporting the nuptial gift hypothesis. Body size also affected the survival period. 4. The results suggest that the nuptial gift is an important factor for the evolution of pre‐overwintering copulation in species in which females mate both before and after overwintering.     

Seasonal changes in plasma testosterone concentrations in response to administration of hCG in a desert rodent, the sand rat (Psammomys obesus)

In adult male sand rats inhabiting the Béni-Abbès area (Algeria), testicular endocrine activity increased in early summer (June-July), was highest in autumn-winter and decreased throughout spring. Testosterone secretion by the testis of the sand rat was stimulated (by 10-60-fold) throughout the year by exogenously administered hCG (25 i.u.). However, the response exhibited annual changes mainly characterized by a marked increase in early summer (June-July); the response to hCG was depressed in autumn and became minimal in winter and in early spring. The results strongly suggest that the summer onset of testicular endocrine activity is, at least in part, due to an increase in the testis sensitivity to LH.     

The population dynamics and sexual strategy of Anodonta woodiana (Bivalvia: Unionacea) in Plover Cove Reservoir, Hong Kong

A 12-month investigation of the population dynamics, growth, ageing, reproduction and sexuality of Anodonta woodiana (Lea, 1834) in Plover Cove Reservoir, Hong Kong, has been undertaken. Recruitment occurs in the summer, May through July being the period of peak incidence of glochidia on captive fish hosts. Juveniles grow rapidly to a shell length of 30 mm and are sexually mature by the following spring. A study of the population structure of A. woodiana indicated that most animals were less than 8 years old. Results of a Walford plot for this species indicated a maximum theoretical shell length (in Plover Cove) of 162·5 mm and a life span of approximately 12 years; the latter is in good agreement with age estimates for European unionids.
Anodonta woodiana is dioecious with females predominating in a ratio of 60:40. A very small number of individuals (0·3%) are hermaphrodite. Males possess mature gonads throughout the year whereas females come into reproductive condition during the spring. There was no evidence of shell form sexual dimorphism, as has been recorded for other unionaceans.
The reproductive strategy and time of gonad maturation of A. woodiana is compared with that of unionids from elsewhere and with other fresh water bivalves from southern China.     

Population structure and dynamics of sympatric Apodemus species (Rodentia: Muridae)

This paper describes the population biology of sympatric populations of Apodemus sylvaticus and A. flavicollis in western England. Annual changes in population siie closely resembled those seen in allopatric populations such that simultaneous peaks in number occurred only in autumn and early winter. Numbers of A. sylvaticus were low and stable in early summer increasing rapidly in late summer and autumn and remaining high throughout winter before a sharp decline in spring. Numbers of A. flavicollis increased after the start of reproduction and continued to rise throughout summer and autumn. The period of major decline of this species was during early winter. Densities of A. sylvaticus and A. flavicollis were comparable with those cited in allopatric studies. Spring populations of both species consisted of cohorts first appearing in the late summer and autumn of the previous year. Few individuals present in spring survived the summer. Reproduction of A. flavicollis commenced four to six weeks before that of A. sylvaticus . Seasonal variation in mean weight of adult males and females also indicates that those changes, due to the acquisition of secondary sex characteristics and reproductive maturity, occur earlier in A. flavicollis than in A. sylvaticus . Data presented contain no evidence of interspecific competition and it is concluded that interspecific differences, chiefly asynchrony in reproduction and annual population cycles, contribute to their stable coexistence.     

SEASONAL LIGHT AND TEMPERATURE INTERACTION EFFECTS ON DEVELOPMENT OF LAMINARIA SACCHARINA (PHAEOPHYTA) GAMETOPHYTES AND JUVENILE SPOROPHYTES1

In previous studies, Laminaria saccharina L. (Lamour.) sporophytes were found to exhibit two major peaks of sporogenesis and an annual life cycle in Long Island Sound, New York. Young sporophytes were observed shortly after the sporogenesis peaks in early autumn and spring, but most of the mature sporophytes decayed during summer. A new study was conducted to determine if the spring sporogenesis activity contributed to the recruitment observed in autumn through oversummering of gametophytic and juvenile sporophytic stages, as previously suggested. Reproduction and growth in gametophytes and growth in juvenile sporophytes were studied under crossed gradients of light and temperature. Periodic outplantings of substrata seeded with gametophytic and sporophytic stages to the field were conducted to assess actual survival. The optimum temperature and light conditions for gametophyte development, growth and reproduction varied with the time of year meiospores were obtained. Most of this variation was attributable to temperature. A seasonal adaptation to temperature in most developmental stages was observed. Higher temperatures resulted in greater numbers of male gametophytes. Gametophytic stages could develop at all times, suggesting that oversummering in this stage was possible. Juvenile sporophytes had a narrower optimum temperature range and again photon fluence rate contributed little to observed variances. Out planting of sporophytic stages at various times during the year indicated only sporophytes prepared from autumn and winter could survive summer conditions. The thalli of these plants grew rapidly in spring and eroded back to the meristematic region in summer. Most of these plants then quickly became reproductive, resulting in another autumn sporogenesis peak. Gametophytic and sporophytic outplantings prepared from spring meiospores did not survive the summer. Thus, the recruitment observed in autumn can only be the result of the previous autumn's sporogenesis activity. The sporogenous activities of spring and early summer appear to be unimportant, despite the fact that all reproductive indices are superior at those times.     

Life history strategy and egg diapause in the intertidal collembolan Anurida maritima

ABSTRACT.

• 1 In a field survey of 1 year the intertidal collembolan Anurida maritima (Guérin) was found to be a univoltine species. Every year in early May a new generation emerges from eggs that overwinter. The first animals become adult in July. Almost all eggs laid in summer do not hatch, but undergo a period of diapause. Diapause is terminated in autumn by temperatures below 5°C. However, due to the low temperatures in winter further egg development is suppressed until spring.
• 2 The adult animals die, mainly in autumn. One of the causes of mortality may be starvation. In late autumn body size decreases and glycogen and lipid content are lower.
• 3 It is argued that in autumn due to low temperatures A.maritima, which forages during low tide on the open shore and seeks refuge underground before the incoming tide, is too sluggish to find food in the limited period of low water. This probably explains why this cosmopolitan species has developed a strategy to survive the winter in the temperate zone in the egg stage.

     

Some aspects of the biology and ecology ofKnipowitschia caucasica (Teleostei: Gobiidae) in the Evros Delta (North Aegean Sea)

Some aspects of the biology and ecology of the gobyKnipowitschia caucasica were studied over a period of 13 months in a poly-to euhaline area in the Evros Delta (North Aegean Sea). This fish grows rapidly in the summer and autumn after hatching, matures after its first winter, breeds from the end of April to the end of July, and grows rapidly again in July–September. The older males perish after their second February, whereas some females have a second breeding season at the end of April/beginning of May, shortly before their death. The fish grows to about 40 mm in total length. There is a positive correlation between the total length (TL) and the standard length (SL) or the cleaned body weight (CW). SL increases slower than TL, whereas CW increases slower than TL in immature individuals and faster in males and females. There is no difference between immature individuals, males and females, in the growth rate of SL, TL and CW, TL. The mean monthly values of the condition factor varies from 0.289 to 0.576 in females and from 0.313 to 0.548 in males. The overall sex ratio of females to males is 1: 1.46. Fecundity ranges from 60 to 217 eggs with a mean value of 109.8 and depends upon size, whereas relative fecundity varies between 968 and 2170 with a mean of 1558. The fish feeds predominantly on benthic amphipods and polychaetes.     

Reproduction and growth of the Arctic hyperiid amphipod Themisto libellula Mandt

Summary The hyperiid amphipod, Themisto libellula, is abundant in Arctic seas and is an important prey for fish, birds and mammals. Characterization of its life cycle has been hampered by the lack of winter collections. In this study, that portion of the hyperiid population inhabiting the upper 30 m of the water column in Frobisher Bay was sampled periodically in winter, spring and summer during five consecutive years. In one year the winter and spring collections included large numbers of ovigerous females. These samples provide new information about timing of reproduction, size at maturity, fecundity, embryonic development, hatching, release and rate of growth of T. libellula in the Arctic. The average length of the ovigerous females was 23.3 mm. This and the summer growth rate of about 6 mm/month indicates that they can reproduce when one year old. Eggs are laid prior to early February, and although some young may be released from the marsupium as early as mid-March, most are liberated during April and May. The fecundity is significantly correlated with female length and an average 23 mm long individual had 275 embryos. The size-specific fecundity is similar to that of hyperiids from temperate and subtropical waters.     

Direct and indirect effects of predation on the genetic structure of a Daphnia population

The effects of fish predation on zooplankton communities arewell documented, but relatively little is known about how predationstructures the genetic composition of individual populations.This study illustrates how a perturbation in the timing andstrength of predation by rainbow trout directly and indirectlyaltered the genetic composition of a Daphnia pulicaria populationin a Minnesota (USA) lake. Trout were stocked in autumn in thefirst 2 years of the study and in spring in the second 2 years.In autumn stocking years, predation was highest over winterand in spring but relatively low during summer stratification.In contrast, in spring stocking years, predation was low overwinter and high during summer stratification. In all years,the Daphnia population became genetically differentiated withrespect to depth, as summer stratification became more pronounced.In addition, allozyme analyses of trout stomach contents revealedselective predation on Daphnia in the metalimnion. In springstocking years, when trout were abundant during the summer,this directly caused a shift in the dominant clone type froma metalimnetic to a hypolimnetic specialist. The fisheries manipulationindirectly affected the genetic composition of the Daphnia populationby altering the importance of recruitment from diapausing embryosin the sediment (the egg bank). In autumn stocking years, whenthe over-wintering population was small, genotype frequenciesin early summer indicated the recent emergence of sexually derivedindividuals from the egg bank. Conversely, in spring stockingyears when over-wintering populations of Daphnia were large,no emergence events were detected.     

Life cycle, production, microdistribution and diet of the damselfly Euphaea decorata (Odonata: Euphaeidae) in a Hong Kong forest stream

Euphaea decorata in Tai Po Kau Forest Stream (Hong Kong) was univoltine. Most recruitment took place in summer, and larval growth proceeded throughout the year. Life-cycles recorded in 1977–78 and 1978–79 were similar. Annual production estimates, using the removal-summation, instantaneous growth and size-frequency methods, were more similar for the 1978–79 generation (ranging from 158.7–174.7; mean 1671 mg dry wt m^-2) than for the 1977–78 generation (93.9-173-6; mean 131.7 mg dry wt m^-2). Mean biomass was similar for both generations (ranging from 33.5–33.9 mg dry wt m^-2), and mean P/B ratios were 3.9 1977–78) and 5.0 (1978–79). These are the first estimates of annual production by an Oriental stream insect.
Larvae were most abundant at microsites in the middle of the stream. Multiple regression analysis indicated that substratum characteristics were a major determinant of microdistribution. Euphaea decorata apparently favoured poorly-sorted sediments with highly peaked grain size-frequency distributions, containing few fine particles.
The carnivorous larvae showed ontogenetic changes in diet. Small individuals consumed mainly chironomid (Diptera) larvae; the diet expanded to include (successively) larvae of Ephemeroptera and Trichoptera as E. decorata grew. Seasonal changes in diet were also apparent, although larval diets during spring and summer were similar. There was also considerable overlap between autumn and winter diets. Ontogenetic influences upon prey consumed were not sufficient to account for the observed seasonal differences.     

The adaptive role of facultative embryonic diapause in the grasshopper Caledia captiva (Orthoptera: Acrididae) in southeastern Australia

In a temperate zone population of the grasshopper Caledia captiva (Fabnerus) (Orthoptera Acrididae), facultative embryonic diapause provided the basis for a number of aspects of life cycle flexibility Most offspring produced during the summer entered diapause, destined to overwinter as embryos and hatch in the spring, but a small percentage developed directly to hatching, overwintered as nymphs and reproduced in the spring The two different developmental pathways often occurred among the offspring of the same mother, and could represent a bet-hedging adaptation Offspring of the small overwintering generation were influenced by a maternal effect which greatly diminished their likelihood of entering diapause This is an adaptation to prevent an inappropriate developmental delay that would negate the fitness advantage of an extra generation Offspring produced in the autumn failed to develop to the stage at which diapause intervenes before winter and overwintered at a pre-diapause stage They did not enter diapause when development resumed in the spring Facultative diapause in this case also prevented an inappropriate developmental delay in the spring, and provided females with the option of reproducing toward the end of the growing season Life cycle flexibility appears to have evolved to exploit a climate with a short growing season but mild winter     

The nature of migration in the red admiral butterfly Vanessa atalanta: evidence from the population ecology in its southern range

1. The migrant Vanessa atalanta (L.) occurs throughout Europe and North Africa. In autumn, populations emigrate from northern and central Europe to the Mediterranean region to overwinter. In the spring, the northern range is recolonised by migrants from the south. The dynamics of the species in the winter range is poorly known. 2. From 1994 to 1999, adults and immatures of V. atalanta were monitored all year round in Mediterranean habitats in north‐east Spain. 3. Data showed that the Catalonia lowlands is an area to which V. atalanta migrates to breed during the winter. Migrants arrive in October and early November and initiate a period of intensive breeding. Larval development occurs throughout the winter until a first annual generation of adults appears in early spring. 4. Most of the butterflies emerging in the spring emigrate and leave the area without breeding. The data suggest strongly that recolonisation of the northern range is by these butterflies not by wintering adults. Altitudinal migration also seems to be a common phenomenon, allowing a further summer generation of adults to occur at high elevations within the Mediterranean region. 5. The complex phenology of V. atalanta in its southern range has evolved as a strategy to track larval resources through space and time. Autumn migration coincides with the greatest availability of the main food plant, Urtica dioica L. Late spring migration occurs by the time food quality is decreasing.     

Why western spruce budworms travel so far for the winter

1. Conifer‐feeding budworms (Choristoneura) hibernate in sheltered locations on their host trees from late summer of 1 year to spring of the next. During this period, they do not feed but rely on sustenance provided in the egg. Overwinter survival is dependent on the rate of consumption of these limited reserves. 2. A process model was developed that quantifies the relationship between the rate of consumption and survival at variable temperatures and exposure times for western spruce budworm. The model supported physiological evidence that warm weather conditions early in the diapause period have a dominant influence on overwinter survival. Output compared favourably with field observations of poorer budworm survival at lower elevations where late‐summer and autumn temperatures were warmer compared to those overwintering at cooler, higher elevations. 3. Field experiments demonstrated these weather‐dependent rates of survival were modulated significantly by the degree of shelter experienced by hibernating budworms. 4. Dissection of whole trees harbouring overwintering western spruce budworms showed a significant portion of the population had travelled a considerable distance from the periphery of the tree canopy where eggs were laid to overwinter successfully on the tree bole where sheltered niches are common. 5. Thus, budworms will travel relatively long distances and risk increased mortality during this dispersal to find adequate shelter to overwinter.     

Seasonality of two gill monogeneans from two freshwater fish from an oligotrophic lake in northeast Finland

The seasonal occurrence of Dactylogyrus amphibothrium and Discocotyle sagittata from the gills of ruffe (Gymnocephalus cernuus) and whitefish (Coregonus acronius), respectively was studied in Lake Yli-Kitka, a large oligotrophic lake in Northeast Finland. The lake, located near the Arctic Circle, is ice-covered for 7-8 months of the year. The prevalence of D. amphibothrium infection was 70.7% and remained high throughout all size-classes of fish. The length distribution and developmental stages of the worms indicated two generations per year. The overwintering generation produces a summer generation which lives for only a few weeks and matures in July. Discocotyle sagittata has only one generation yearly. Its worms mature and produce eggs in July-August and recruitment of the new generation starts in the autumn. The prevalence of D. sagittata infection varied between 40% (in January) and 6.7% (in September); smaller fish were more heavily infected than larger fish.     

Development of a Marine Subtidal Epibiotic Community in Hong Kong: Implications for Deployment of Artificial Reefs

A 2-year study was conducted in Hong Kong to examine the effects of substratum, season and length of submersion on the development of a subtidal epibiotic community using four types of settlement panels (concrete, steel, wood and tyre). The season and length of submersion had a strong influence on the total biomass and on community structure while the type of substratum had very little impact on the total biomass or the structure of the epibiotic community. The season of submersion determined the species composition of the newly submerged surfaces. In the spring and summer, tubeworms were the most abundant. In the autumn and winter, barnacles and tunicates dominated. Community succession was not obvious in the first year of submersion as it was intermingled with strong seasonal settlement, growth and death of barnacles and tunicates. In the second year of submersion, green mussels and tunicates settled and grew to occupy most of the panel surfaces, forming an assemblage that was characteristic of climax communities in the local subtidal waters. The results suggest that the type of construction material has limited impact on the development of epibiotic communities on artificial reefs deployed in Hong Kong; the season of submersion may affect community structure in the early successional stage, but not the characteristics of the climax communities. This study indicates that the type of substratum should not be of concern when deploying artificial reefs in the subtidal waters in this region. The design of artificial reefs should focus more on other physical and economical aspects such as durability, flow dynamics, stability, cost, and effects on the ambient environment.