Growth and reproductive responses of Laminaria longicruris (Laminariales,Phaeophyta) to nutrient enrichment

A series of comparative culture experiments were conducted in order to determine responses of Laminaria longicruris male and female gametophytes and juvenile sporophytes to several temperatures (5, 10, 15, 20 °C), light levels (10, 35, 75 µmol m^–2 s^–1) and media nitrogen concentrations (0, 20, 100 µM ammonium-nitrogen). Responses were measured as numbers of male and female gametophytes producing gametangia and number of sporophytes produced following fertilization. Both male and female gametogenesis was reduced at 5 and 20 °C versus 10 and 15 °C. At 20 °C gametogenesis inhibition was greater with higher levels of ammonium-nitrogen concentration (100 µM). Sporophyte production was more sensitive to light, temperature and nitrogen concentration than gametogenesis. Production of sporophytes was inhibited completely at 20 °C. At lower temperatures, increasingly higher nutrient concentrations produced greater inhibition of production of sporophytes.     

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.     

Growth and survival rates of large-type sporophytes of Ecklonia cava transplanted to a growth environment with small-type sporophytes

Stipe lengths of sporophytes of Ecklonia cava Kjellman have been reported to be longer along the southeast than southwest coast of the Izu Peninsula, central Japan. Two bays in this region that have natural populations of E. cava, but with different stipe lengths, were chosen for transplant experiments to examine if stipe length was an environmentally controlled trait. Transplant experiments were carried out in order to determine whether large-type sporophytes of E. cava with long stipes growing in Nabeta Bay (southeast Izu Peninsula, Japan) would turn into small-type sporophytes with short stipes when transplanted to Nakagi Bay (southwest Izu Peninsula). Ten juvenile sporophytes of E. cava (stipe length < 5 cm) were collected from Nabeta Bay (large-type habitat) and transplanted to Nakagi Bay (short-type habitat) in December 1995. As a transplant control, ten juvenile sporophytes of E. cava growing in Nakagi Bay were also transplanted to the same artificial reefs. Growth and survival rates of the sporophytes were monitored monthly for 3 y until December 1998. The transplanted sporophytes showed an increase in their stipe length and diameter from winter to spring, whereas almost no increase was observed from summer to autumn. However, the elongation was greater in Nabeta sporophytes than in Nakagi sporophytes. The primary blade length increased mainly from winter to early spring and decreased largely in autumn. Average primary blade lengths were similar in both Nabeta and Nakagi sporophytes from the end of the first year of transplanting. Although ca. 70% of both Nabeta and Nakagi sporophytes survived during the first 2 y after transplantation, no Nakagi sporophytes and only two Nabeta sporophytes survived to the end of the 3 y study period. Despite transplantation to Nakagi Bay, where short sitpes are naturally present, the sporophytes from Nabeta Bay persisted in having longer stipes, which suggests that stipe length is genetically, rather than environmentally, controlled.     

Growth of Ecklonia cava (Laminariales, Phaeophyta) sporophytes transplanted to a locality with different temperature conditions

Transplanting experiments were carried out to determine whether the small type sporophytes with short stipe of Ecklonia cava Kjellman (Laminariales, Phaeophyta) growing in a locality with warm temperatures, change into larger type with a long stipe when transplanted to a locality with cooler temperatures. Juvenile E. cava sporophytes, having a stipe shorter than 5 cm long were collected from Tei in Tosa Bay (southern Japan) (seawater temperature 15–29°C) and transplanted to Nabeta Bay (central Japan) (seawater temperature 13–25°C), where larger type E. cava sporophytes characterized by long stipe (ca 1 m) grow. They were attached to artificial reefs at the sea bottom (9 m depth) in Nabeta Bay to monitor their growth. For comparison, juvenile E. cava sporophytes of almost similar size growing in Nabeta Bay were also transplanted in the same way to the same experimental site. Observations of growth of sporophytes from Tei and Nabeta were carried out monthly for 2 years from November 1995 to October 1997. The transplanted Tei and Nabeta sporophytes showed an increase in stipe length and diameter from winter to spring, whereas almost no increase was observed during summer and autumn. At the end of the study period, the stipe of Nabeta sporophytes reached 25.6 cm in length and 17.0 mm in diameter, whereas that of Tei sporophytes reached 11.1 cm in length and 11.2 mm in diameter. The primary blade length was 16.0 cm in Nabeta sporophytes, whereas it was 5.5 cm in Tei sporophytes. Thus, Tei sporophytes still remained smaller than Nabeta sporophytes even under the same environmental conditions.     

PHOTOINHIBITION AND RECOVERY AFTER HIGH LIGHT STRESS IN DIFFERENT DEVELOPMENTAL AND LIFE-HISTORY STAGES OF LAMINARIA SACCHARINA (PHAEOPHYTA)1,2

The capacity to cope with high light stress was investigated in different life-history and developmental stages of Laminaria saccharina Lamour. sporophytes and gametophytes. Changes in photosynthetic efficiency and in the level of photoinhibition were measured by in vivo fluorescence changes of photosystem II. Pigment content was studied using high performance liquid chromatography. Additionally, the morphology of the various developmental stages during the life cycle was studied by light microscopy in relation to the photosynthetic parameters. High light stress (2 h, 500 μmol.m-².s^?1) induced photoinhibition of photosynthesis with fast kinetics in older sporophytes and gametophytes. In contrast, the absolute degree of photoinhibition after light stress was higher in younger than in older sporophytes. Photosynthesis recovered faster in older sporophytes and gametophytes compared to young sporophytes. In very young sporophytes, photosynthesis did not recover fully even after 12 h exposure to low light, indicating severe photodamage. Kinetics of recovery in old sporophytes and in gametophytes showed a fast and a slow phase, whereas younger sporophytes recovered only with a slow phase, The fast phase is indicative of a decline of the photoprotective process, whereas the slow phase indicates a recovery from photodamage. The capacity to cope with high light stress in Laminaria sporophytes increased with increasing age of the thalli. The gametophytes are less sensitive to high light stress and may be selected to endure unfavorable white light conditions. Investigation of the xanthophylls showed that the higher resistance to high light is not caused solely by a higher content of xanthophyll cycle pigments. Additionally, changes in the thallus structure during the development of the sporophytes seemed to cause a higher resistance to high light. The observed changes in the ability to cope with high light in the different life-history and developmental stages of Laminaria saccharina may influence the distribution of the species on the shore.     

REPRODUCTIVE PHENOLOGY OF LAMINARIA SACCHARINA (L.) LAMOUR. (PHAEOPHYTA) AT THE SOUTHERN LIMIT OF ITS DISTRIBUTION IN THE NOTHWESTERN ATLANTIC OCEAN1,2

Laminaria saccharina (L) Lamour. Sporophytes were monitored monthly from October 1982 to September 1983 to investigate reproduction phenology and relationshiops to growth paatterns aaat its southern limit of distribution in the northwest Atlantic Ocean (Long Island Soundd). Plants exhibited an annual growth pattern. Growth raate, bladelehgth, maximum width, area, stiipe lehgth an wet weight swhoed the same seasonal pattern and reached maximum values between May and June. Only blade thickness continued to increase tthrooughout the ovservation period. Blade dissintegration occurred dduring August and September. Reproductive sporophytes occurred throughout the observation period; the greatest frequency of appearance occurred in October (43.8%) and June (37.8%). The blade area covered by sori ranged from 2.4% (Janaury) to 6.1% (August). Meiospore release under laboratory conditions was maximum in May and minimum in July. No meio-spores were released in August Sporulation was not correlated with meristematic growth of nitrogen content How ever, reproductive plants were generally larger and thicker throughout the sporulation period, and had a greater carbort content is spring than nonreproductive plants. Fecundity and reproductive success of female gametophytes were maximum in spring and minimum in winter. The growth of early sporophyte stages in the laboratory was greatest in early spring; however, juvenile macroscopic stages were hardly observed in the field during summer months due to warm water temperatures. “Over-summering” of gamelophytes and / or microscopic sporophytes may account for the annual cycle of Laminaria at its southern limit of distribution.     

Morphology and life history of Scytosiphon canaliculatus comb. nov. (Scytosiphonales,Phaeophyceae) from Japan*

Morphology of field material and life history in culture were studied in Scytosiphon canaliculatus (Setchell et Gardner) comb. nov. from northern Japan. Erect gametophytes of S. canaliculatus are cylindrical, tubular, up to 7 mm wide and 40 cm long, and without regular constrictions. S. canaliculatus has pronounced anisogamy and ascocysts accompanied with plurilocular gametangia. The life history of S. canaliculatus showed an alternation between erect gametophytes and crustose prostrate sporophytes bearing unilocular sporangia. Since field sporophytes of S. canaliculatus were found to be identical with Hapterophycus canaliculatus Setchell et Gardner (Ralfsiaceae, Phaeophyceae), it is proposed to transfer H. canaliculatus to the genus Scytosiphon. In the field, gametophytes with plurilocular gametangia appeared in spring and disappeared in summer. Sporophytes with unilocular sporangia were collected in late autumn and winter. Unilocular sporangia were produced at 15°C in short-day culture conditions and unispores developed into erect gametophytes at 5–15°C. It is suggested that the seasonal Occurrence of gametophytes in the field is due to the seasonal formation of unilocular sporangia, which is regulated by temperature and photoperiod.     

Ecophysiological plasticity of annual populations of giant kelp (Macrocystis pyrifera) in a seasonally variable coastal environment in the Northern Patagonian Inner Seas of Southern Chile

Annual populations of Macrocystis pyrifera in Southern Chile have been the main focus of studies intending to understand how these populations can couple consecutive sporophytic generations. Research has included studying the population dynamics and gametophytic responses to environmental conditions and the role of recruitment, grazing, and the use of benthic filter feeders as secondary substrate. Adult sporophytes undergo senescence due to changes in abiotic factors during summer and autumn producing 100 % mortality. This study provides evidence about the environmental factors driving the decline in sporophyte populations occurring in summer and fall by monitoring two independent kelp populations and also by running experiments using 400 L tubular photobioreactors with semicontrolled environmental factors for testing the capacity for new recruits to recover population levels under winter conditions. The study of natural populations of giant kelp indicates that high temperatures (>15–17 °C) explain the high mortality of adult plants in summer. On the other hand, the sporophytes established in late winter/early spring are able, under high nitrogen availability, to increase their chlorophyll content significantly, allowing the individuals to reduce their light saturation point and thus allow a higher productivity under the low light conditions that exist in late winter and early spring. These results, in addition to the recruitment facilitation produced by filter feeders, help to explain how giant kelp can deal with, and couple sporophytic generations, in variable environments. These results also emphasize the highly plastic physiology of giant kelp that enables this species to colonize diverse habitats across its large distributional range.     

THE LIGHT FACTOR IN THE GERMINATION ECOLOGY OF DRABA VERNA

The majority of the seeds of the winter annual Draba verna L. require light for germination, but light can be given before they are fully after-ripened and as long as 4 months before temperature and moisture conditions are favorable for germination. Seeds that are exposed to light in late spring, or in late spring and early summer, and then removed to darkness can germinate under favorable temperature and moisture conditions in autumn, even after passing through a long unfavorable (for germination) wetting and drying period at high summer temperatures in darkness. The light requirement for germination probably is not an important factor restricting D. verna to open, well-lighted habitats.     

THE INFLUENCE OF CULTURAL CONDITIONS ON THE INDUCTION OF APOGAMY IN PTERIDIUM GAMETOPHYTES

Apogamous sporophytes formed on Pteridium gametophytes in response to concentrations of certain sugars which supported gametophytic growth. High osmotic concentration of the medium inhibited apogamy, while variations in the basic medium were not stimulatory. Agar, autoclaving, the ammonium ion, and dry media were not required for apogamy. Renewing the medium during an experiment enhanced the apogamous response. Changing the medium at set intervals facilitated the separation of apogamous plant development into gametophytic, initiative, and developmental phases, thus enabling testing of various factors at each of these stages. Apogamy was light-initiated, while the actual development of apogamous sporophytes was caused by light, succinic acid or sugar.     

SPECIES COMPOSITION,BIOMASS, AND NUTRIENT CONTENT OF PERIPHYTON IN THE FLORIDA EVERGLADES1

Periphyton biomass, nutrient dynamics in the biomass, and species composition were studied in two Florida Everglades sloughs from August 1991 to August 1992. Periphyton biomass on macrophytes was strongly season-dependent. Maximum biomasses, 1180, 161, and 59 g dry mass.m^?2 on Eleocharis vivipara, E. cellulosa, and Nymphaea odorata, respectively, occurred in summer and early autumn; winter and spring periphyton biomass was very low (practically not measurable). Periphyton was dominated by blue-green algae (cyanobacteria) during the summer and autumn; diatoms dominated during the winter and spring. Green algae occurred mostly during the summer and autumn, but their growth was sparse and did not contribute significantly to periphyton biomass. Nitrogen-to-phosphorus ratios in the periphyton were very high (59–121:1), suggesting phosphorus limitation of periphyton growth. The periphyton contained large concentrations of calcium (up to 22.3% on dry mass basis) especially in late summer and autumn.     

THE GERMINATION STRATEGY OF OLDFIELD ASTER (ASTER PILOSUS)

At maturity in November, a high percentage of Aster pilosus Willd. seeds germinated in light at high temperatures (30/15, 35/20 and 40/25 C). Stratification during winter lowered the temperature requirement for germination, and high percentages of germination were obtained in light at 15/6 and 20/10 C., as well as at 30/15, 35/20 and 40/25 C. Stratification in darkness was completely ineffective, but stratification in light was partially effective in overcoming the light requirement for germination. Inability of seeds to germinate at low temperatures prevents germination after dispersal in late autumn and winter, when freezing temperatures could kill the seedlings. The lowering of the temperature requirement for germination during winter stratification allows the seeds to germinate and the resulting vegetative rosettes to become well established before the onset of the periodic summer droughts that occur in habitats occupied by A. pilosus.     

Population dynamics of the cattle tick Rhipicephalus (Boophilus) microplus in a subtropical subhumid region of Argentina for use in the design of control strategies

The population dynamics of Rhipicephalus microplus (Ixodida: Ixodidae) in northwest Argentina was analysed to support the design of strategic methods for its control. Both parasitic and non‐parasitic phases were studied. The seasonal activity of R. microplus in its parasitic phase was characterized by three peaks in abundance: the first in mid–late spring; the second in summer, and the third in autumn. The non‐parasitic phase of R. microplus was characterized by a long total non‐parasitic period observed after exposures of females from mid‐summer to early autumn, a short total non‐parasitic period observed after exposures of females from late winter to late spring, a short period of larval longevity in early and mid‐summer, and no hatch of the eggs produced by females exposed in mid‐ and late autumn and winter. Treatments of cattle administered during the period from late winter to late spring will act on small cohorts of R. microplus, preventing the emergence of larger generations in summer and autumn. A 17‐week spelling period starting in late spring and early summer will be necessary to achieve optimal control of R. microplus free‐living larvae. If spelling begins in mid‐ or late summer or in autumn, the required period will be 26–27 weeks.     

LIFE CYCLE OF A STEPHANODISCUS SP. (BACILLARIOPHYTA)1

Studies of the life cycle of a centric diatom, tentatively identified as Stephanodiscus neoastraea Håkansson & Hickel, showed that sexual reproduction occurred every year in a freshwater lake (Lough Neagh, Northern Ireland). Male and female gametes were produced in cells below 55% of the maximum diameter during a 3–4-week period in late summer, following the return of nitrate concentrations above 10 μM NO_3-N. The frequency of sexual reproduction was linked to the cycle of diameter size reduction and regeneration. The times of largest decreases in cell diameter were during nutrient stress in summer and low light conditions in late autumn, rather than during the main spring growth period. So, environmental conditions (combined with the limited life-spans of individual cells) affected the rate of diameter reduction and, therefore, the length of the life cycle (3–4 years).     

On the sexual reproduction ofDictyosiphon foeniculaceus (Phaeophyceae,Dictyosiphonales)

Dictyosiphon foeniculaceus from Sweden and Newfoundland was studied in laboratory culture. Zoids from unilocular sporangia developed into dioecious microscopic filamentous gametophytes which produced uniseriate plurilocular gametangia in low temperatures (0 to 8 °C). Zygotes and unfused isogametes gave rise to filamentous protonemata on which parenchymatous macroscopic sporophytes were formed. Isolates from Sweden and Newfoundland were interfertile. Although formed in culture, genetically unisexual sporophytes were not detected in nature. Female gametes ofD. foeniculaceus produced a sexual pheromone. It was identified as finavarrene, which is also known as the sperm attractant inAscophyllum nodosum.     

SEASONAL CHANGE IN THE DISTRIBUTION OF CELL SIZE OF COCCONEIS SCUTELLUM VAR. ORNATA (BACILLARIOPHYCEAE) IN RELATION TO GROWTH AND SEXUAL REPRODUCTION1

Growth and sexual reproduction of the marine littoral diatom Cocconeis scutellum Ehrenb. var. ornata Grun. were investigated at 30 different combinations of temperature (5, 10, 14, 18, 22° C), irradiance (20, 60, 100 μE·m^?2·s^?1) and daylength (14:10 and 10:14 h LD cycle). Growth occurred at all combinations. The optimal growth was observed at 14–18° C, long daylength and highest-to-moderate irradiance, and at 18° C, short daylength and highest irradiance. Sexual reproduction on the other hand occurred between 5 and 18° C, and the optimal condition was 10–14° C and short daylength. Annual cyclic, and sesonal changes in the distribution of cell size (valve length) were observed in a field population. These changes were characterized by an annual minimum in mean cell size in autumn, an annual maximum in winter, a slight decrease from the mean in spring–middle summer, a rapid decrease from the mean in late summer–early autumn, and appearance of bimodal distribution of cell size in winter. These changes were caused by sexual reproduction in autumn, rapid growth in late summer–early autumn and slow growth in other seasons, and poor viability of small cells near the lower end of the size range.     

ROLE OF NUTRIENT FLUCTUATIONS AND DELAYED DEVELOPMENT IN GAMETOPHYTE REPRODUCTION BY MACROCYSTIS PYRIFERA (PHAEOPHYCEAE) IN SOUTHERN CALIFORNIA1

Organisms occurring in environments subject to severe disturbance and/or periods of poor environmental quality that result in severe adult mortality can survive these periods by relying on alternate life stages that delay their development in a resistant state until conditions improve. In the northeast Pacific, the forest‐forming giant kelp Macrocystis pyrifera (L.) C. Agardh periodically experiences widespread adult mortality during extended periods of extremely low nutrients and high temperatures, such as those associated with El Niño. Recovery following these periods is hypothesized to occur from microscopic life stages that delay their development until the return of favorable conditions. In the laboratory, we experimentally examined the environmental conditions responsible for regulating delayed development of the microscopic stages of M. pyrifera from Southern California, USA. Nutrients controlled the delay and resumption of gametophyte growth and reproduction, perhaps linked to the large fluctuations in nutrients occurring seasonally and interannually in this region. Although growth of gametophytes proceeded in the virtual absence of nitrate, both nitrate and other trace nutrients were necessary for gametogenesis. Upon exposure to elevated nutrients, delayed gametophytes produced sporophytes more quickly (5–20 d) and at smaller sizes (10–200 μm) than gametophytes that had never been delayed (18–80 d, 80–400 μm, respectively), reducing negative density‐dependent effects. This finding demonstrates that delayed gametophytes of M. pyrifera rapidly utilize increased resources to consistently produce sporophytes. Further work is needed to assess their potential role in population recovery following periods of poor environmental quality.     

LIFE HISTORY AND SYSTEMATIC POSITION OF AKKESIPHYCUS LUBRICUS (PHAEOPHYCEAE)1

The life history in culture of Akkesiphycus lubricus Yamada et Tanaka, an alga which has been placed in the Coilodesmaceae or the Punclariaceae, Dictysiphonales, was studied. In culture the species alternates between a microscopic filamentous gametophyte and a macroscopic polystichous sporophyte, a pattern common to the Dictyasiphonales and Laminariales. However, it has a unique anisogamous dioecious gametophyte. Fusions between mac-ro-gametes and micro-gametes were not observed, Macro-gametes or zygotes germinated, mostly developing into sporophytes that formed unilocular sporangia and the rest developed into reduced gametophytic flaments again. The gametophyte matures in 50C short-day conditions, corresponding to winter in Hokkaido. The sporophyte develops normally and matures only in low-temperature conditions irrespective of daylength. In regard to iits systematic position, Akkesiphycus lubricus is considered to have a closer relationship with the Laminariales than with the Dietyosiphonales in the following characters; lack of pyrenoids; early stages of parenchyma formation in the sporophyte; direct development of sporophytes from gametes or zygotes without forming a besal system zoospore becomes almost empty after germination by the migration of cell contents into a germ lube; formation of macro-gametangia by direct conversions of mother cells of mother cells of fertile branches; and micro-gametangia formed in clusters showing closeresemblance to the antheridia of Pseudochorda nagii (Tokida) Inagaki.     

HAPLOID PARTHENOGENETIC SPOROPHYTES OF LAMINARIA JAPONICA (PHAEOPHYCEAE)1

Parthenogenetic sporophytes were obtained from three strains of Laminaria japonica Areschoug. These sporophytes grew to maturity in the sea, producine spores that all grew into female gametophytes. These female gametophytes gave rise to another generation of parthenogenetic sporophytes during the next year, so that by the year 1990 parthenogenetic sporophytes had been cultivated for 12, 9, and 7 generations, respectively, for the three strains. When female gametophytes from parthenogenetic sporophytes were combined with normal male gametophytes, normal sporophytes that reproduced and gave rise to both female and male gametophytes were obtained. The parthenogenetic sporophytes were shorter and narrower than the normal sporophytes of the same strain. Chromosome counts on mature sporophytes showed that normal sporophytes (from fertilized eggs) were diploid (2n = approximately 40) and that the spores they produced were haploid (n = approximately 20), while nuclei from both somatic and sporangial cells in parthenogenetic sporophytes were haploid. All gametophytes were haploid. Young sporophytes derived from cultures with both female and male gametophytes were diploid, while young, sporophytes obtained from female gametophytes from parthenogenetic sporophytes had haploid, diploid, or polyploidy chromosome numbers. Polyploidy was associated with abnormal cell shapes. The presence of haploid parthenogenetic sporophytes should be use in breeding kelp strains with useful characteristics, since the sporophyte phenotype is expressed from a haploid genotype which can be more readily selected.     

SEASONAL CHANGES IN BODY-WEIGHT OF OYSTERCATCHERS HAEMATOPUS OSTRALEGUS

The body-weights of Oystercatchers Haematopus ostralegus wintering in Morecambe Bay, north-west England, showed marked seasonal changes between late summer and late winter, with considerable differences apparent between adult and immature birds. An attempt is made to relate these changes to recorded seasonal variations in prey biomass and to the annual cycles of breeding, moult and migration of the Oystercatcher. The mean weight of females invariably exceeded the mean weight of males in samples collected on the same dates, regardless of age. Adults returned from northern breeding areas in very lean condition, with mean weights ranging from 526 g in males to 540 g in females. Mean weight then increased progressively, due mainly to fat deposition, to a peak in March (up to 662 g in males and 675 g in females) around the time of their main departures for breeding. Heaviest birds then exceeded 800 g. Birds migrating to Iceland in spring would need to be of above average weight in March to make the shortest crossing (850 km, 13 h), via Scotland, while Oystercatchers of 700 g and over could probably make a direct flight (1500 km, 25 h) from Morecambe Bay in favourable weather. Breeding weights of British Oystercatchers were similar to those of post-breeders returning to Morecambe Bay in late August. The mean weights of first-year Oystercatchers arriving in August were very low, 449 g in males and 478 g in females. Their weights, and those of second- and third-year immatures, then rose rapidly in autumn, with some fat deposition, and reached mean values ranging between 551 g (males) and 597 g (females) by November-December. Mean weight then fell by 10–17% from December to March returning close to or below the September levels, whereas adults gained a further 6% during these winter months. Summer and autumn weight gains, and the major moult of adults and older immatures, occurred when the biomass of their two staple mollusc preys, Mytilus edulis (mussel) and Cardium edule (cockle), was maximal. Winter loss in mean weight of immatures corresponded with declining prey biomass, suggesting either that they were less efficient than adults in coping with deteriorating winter food supplies, or that they had no need to accumulate further (premigratory) fat reserves. The autumnal increases in mean weight of immatures are interpreted as an adaptation for withstanding adverse feeding conditions in winter. The Oystercatcher appears to be the only wader species in Britain in which adults increase, rather than lose, weight during the winter. This may be a consequence of an early breeding season, but it may be regarded also as a measure of the success Oystercatchers have achieved by specializing on a difficult but plentiful prey source.