Peak Recruitment Research Articles

Preliminary recruitment studies of the eastern oyster, Crassostrea virginica, and their potential applications, in coastal Georgia

Oyster recruitment was monitored in Wassaw Sound, Georgia from April through October, 1991. The study was initiated to determine precise recruitment patterns of oysters over the 7 month spawning season. One of the goals was to determine the most suitable time for the collection of natural spat for maricultural, fisheries and recreational purposes. Three sites of varying hydrographic characteristics were chosen in Wassaw Sound. The sites varied in terms of temperature and salinity regimes and in their exposure to wind and wave action. Within each site, oyster recruitment was estimated at three tidal heights; subtidally, at mean low water and intertidally, approximately 2 h above the mean low water mark. Sampling took place so as to measure net re cruitment over biweekly (BW), monthly (M) and seasonal (S) periods. The results indicated that the recruitment of oysters in this region of coastal Georgia is protracted, lasting 6 months (May through October). The levels of recruitment were very high relative to other regions on the east coast of the USA, with peak recruitment for the entire study area ( x − = 2800 spat m −2 for BW and x − = 3020 spat m −2 for M) occurring between July and September. The intensity of recruitment varied significantly among the three sites. The most sheltered site in terms of wave exposure experienced the highest recruitment ( x − = 4380 spat m −2 BW; x − = 6260 spat m −2 M). This site also had higher overall water temperatures as well as greater daily temperature fluctuations. The least sheltered site had the lowest numbers of young oysters ( x − = 1000 spat m −2 BW; x − = 686 spat m −2 M), as well as having the most stable temperature regime. The biweekly samples generally experienced higher recruitment subtidally at the three sites. The monthly samples had higher numbers of oyster spat at the mean low water mark while the seasonal samples showed significantly higher recruitment intertidally (up to x − = 7353 spat m −2). The shift in recruitment patterns over time is partially attributed to increased subtidal predation pressure on the recruits. The collection of natural spat in relation to the natural fishery, potential maricultural activity and the recreational fishery is discussed.

Regeneration Pulses and Climate-Vegetation Interactions in Nonpyrogenic Boreal Scots Pine Stands

1 Age and diameter distributions of nonpyrogenic, virgin Scots pine (Pinus sylvestris) populations were studied at six different sites of the Empetrum-Vaccinium type along a latitudinal gradient (65?54'-68?15'N) in northern Sweden. 1910 trees, including saplings and seedlings, were aged at their root collars in order to reconstruct regeneration patterns. 2 All populations had multimodal age structures. Obvious regeneration pulses occurred in the 1770s-80s and 1950s-70s. The three southernmost stands exhibited an additional regeneration pulse in the 1870s-80s. A regeneration cycle of about 100 years in the south and 200 years in the north were thus clearly expressed. Correlation with temperature changes over the past 300 years showed that recruitment peaks of Scots pine lagged 20-30 years behind the warm climate periods and fell to zero in the intervening cold periods. 3 A much smaller number of experimentally planted pine and spruce seeds established and survived in vegetation dominated by Empetrum hermaphroditum than in that dominated by Cladina spp. After two growing seasons, fresh weights of pine seedlings were also much lower in E. hermaphroditum vegetation than in Cladina spp. Spruce established less frequently than pine in both vegetation types. Naturally established pine seedlings were found almost exclusively in vegetation dominated by Cladina spp. 4 The wave-like regeneration pattern of pine may therefore result from successful establishment in periods with good seed production on ground dominated by Vaccinium spp. and Cladina spp. This vegetation type is favoured by warm dry conditions but its establishment possibly lagged behind climate change. The extreme amplitude between regeneration failures and successes found over the period studied may be caused by the specific complex of vegetation responses to climate variability found in this forest type. 5 The results have implications for predicting the effect of climatic warming.

Life histories ofNeomysis integer, and its copepod prey,Eurytemora affinis, in a eutrophic and brackish shallow lake

Data was collected on the population structure and fecundity of the mysidNeomysis integer and the calanoid copepodEurytemora affinis in Hickling Broad, a shallow and eutrophic brackish lake, over a two-year period 1988/89. Standing biomass and production rate estimates were made using estimates of size-specific dry weights and development times obtained from laboratory measurements, field observations and information found in the literature. Both mysid and copepod are capable of a rapid response to favourable conditions and have high rates of birth, growth and production.E. affinis reproduces throughout the year with an estimated annual (May 1988–May 1989) production of 20 g dry wt m−2. Copepod standing biomass was less in 1989 compared with 1988 owing to an overall reduction in copepod body-size and a reduction of size at maturity. There was a suggestion from the data that this was caused by predation from a greater number of large (>9 mm body-length) mysids in 1989 compared with 1988.N. integer is highly seasonal in its growth with distinct peaks of recruitment in May and July. Annual (May 1988–May 1989) production of the mysid was estimated as 5.8 g dry wt m−2. AlthoughE. affinis is the only available prey ofN. integer in the lake, the mysid population appears independent of changes in that of the copepod and probably avoids negative predator-prey feedback mechanisms owing to an ability to feed on epiphytic algae.

The influence of adult conspecifics and shore level on recruitment of the ribbed mussel Geukensia demissa (Dillwyn)

The roles of recruitment variability and adult conspecifics on the distribution of Geukensia demissa (Dillwyn) in a Spartina alterniflora (Loisel) marsh were examined in a field study. Recruitment to experimental collectors was compared to recruitment onto the marsh surface at three shore levels. Peak recruitment of mussels in the smallest size class (0–5 mm) occurred in October. Recruitment was greatest on the marsh edge and to collectors containing adult mussels, suggesting that larval mussels either preferentially settle or have higher survivorship among adult conspecifics. Mussels in the largest size class (5–15 mm) recruited to collectors in early July suggesting that the previous year's cohort was still actively involved in post-settlement microhabitat selection.

Structure, radial growth dynamics and recent climatic variations of a 320-year-old Pinus rigida rock outcrop community.

The composition, structure and dendroecology of a 320-year-old Pinus rigida rock outcrop community was studied in the Shawangunk Mountains of southeastern New York. This represents one of the oldest known examples of this forest type and it is located on one of the most extreme sites in the northeastern United States. P. rigida represented 88% of all sampled trees, which typically grew on individual soil islands with soil depths of 8-35 cm surrounded by exposed bedrock. The forest was uneven-aged and P. rigida exhibited continuous recruitment into the tree size classes since the late 1600s, suggesting that it represents a physiographic climax for this species. However, a limited amount of Nyssa sylvatica and Quercus prinus recruitment started after 1830. Peak recruitment of P. rigida trees in 1720-1760 and 1860-1890 coincided with parabolic-shaped releases in their radial growth, possibly in response to disturbances. Tree ring growth was typically <0.4 mm/year since the 1850s and <0.3 mm/year during a prolonged and severe drought in the 1960s. However, large increases in precipitation and temperature from 1970 to 1993 were correlated with a dramatic post-drought growth response producing the highest ring width index values throughout the life of 260 to 280-year-old trees. Thus, we attribute certain moderate growth releases (>50%) lasting 10-15 years to climate, rather than disturbance. Tree growth and recruitment at the study site were influenced by a complex interaction of climate, soil and disturbance factors. Coupling of species recruitment, tree ring and climatic data in this study provided an improved technique for understanding forest growth and dynamics.

Dendroecological Analysis of Successional Dynamics for a Presettlement- Origin White-Pine-Mixed-Oak Forest in the Southern Appalachians, USA

1 Prior to European settlement, Quercus alba, Castanea dentata, Carya spp. and Pinus strobus dominated Ridge and Valley forests in southern West Virginia. Dendroecological techniques were used to study the disturbance history and successional dynamics of one of the few remaining valley-floor remnants of this primary forest type. 2 The uneven-aged forest is presently dominated by P. strobus, Acer rubrum, Q. alba, Q. rubra and Q. velutina. Quercus alba and P. strobus represented most of the oldest and largest trees in the forest, while Acer, Fagus and Tsuga were the youngest and smallest. Maximum age was 295 years for Q. alba and 231 years for P. strobus. 3 There was continuous recruitment of Q. alba trees from 1700 to 1900. The abundance of P. strobus increased between 1830 and 1900, with a peak in the 1870s associated with releases in radial growth. The subsequent peak recruitment of Q. rubra and Q. velutina from 1880 to 1900 is suggestive of possible facilitation of these species by P. strobus. Following the cessation of Pinus and Quercus recruitment in 1900, Acer, Fagus and Tsuga abundance increased, particularly A. rubrum. 4 Radial growth chronologies across all species and age classes exhibited a series of major or moderate releases at regular intervals (typically every 20-30 years). However, the asynchronous nature of these releases suggests that they were caused by a series of small-scale disturbances, such as fire and wind-throw, each of which had localized impacts within the stand. 5 We believe that Pinus and Quercus were maintained in this stand during the 18th and 19th centuries by periodic disturbance, in particular fire, which would have eliminated later successional species. The lack of Pinus and Quercus recruitment after 1900 and the subsequent increase in Acer, Fagus and Tsuga indicates the transitional nature of this forest in the absence of fire. 6 Coupling of tree-ring chronologies and species establishment dates greatly increased our understanding of the disturbance history and dynamics of this old-growth forest, and we believe this represents an important approach to the study of species lifehistory attributes and ecological history in general.

Occurrence of Paraprionospio sp. (form A) larvae (polychaeta: spionidae) in hypoxic water of an enclosed bay

Spatial and temporal distribution of larvae of Paraprionospio sp. (form A), an indicator of organic pollution in Japanese coastal waters, was investigated from May 1982 to November 1982 in Kumihama Bay, an area where oxygen-deficient water develops from late spring through autumn. This species has a less opportunistic life history represented by a seasonal peak in recruitment (September–October 1982). Simultaneously with the deoxygenation in May, larvae appeared in the bottom layer and were subsequently found through November almost exclusively in water layers with oxygen levels below 1 mg l −1. Larvae displayed no changes in vertical distribution in a 24-h study conducted in August 1980. These observations indicate that larvae have the ability to tolerate anoxic or hypoxic water throughout their entire development. It is hypothesized that the adaptation of larvae to hypoxic water may explain the high abundances of Paraprionospio sp. (form A) in organically polluted sediments.

Fishery biology of northeast Atlantic squid: an overview

The aims of this project were to improve understanding of the basic life-cycle biology, stock structure, trophic interactions and fisheries exploitation of northeast Atlantic squid, in particular the loliginids Loligo forbesi and Loligo vulgaris. Multivariate analysis of morphometric data and isozyme electrophoresis shows that L. forbesi populations from the coast of mainland Europe are fairly homogeneous, but that those from the Azores are a distinct stock. Loligo forbesi and L. vulgaris are annual species with a peak of breeding in the winter (December–May) throughout the geographical range, and one or more seasonal peaks of recruitment. Loligo are probably batch spawners. Males grow larger than females, mature about 1 month earlier and recruit earlier to the fishery. Sexual maturity occurred at two distinct size classes, more obviously in males than females. Fish, including many commercially important species, and Crustacea, were the dominant prey of Loligo. The incidence of fish in the diet increases with squid size. Squid of all types are eaten by fish, seals, cetaceans and seabirds, although quantitative estimates of consumption by these trophic levels were low. Catches of squid from northeast Atlantic waters increased over the period 1980–1990 but currently supply only a minor component of the European market for squid, the balance being made up by imports. The squid catch in the northern part of the range shows wide fluctuations in abundance by area and by season. The applicability of assessment methods for these stocks is limited by inadequate and inaccurate statistical information, and because nearly all of the catch arises as a by-catch from finfisheries.

Cell death and neuronal recruitment in the high vocal center of adult male canaries are temporally related to changes in song.

Adult male canaries modify their song every year. Most of these changes occur during late summer and early fall, after the end of the breeding season, and in late winter, immediately before the onset of the next breeding season. The high vocal center (HVC) is an important nucleus in the brain pathway that controls this learned behavior. New neurons continue to be added to the HVC of adult male canaries, where they replace older neurons that have died. The present report describes the monthly incidence of cell death and neuronal addition in the HVC of such birds. Different groups of 1- to 2-year-old male canaries were treated with [3H]thymidine, a marker of cell birth, during each month of the year and killed 27 days later. The ratio of 3H-labeled neurons to all neurons in the HVC showed seasonal peaks and troughs. This ratio was highest in October and March. Peaks in the ratio of pycnotic (dying) HVC cells to all neurons in HVC preceded the peaks in the ratio of 3H-labeled neurons. We suggest that seasonal peaks in cell loss and neuronal recruitment in HVC are related to endocrine changes and that all three play a role in the seasonality of song modification.

Larval durations and recruitment patterns of two Caribbean gobies (Gobiidae): contrasting early life histories in demersal spawners

Two demersally-spawning fishes were selected to examine the relationship between adult spawning strategies and the early life histories of larvae and juveniles. Although the Caribbean gobies, Coryphopterus glaucofraenum Gill and Gnatholepis thompsoni Jordan are common, co-occurring constituents of nearshore coral reefs, relatively little is known about the early life history and recruitment of their pelagic larvae to the reef environment. To examine patterns of recruitment, recently settled juveniles (7.5 to 16 mm standard length, SL) were censused and collected biweekly from reefs of Barbados, West Indies, during the spring recruitment peak for 3 yr (March to June 1990 to 1992). Otolith analyses were used to determine larval durations and to back-calculate settlement patterns and spawning dates. G. thompsoni spent more than twice as long in the plankton (60 d) than did C. glaucofraenum (27 d). Variation in larval duration of G. thompsoni [coefficient of variation (CV)=22.4%; durations up to 112 d] was significantly greater than that of C. glaucofraenum (CV=9.4%), which suggests that G. thompsoni has a greater capacity for delaying settlement and metamorphosis. Estimated size at settlement for G. thompsoni (SL=8.4 mm) was significantly larger than for C. glaucofraenum (SL=6.4 mm), but post-settlement growth rates were virtually identical (0.21 to 0.22 mm d−1). Settlement of both gobies occurred predominantly around the third quarter moon (ca. Day 21 to 22), but there was much greater spread in the settlement of C. glaucofraenum. Back-calculated spawning frequency for C. glaucofraenum was not closely linked to the lunar cycle, whereas spawning of G. thompsoni occurred more frequently during the third quarter moon. These observations demonstrate that two confamilial demersal spawners may have larvae with contrasting life history traits which can influence patterns of juvenile recruitment. A secondary peak in larval duration of G. thompsoni approximately 1 mo after the major mode (Day 87) suggests that if settlement does not occur two lunar cycles after hatching, G. thompsoni larvae may delay settlement until the following third quarter moon. This coupling suggests that settlement during a particular lunar phase may be advantageous for new recruits, and the capacity to delay metamorphosis may enable tighter synchronization to the lunar cycle.

Land-use history (1720–1992), composition, and dynamics of oak–pine forests within the Piedmont and Coastal Plain of northern Virginia

Woody vegetation was surveyed in 58 forest stands in northern Virginia to examine the effects of previous land-use history on past and present-day forest composition and dynamics. Stands were separated using detrended correspondence analysis (DCA) and overstory importance values into three forest groups: (i) white oak (Quercusalba L.)–tulip-poplar (Liriodendrontulipifera L.) (ii) white oak–scarlet oak (Quercuscoccinea Muenchh.) and (iii) Virginia pine (Pinusvirginiana Mill.) The first DCA axis represents a successional continuum from more recently disturbed areas containing young pine forests to less disturbed mature oak stands, and is negatively correlated with stand age and species diversity. White oak and red oak (Quercusrubra L.) dominated presettlement forests in the area. Following European settlement, forests experienced intense logging associated with the charcoal iron industry, large-scale clearing for agriculture, and subsequent land abandonment. By coupling radial growth analysis with age–diameter figures, we evaluated the responses of stands to disturbances associated with various land-use practices. This analysis indicated that many Virginia pine stands resulted from agricultural abandonment during the early 1900s, while a majority of oak stands experienced peak recruitment and radial growth following periodic logging disturbances in the 1800s. Canopy closure, forest protection, and reduced fire and logging disturbance this century led to increases in dogwood (Cornusflorida L.) and blackgum (Nyssasylvatica Marsh.) in area forests. The oldest stands exhibited a lack of tall oak regeneration; however, they also contained a scarcity of potential oak replacement species. Therefore, oak will seemingly share future dominance with several mixed-mesophytic species, although the exact successional status of these stands is unresolved.

Tropical tree species diversity: a test of the Janzen-Connell model.

To test the premises and predictions of the Janzen-Connell model (Janzen's spacing mechanism), seeds of the rainforest canopy tree, Brosimum alicastrum, were placed at different distances from the parent tree and their removal observed over 3 weeks. The number and density of naturally occurring seeds at different distances from the parent tree were also estimated. Predation was not greater near the parent tree, except on the very small spatial scale: the proportion of experimental seeds removed was greater 1 m from the trunk than it was 5-25 m from the trunk. Predation was negatively correlated with seed density, not positively as the Janzen-Connell model assumes-presumably due to predator satiation. The density of seeds after predation peaked 5 m from the tree trunk, but this is well within the crown radius of the parent tree. There is a peak in the number of potential recruits at a distance of 10 m from the parent tree, due to the peaked initial distribution of seeds. This peak is caused by the interaction between the seed density curve and the increasing area of an annulus around the parent tree at increasing distances, not by the product of the density curve and the predation curve. However, it is important to realize that it is not the presence of a peak in recruitment away from the parent that is essential to maintaining tropical tree species diversity, but frequency-dependent recruitment induced by poor recruitment near conspecifics. Predator satiation seems to be an important factor in the survival of B. alicastrum seeds, possibly at several spatial scales. The number of seeds produced by the tree is negatively correlated with the loss to predators, and trees that have a fruiting conspecific nearby also suffer lower levels of predation. Seed predation increases as one moves from the forest edge into the interior, creating an edge effect that may have long-term effects on the forest composition and tree species diversity. More studies are needed, for other species, other localities, and larger spatial and temporal scales, on both the Janzen-Connell mechanism and this edge effect.

Spatial variability in the abundance of the corallivorous gastropod Drupella cornus

The corallivorous gastropod Drupella has been responsible for extensive coral mortality on a number of reefs in the Indo-West Pacific region. Seasonal and spatial variability in the abundance of juvenile ( 2.0 cm) Drupella cornus along the Ningaloo Reef, Western Australia, were documented during 1990/1991. Numbers of D. cornus were found to vary significantly between the different sites and reef habitats sampled, with the highest numbers recorded at a backreef edge site at the southern end of the reef. There was no evidence that the densities of D. cornus varied seasonally, although size-frequency distributions suggest that there may have been a recruitment peak in January/February 1991 following a major spawning event in November/December 1990. Drupella cornus were found predominantly on Acropora species with a caespitose/corymbose growth form; however the abundance of these corals at different sites did not explain the observed distribution patterns of the gastropod. Drupella cornus recruits (<1.0 cm shell length) frequently occurred on corals which were also occupied by larger conspecifics, suggesting that either larval settlement or post-settlement survival were increased in the presence of adult D. cornus.

Age Structure and Successional Dynamics of a Presettlement-Origin Chestnut Oak Forest in the Pennsylvania Piedmont

MIKAN, C. J., D. A. ORWIG AND M. D. ABRAMS (School of Forest Resources, Forest Resources Laboratory, Pennsylvania State University, University Park, PA 16802). Age structure and successional dynamics of a presettlement-origin chestnut oak forest in the Pennsylvania Piedmont. Bull. Torrey Bot. Club 121: 13-23. 1994.-This study investigated the community ecology of an old-growth Quercus prinus L. forest on a dry talus slope in southeastern Pennsylvania. Quercus prinus trees up to 367 years in age dominated the canopy layer. Acer rubrum L., Betula alleghaniensis Britton, Betula lenta L. and Nyssa sylvatica Marsh. occupied a small share of canopy space but dominated understory positions. Acer rubrum was the only canopy species present in significant numbers in the sapling layer. Presettlement forests of the region were dominated by Quercus velutina Lam., Quercus alba L., Castanea dentata (Marsh.) Borkh. and Carya spp. Following European settlement, frequent cutting and burning associated with the charcoal iron industry promoted oak coppice stands during the 19th century and prevented the development of a shade tolerant understory. Age determinations indicated that Q. prinus recruited continuously from 1625 until 1920. However, radial growth analysis indicated that peak recruitment of both the oak and mixed mesophytic tree species was associated with past disturbances. Cessation of frequent cutting, reduced occurrence of fire and the loss of C. dentata facilitated establishment of A. rubrum, B. lenta and B. alleghaniensis in the understory early this century. Results of this study indicate the potential for replacement of oak by more tolerant species on this and similar dry sites in the northern mid-Atlantic region which are traditionally considered to be physiographic or edaphic climaxes.

Reproductive biology of Orchestia gammarellus (Crustacea: Amphipoda) living in a sewage treatment works

A population of the supralittoral Orchestia gammarellus (Pallas) (Amphipoda: Talitridae), living in a percolating filter of a sewage treatment works has been studied over 12 months. The population showed a seasonal pattern of occurrence, few animals being collected during the winter. The sex ratio was biased significantly to females except during the breeding season when it did not deviate significantly from 1:1. Ovigerous females, never collected in high numbers (1–14 per month), were found from April to August, with peak occurrence in May/June. Juveniles were present throughout the year, but peak recruitment occurred in summer, following the period of reproduction by females. The smallest ovigerous female had a body length of 12.6 mm and the largest measured 20.0 mm. Egg numbers (5–31 per brood) increased with increasing female size and egg volume more than doubled during embryonic development. These findings are discussed in relation to the possible methods of colonization of this unique habitat and the influence of sewage on crustacean life-history traits.

Role of CD4+ T lymphocytes and interleukin-5 in antigen-induced eosinophil recruitment into the site of cutaneous late-phase reaction in mice.

Previous studies suggested that the eosinophil recruitment into the site of cutaneous late-phase reaction (LPR) was dependent on IgE antibody and mast cells. In this study, we determined the role of CD4+ T cells and CD8+ T cells in causing antigen-induced eosinophil recruitment of LPR in mouse skin. Eosinophil infiltration into the subcutaneous tissue of ovalbumin (OVA)-sensitized BALB/c mice was biphasic, reaching the first peak at 6 h after the subcutaneous challenge with OVA and the second peak at 24 to 48 h. The in vivo depletion of CD4+ T cells by pretreatment with anti-L3T4 monoclonal antibody (mAb) significantly decreased the second peak (at 24 h and 48 h), but not the first peak (at 6 h), of OVA-induced eosinophil infiltration into the skin of OVA-sensitized mice. However, the depletion of CD8+ T cells by pretreatment with anti-Lyt-2 mAb had no significant effect on either the first peak or second peak of OVA-induced cutaneous eosinophilia. Pretreatment with anti-murine interleukin-5 (IL-5) mAb also decreased the second peak, but not the first peak, of OVA-induced cutaneous eosinophilia. In contrast to the inhibitory effects of depletion of CD4+ T cells and of anti-IL-5 mAb on the second peak of antigen-induced cutaneous eosinophilia, disodium cromoglycate and a selective antagonist for platelet activating factor (PAF) CV-6209 decreased the first peak of OVA-induced cutaneous eosinophilia in the mouse. These results indicate that CD4+ T cells, but not CD8+ T cells, cause the second peak of antigen-induced eosinophil recruitment of cutaneous LPR and that IL-5 mediates this eosinophil recruitment. In contrast, the first peak of antigen-induced eosinophil recruitment of cutaneous LPR is mediated by mast cells and PAF.

Spatial and temporal patterns of Crassostrea virginica (Gmelin) recruitment: relationship to scale and substratum

A series of experiments were performed in a euhaline (North Inlet, South Carolina) and mesohaline estuary (Wando River, South Carolina) to examine recruitment variability across several spatial and temporal scales. Crassostrea virginica (Gmelin) recruitment varied across all temporal and spatial scales with the exception of among reefs within restricted zones of the Wando River Estuary. Both biological and physical factors could be related to the variability observed at different scales. At the finest spatial scales, oyster larvae indicate a preference for crevices and shells or tests of other organisms as attachment sites. Higher recruitment on the shaded sides of plates in the high intertidal zone was consistent with observations documenting the preference of larvae for shaded attachment sites, adverse effects of direct exposure to light and heat, and sedimentation. Observations made at broader spatial scales (100s m) suggested that the availability of potential settlement surfaces may be the underlying factor contributing to the observed distribution of intertidal oyster reefs. Recruitment patterns were affected by choice and position of substrata. Sampling frequency was observed to influence the number of peak recruitment events observed within a season, estimates of total net recruitment, and the variability associated with recruitment observations made at different levels in the intertidal zone. Gregarious settlement behavior, sedimentation, and biophysical modification of the substrata were implicated in the regulation of recruitment patterns at shorter time scales. Competition and predation appeared to be important in regulating recruitment patterns at longer time scales, although the effects varied across the intertidal gradient. The physical and biotic processes which normally operate to structure intrareef spatial recruitment patterns were not observed to be as important in reefs which are physically restricted by tidal factors to very narrow zones within the intertidal gradient.

Kelp-sandhopper interactions on a sand beach in New Zealand. I. Drift composition and distribution

Abstract The composition of organic matter deposited on a New Zealand sand beach was measured monthly during neap tide periods over an annual cycle. The main components of the drift were wood, grass and two kelp species, Macrocystis pyrifera (L.) C.Ag. and Durvillaea antarctica (Chamisso) Hariot. Drift composition was measured from five transects, 5 m in width extending from the base of the dunes (EHWS) to the low tide level. The average wet weight of material collected from the 5-m beacg strip was 11.23 kg (SE 3.78), with low values in summer (December and January) and highest biomass in July. Variability in organic matter was due to fluctuating levels of M. pyrifera, which dominated the summer periods, ranging from 0.07 to 2.2 kg·m −1 strip of beach. The biomass of D. antarctica decreased downshore, contrasting with M. pyrifera which was generally distributed evenly down to 20 m from the base of the dunes. The amphipod Talorchestia quoyana (Milne-Edwards) was closely asociated with drift material, especially fresh M. pyrifera. Distribution patterns and density of T. quoyana varied with season, shore level and substratum type. Exceptional densities of T. quoyana ( x= 121750m −2 ) were found associated with M. pyrifera on the drift line. There were two peaks of juvenile recruitment during which juveniles were also associated with sand away from drift material. Results from core samples suggest no direct relationship between sandhopper density and algal biomass on a seasonal basis. Annual deposition of kelp onto the sand beach was estimated as < 400 kg·m −1, characterized by regular input due to storm and seasonal events. The spatial distribution of T. quoyana was dependent on the availability of drift wrack and this is discussed in relation to its behavioural and physiological tolerances of the environmental factors operating on the sand beach.

Seasonal changes in populations of three sympatric isopods in a Hong Kong forest

Data on the population dynamics of tropical isopods in general, and those inhabiting forests in particular, are scarce. Consequently, the population dynamics of three sympatric isopods, Burmoniscus ocellatus (Philosciidae), Formosillo raffaelei and Orodillo maculatus (Armadillidae), were studied at two neighbouring sites in a mixed forest in Hong Kong between March 1985 and December 1986. Isopod population densities varied with the species, site and season, with mean densities ranging from 100–150 m‐2. Burmoniscus ocellatus and F. raffaelei had a single recruitment peak per year, while O. maculatus had two. Results of the present study revealed that air temperature was the main factor explaining seasonal variations in population density, while rainfall exerted its effect with a two‐ to three‐week time‐lag, depending on the species studied. Approximately 20% of Formosillo raffaelei and O. maculatus sampled were found infected by Rickettsiella bacteria. The possible influence of pathogens, and other biotic factors, on the dynamics of the Hong Kong isopod populations was discussed

Oviposition, larval abundance, in situ larval growth and recruitment of the herbivorous gastropodLacuna vincta in kelp canopies in Barkley Sound, Vancouver Island (British Columbia)

We investigated recruitment of the herbivorous gastropodLacuna vincta (Montagu, 1803) in the canopies ofMacrocystis integrifolia andNereocystis luetkeana beds in Barkley Sound, Vancouver Island (British Colombia), from 1987 to 1989. Four factors influencing intensity and patterns of recruitment were studied: (1) seasonality of oviposition, (2) larval abundance, (3) growth of larvae in the field and (4) larval settlement. Egg masses were abundant on low intertidal algae but were scarce in kelp canopies. Although egg masses could be found almost year-round, a distinct and intense period of oviposition occurred during winter and spring. Intracapsular development lasted 2.5 to 3.5 wk before planktotrophic veligers emerged. The duration of the planktonic period, 7 to 9 wk, was determined through an in situ study of cohorts ofLacuna spp. larvae present in the plankton between January and June 1988. The general timing of the onset of the spring peak recruitment period was predicted from these cohorts. Primary periods of recruitment ofL. vincta in the canopy occurred in April–May (average density up to 383.9 juveniles m−2 blades), with a second period of lower intensity in the late summer—fall period. We observed similar trends between abundance of advanced larvae (> 500µm) in the plankton and recruitment rates in kelp canopies. Although adults were occasionally observed in the canopy, newly metamorphosed juveniles consistently dominated the habitat. The persistance of small juveniles (0.7 to 1.5 mm), rapid declines in density shortly after recruitment, and SCUBA observations of drifting individuals suggest that juveniles migrate to the under-canopy or low intertidal area after a brief period of growth on kelp blades.