Shing Yip Lee

Mangrove outwelling: a review

The export of detritus and faunal biomass from mangroves has long been considered as an important support for offshore biological production and has been widely used as an argument for mangrove conservation. This functional role of the mangroves, like many other paradigms in mangrove ecology, has seldom been put to rigorous test since the hypothesis was postulated about 25 years ago. Past studies on which the hypothesis was based were mostly carried out in mangrove or other wetland environments, little is known about the fate and effects of outwelled detritus on oceanic, offshore, communities. Mass balance studies carried out in the last 15 years tend to suggest that export is common from tidal mangroves, the direction of flow depends, however, on the identity of the chemical species in question. Pore water and groundwater flow can affect tidal material exchange but are poorly studied. Generally, tracer methods using stable isotope ratios or other signatures have suggested that outwelling may be much less significant than expected. Further, most past studies have focussed on particulate matter while it is increasingly apparent that dissolved organics may play a more important role in matter exchange between offshore and mangrove communities. There is also evidence that benthic biomass and richness may not bear any positive or significant relationship with detritus availability.

The effects of seagrass (Zostera japonica) canopy structure on associated fauna : a study using artificial seagrass units and sampling of natural beds

The importance of seagrass canopy to associated fauna was assessed by comparing the species richness, abundance and diversity of the epi- and infaunal macroinvertebrate assemblages in a seagrass (Zostera japonica Ascherson and Graebner) bed and the adjacent unvegetated area in Hong Kong. Seagrass cover had significant effects on the composition and abundance of the associated fauna and the amount of detritus accumulated on the sediment surface. Detritus abundance was significantly higher in the seagrass bed, and was positively correlated with both the above- and belowground biomass of Z. japonica. Both the abundance and species richness of the epi- and infauna were significantly positively correlated with the belowground biomass of the seagrass and detritus standing crop. Macrofaunal species richness was higher (118) in the seagrass bed than the adjacent unvegetated areas (70), with a higher degree of similarity between the infauna than the epifauna of the two habitats. While all species recorded from the unvegetated areas were found in the seagrass bed, 48 species occurred only in the seagrass-covered areas. Species richness of epifauna was significantly higher in the seagrass bed, but there was no difference between infaunal species of the two habitats. On the contrary, faunal (epi- and infauna) abundance was significantly higher in seagrass areas. The seagrass bed also supported species of small tellinid bivalves previously not recorded from Hong Kong. Artificial seagrass units (ASUs, 0.2 m(2)) with four combinations of leaf density and leaf length and a control (bare sand) were placed at short distances from natural patches of Z. japonica. The composition, abundance and biomass of the epibenthos associated with the ASUs and the control were recorded after 3 months in the field. While species richness did not differ among the treatments, total abundance of epibenthos was significantly higher in the high density-long leaves (HL) treatment than in the control. Results of a discriminant analysis using log-transformed abundance data suggest that the gastropod Clithon oualaniensis, the mussel Musculista senhousia and the crab Thalamita sp. were important species distinguishing the assemblages in the various treatments. All the three species were significantly more abundant in the HL treatment than in the low density-short leaves (LS) treatment and the control. By contrast, there was no significant difference in the biomass of the epifauna, but discriminant analysis again separated the five treatments based on the composition of the biomass, with the same three species identified as the most important discriminating species. The species richness and abundance of the epifauna associated with the ASUs were similar to the adjacent unvegetated areas, but significantly lower than in the Zostera patches. The physical canopy structure of Z. japonica beds increased the abundance of the epibenthos, potentially through provision of canopy and indirectly through trapping of detritus.

Ecology of mangrove fungi and their role in nutrient cycling: what gaps occur in our knowledge?

Recent investigations have increased our knowledge of the ecology of mangrove fungi. In this paper this information is reviewed with emphasis on biogeography, biodiversity, differences in the tropical and subtropical mycoflora, fungal distribution on mangroves trees, host specificity, vertical zonation and distribution with salinity. Gaps in our knowledge are discussed. There is little knowledge of the role of mangrove fungi in nutrient cycling which is also reviewed. Areas in which knowledge is deficient include quantification techniques for fungal abundance, the nature and activities of fungal extracellular enzymes and fungal modification of mangrove detritus matter, especially the dissolved form.

Primary productivity and particulate organic matter flow in an estuarine mangrove-wetland in Hong Kong.

Net primary productivity and organic matter flow of a mangrove-dominated wetland was estimated by following production and detritus dynamics in a tidal pond in north west Hong Kong in 1986–1988 (9.1 ha). Total productivity was 12.47 t dry wt ha−1 yr−1, of which >90% was from emergent macrophytes (the mangroveKandelia candel and the reedPhragmites communis). High turbidity and high summer temperatures probably limited respective production by phytoplankton and benthic macroalgae (dominated byEnteromorpha crinata). Despite the high total productivity, little detritus was exported from the emergent macrophyte stands, due to the low inundation frequency. This created a net water column carbon deficit which was provided for by the high organic matter import (mean = 4.42 g ash free dry wt m−2 d−1) from the incoming water. This same sediment and particulate organic carbon input giving a high accretion rate of 1.7 cm yr−1 was probably also the force behind progressive dis-coupling of emergent macrophyte production from water column consumers. This resulted in a tendency to retain production in the emergent macrophyte stands while the water column community increasingly relied on allochthonous carbon. This shift from a net exporter to a net importer of carbon in landward wetlands is probably characteristic of the transition into nutrient-conservative terrestrial systems.

Cheliped size and structure: the evolution of a multi-functional decapod organ

Abstract Past studies on the evolution of the structure of the decapod cheliped relied strongly on intraspecific allometric comparisons. This approach is probably inadequate in understand- ing the interplay of the various selection pressures shaping the evolution of such multi- functional organs as the decapod cheliped. A framework employing intersexual and interspecific allometric comparisons as well as manipulative experiments is proposed to allow a better understanding of the interaction and possibly, the relative importance of foraging, agonistic interactions, phenotypic variations and sexual selection in cheliped evolution. Comparative allometric data are presented for seven Charybdis and Portunus spp. from Hong Kong. The uncertainty about the role of intersexual niche partitioning in the evolution of sexual dimorphism, the lack of comprehensive fossil data on cheliped evolution and timing of the development of morphometric characteristics, and questions about the importance of phenotypic plasticity are identified as major problems met in the study of cheliped evolution.

The importance of sesarminae crabs Chiromanthes spp. and inundation frequency on mangrove (Kandelia candel (L.) Druce) leaf litter turnover in a Hong Kong tidal shrimp pond

Abstract In high-zoned ( Kandelia candel (L.) Druce (Rhizophoraceae) mangrove stands in a Hong Kong tidal shrimp pond, sesarminae crabs Chiromanthes spp. selectively feed on decaying leaves of lower C:N ratios and are capable of consuming >57% of the daily leaf litter production during the active season. Leaf litter turnover rate is significantly negatively correlated with crab density. Turnover at more frequently inundated sites is affected by detritivorous gastropods, with cumulative day-degree ( D °) and cumulative immersion time ( I ) values acting as better predictors of decomposition rates (based on the negative exponential model) than time. Turnover rates at these sites are, however, much slower than those affected by crab consumption at less frequently inundated sites. Litter turnover is, thus, affected by different agents in different inundation regimes in the mangal.

Growth and Reproduction of Double-Ended Pipefish, Syngnathoides biaculeatus, in Moreton Bay, Queensland, Australia

Life-history characteristics of the double-ended pipefish, Syngnathoides biaculeatus (Bloch), were investigated to determine growth rate, degree of sexual dimorphism, size at maturity, and reproductive biology. Growth rates of wild juveniles and adults calculated from monthly progression of length-frequency modes ranged from 0.8 mm d−1 (fish lengths 120–145 mm standard length (SL)) in summer to 0.2 mm d−1 in winter (185–200 mm SL). Growth of laboratory-reared juveniles up to 63 d old was greater, ranging from 0.8 to 2.3 mm d-1. The von Bertalanffy growth constant K was estimated at 0.0076 d- 1, or 2.8 year−1. Morphological differentiation between the sexes based upon abdominal pattern was possible for fish larger than 120 mm SL, with females possessing a zigzag pattern on the abdomen. The association between this pattern and sex was confirmed by histological gonad analysis. Males were significantly longer than females during four of seven seasons examined, and a 1 : 1 sex ratio was determined for all seasons except autumn when the ratio was female biased. The breeding season was marked by the appearance of pregnant males between October and April, and during courtship both species exhibited increased pigmentation. The minimum paternal size at maturity was 185 mm, the maximum length recorded 260 mm. Clutch size ranged between 60 and 200 eggs, with a mean of 153. Ovaries had a sequential pattern of egg development, resulting in egg batches that approximated the number of eggs carried by brooding males. Additionally, all eggs in a brood were at the same developmental stage. This suggests that one female provides all of the eggs for one male per breeding event in a monogamous mating system.

Macrobenthos of a tidal impoundment at the Mai Po Marshes Nature Reserve, Hong Kong

The composition and spatial distribution patterns of the macrobenthic faunal assemblages of an 8-ha tidal impoundment operated as a traditional shrimp pond at the Mai Po Marshes Nature Reserve, Hong Kong, were studied in relation to temporal changes in local environmental conditions. Species richness, abundance and biomass of macrobenthos across 5 different sub-habitats (seaward, middle, and landward parts of open water unvegetated areas, and Phragmites- and Kandelia-dominated, vegetated areas) within the pond were examined bimonthly between January 1997 and January 1998. Grab samples were collected randomly within the sub-habitats. Key physical environmental parameters of the sampling sites were also measured. A total of 46 species of macrobenthos was recorded: 11 polychaetes, 11 molluscs, 13 crustaceans and 11 insects. Mean species density in the five sub-habitats ranged from 0 to 3907 indċm−2, with mean biomass ranging from 0 to 96.9 gċm−2. The macrobenthos showed spatial and temporal differences among the sub-habitats and across sampling times. Species abundances of Polychaeta, Mollusca and Crustacea were significantly higher in the three open water areas than in the two vegetated (Phragmites- and Kandelia-dominated) areas, with an inverse pattern for Insecta. There were no clear temporal patterns although abundance and biomass generally increased in the cooler months. Results of a canonical correspondence analysis suggest that macrobenthic species richness, abundance and biomass in the open areas were positively correlated with salinity, while water depth, dissolved oxygen and sediment organic matter content had little relationship with the macrobenthic assemblage parameters. Ordination by multi-dimensional scaling suggests that different habitats supported distinct macrobenthic assemblages. The macrobenthic assemblage in the tidal pond was less species rich but denser than those in the neighboring tidal mangrove and mudflat, suggesting that conversion of these areas into extensively managed tidal aquaculture ponds results in reduced species richness in tropical mangrove habitats.

Adult–juvenile interactions in the crabs Sesarma (Perisesarma) bidens and S. (Holometopus) dehaani (Decapoda:Grapsidae) from intertidal mangrove habitats in Hong Kong

Grapsid crabs in the subfamily Sesarminae are key faunal components of many intertidal mangrove ecosystems. Although their feeding and burrowing activities play important roles in the processing of plant material and nutrient cycling, relatively little is known about ecological interactions that regulate populations or influence species composition of intertidal crab assemblages. We used manipulative microcosm experiments to explore the potential effects of both intra- and interspecific adult–juvenile interactions in populations of Sesarma (Perisesarma) bidens (de Haan 1835) and S. (Holometopus) dehaani H. Milne Edwards 1853, two species of crabs common in mangrove habitats of Hong Kong. Survival of juveniles (3–10 mm carapace width, CW) held alone and exposed to either a conspecific or heterospecific adult crab (14–30 mm CW) was measured in microcosms (ca. 2.8 l) containing both physical structure and alternative food (mangrove litter). In the absence of adults, juvenile survival of both species after 3 days was 95–100% at densities of one, two and four juveniles per microcosm. Adults functioned as cannibals and intraguild predators by consuming both conspecific and heterospecific juveniles, but the proportion of juveniles consumed was not density dependent (i.e. predatory adults did not exhibit a functional response). There was no difference in the effect of male and female crabs on juvenile survival, but there was an asymmetrical interspecific effect. Fewer H. dehaani juveniles survived in the presence of heterospecific (52%) than conspecific (75%) adults, while survival of P. bidens juveniles did not differ between adult species (85% with heterospecific adults and 78% with conspecific adults). If cannibalism is important in population regulation of these semi-terrestrial crabs, our findings suggest that the mechanism is unlikely to involve a strong functional response of adults to changes in juvenile density. However, juvenile survival in the intertidal environment may be regulated by changes in adult density and availability of refugia. Based on the observed asymmetrical interspecific effect of adult species on juveniles, we predict that P. bidens should tend to reduce H. dehaani populations in areas where the two species overlap through a mechanism of stage-structured intraguild predation.

Distribution pattern and interaction of two infaunal bivalves, Tapes philippinarum (Adams and Reeve) and Anomalocardia squamosa (Linnaeus) (Bivalvia: Veneridae)

Abstract The vertical and horizontal distribution patterns of two infaunal bivalves, Tapes philippinarum (Adams and Reeve) and Anomalocardia squamosa (Linnaeus) were investigated on a sheltered sandy shore in Hong Kong. Tapes philippinarum occurred at significantly higher densities and occupied a deeper stratum in the sediment than Anomalocardia squamosa , which is a species protected by a much thicker shell. Both species showed an aggregated pattern of spatial distribution over the scale of 10 1 to 10 −2 m. There was no correlation between the densities of the two bivalves nor was there any relationship between body condition and burying depth of Tapes philippinarum . However, individuals of Anomalocardia squamosa residing in the 2–4 cm depth stratum had a significantly higher condition index than their counterparts in the 0–2 cm stratum. The three-dimensional distribution pattern of the two bivalves in relation to density and presence of the other species was investigated by using tagging experiments in the laboratory whereby the exact three-dimensional position of the individuals was recorded in the form of an x , y , z coordinate. The same relative vertical distribution pattern of the bivalves as in the field was maintained while nearest neighbour analysis using the three- ( x , y , z coordinates) or two-dimensional (only the x , y coordinates) data yielded different conclusions about the distribution pattern of the bivalves. The three-dimensional analysis consistently suggested an aggregated pattern whereas horizontal distribution was mostly random in the sediment matrix. Both horizontal, vertical and total displacement by individuals of Tapes philippinarum , but not Anomalocardia squamosa , were significantly affected by manipulations of the densities of conspecifics and/or A. squamosa . The latter species, however, was responsive to manipulations in terms of their burying depth. Both species showed considerable movements over the 2-week experimental period. The presence of a predator Portunus pelagicus did not significantly change the vertical distribution pattern of the two bivalves but the mortality of Tapes philippinarum was significantly higher than that of Anomalocardia squamosa . The results suggest that short-term responses such as emigration may be an important response to potential competitive interaction among infaunal bivalves and that such interaction may reduce the usefulness of long-term indicators such as growth and mortality. Whereas burying depth of infaunal bivalves may be influenced intra-specifically by trade-off between predation risk and hunger level, the soft tissue: shell weight (≈ defence) ratio seems to be a more general predictor of vertical distribution in intra- as well as inter-specific comparisons.