Sabine Dittmann

Mytilid mussels: global habitat engineers in coastal sediments

Dense beds of mussels of the family Mytilidae occur worldwide on soft-bottoms in cold and warm temperate coastal waters and have usually been considered hot spots of biodiversity. We examined intertidal mussel beds at four distant locations around the globe with the same sampling method, to find out whether this “hot spot” designation holds universally. We studied species assemblages within the matrices of byssally interconnected mussels engineered by Mytilus edulis in the North Sea, by mixed Perumytilus purpuratus and Mytilus chilensis at the southern Chilean coast, by Musculista senhousia in the Yellow Sea and by Xenostrobusinconstans at the coast of southern Australia. In all cases, species assemblages inside mussel beds were significantly different from those outside with many species being restricted to one habitat type. However, species richness and diversity were not generally higher in mussel beds than in ambient sediments without mussels. In the North Sea (M. edulis) and at the Chilean coast (P. purpuratus, M. chilensis), mussel beds have markedly higher species numbers and diversities than surrounding sediments, but this was not the case for mussel beds in Australia (X. inconstans) and the Yellow Sea (M. senhousia) where numbers of associated species were only slightly higher and somewhat lower than in adjacent sediments, respectively. In conclusion, although soft bottom mytilid mussels generally enhance habitat heterogeneity and species diversity at the ecosystem level, mussel beds themselves are not universal centres of biodiversity, but the effects on associated species are site specific.

Benthos structure on tropical tidal flats of Australia

General features of benthos communities of tropical tidal flats are defined from northeast Australia, based on surveys from 1988 to 1991 in Hinchinbrook Channel and in the Haughton River estuary. A zonation of benthic communities is described. Total abundances for macrofauna averaged 31 individuals 200 cm−2, mesofauna (defined as infauna smaller than 0.5 mm and retained on a 0.25 mm sieve) averaged 16 individuals 10 cm−2 and meiofauna averaged 231 individuals 5 cm−2. The two study sites were similar in their abundance structures, but species due to a higher diversity of polychaeta in the Channel (120 species, H′=3.80 vs. 29 species and H′=1.78 in the Haughton estuary). Species densities were comparable at both sites and rather low (6 species 177 cm−2, 2–7 species 10 cm−2 and 5 meiobenthic Plathelminth species 5 cm−2). Over 50% of the species encountered were represented by less than three individuals. Deposit feeders, dominated the assemblages both in terms of individuals as well as species numbers. Tropical tidal flats are compared with their temperate counterparts and approaches for future research in tropical benthos communities are recommended.

Benthic fauna in tropical tidal flats - a comparative perspective

Tidal flats extend seaward from mangrove forests along many tropicalcoastlines. This contribution compiles the current knowledge on tropicaltidal flats with regard to species richness, abundance, spatial distributionpatterns and ecological roles taken by major organisms. Tidal flatsencompass a variety of soft-sediment habitats which are inhabited by aspecies-rich fauna. Species numbers are generally higher than in temperatetidal flats, but vary widely between tropical sites where they have not yetbeen fully assessed due to lack of research and taxonomic problems. Theassessment of biodiversity is further complicated by a low species frequency,the small size of macroinfaunal organisms and variations in the occurrence ofspecies between sites. Mean abundance ranges from 1000–2000individuals m-2, although figures ten times higher have beenrecorded in a mudflat in Costa Rica, and there are great variations in general.Individual densities of small-sized macrofauna are about ten times higher than for larger macrobenthos. A generalized zonation scheme for intertidaldistribution patterns of tropical tidal flat fauna is presented. The comparisonshows that the occurrence of macrofauna and especially of ecosystemengineers varies between sites, which can in turn explain the great variabilityin species abundance patterns recorded in benthic surveys of tidal flatswithin and between regions. This review revealed similarities as well asdifferences between various tropical tidal flats, which underlines the need for further comparative studies to be done, using the same methodology, before generalizations can be made.

Spatial variation in burrow morphology of the mud shore crab, Helograpsus haswellianus (Brachyura, Grapsidae) in South Australian saltmarshes

Burrowing by crabs is an important component of their functional role in mangrove and saltmarsh habitats. The grapsid crab Helograpsus haswellianus (Whitelegge, 1889) is one of the more conspicuous burrowing organisms in the saltmarshes of southern Australia. To evaluate intraspecific differences in burrowing behaviour among saltmarshes on a regional scale, we compared vegetation cover, sediment composition and burrow morphology at four sites using resin casts. Six burrow morphology characters were measured (burrow depth, number, lengths and diameter of the shafts, ratio of the shafts, number and diameter of the burrow openings), and the overall 3D burrow complexity was described using a single parameter, the fractal dimension D. Apart from the number of shafts, all morphological characters of the burrows differed significantly among sites. Analyses of the fractal dimensions lead to the identification of three groups of burrows based on D: a group of highly complex burrows (one site), a group of burrows of intermediate complexity (two sites) and a group of less complex burrows (one site). Burrow morphology variation was correlated with non-dominant vegetation, plant matter in the soil and very coarse sand in the sediment. Site-specific differences in burrows caution against generalising the functional role of crabs across sites.

Burrow morphology and utilization of the goby (Parapocryptes serperaster) in the Mekong Delta, Vietnam

Some fish species living in mudflats construct burrows for dwelling and hiding. The goby Parapocryptes serperaster is a burrowing fish in mudflats of many estuaries in South East Asia. This study was carried out in the Mekong Delta, Vietnam, to examine burrow morphology and usage by this species. Morphology of the burrows constructed by P. serperaster was investigated by resin castings in situ to obtain the physical structure and configuration of each burrow. Fish from the burrows were caught and measured before burrow casts were made. Fish burrows comprised several openings, a few branching tunnels and multi-bulbous chambers. The surface openings were circular, and the shapes of branching tunnels were nearly round. The burrows had interconnected tunnels and various short cul-de-sac side branches. The burrow structure differed between fish sizes, but burrow dimensions were positively correlated with fish size, indicating that larger fish can make larger and more sophisticated burrow. The burrow structure and dimensions were not different between the dry and wet seasons. Laboratory observations showed that P. serperaster used body movements to dig burrows in the sediment. Burrows could provide a low-tide retreat and protection from predators, but were not used for spawning and feeding for this goby species. This study indicates that the burrowing activity of gobies is an important adaptation for living in shallow and muddy habitats.

Temporal variation in feeding behaviour and trophic ecology of the temperate hemiramphid, Hyporhamphus melanochir

The feeding ecology of southern garfish, Hyporhamphus melanochir, in South Australia was investigated to determine the dietary composition and diurnal feeding patterns, especially in relation to a hypothesised relationship between prey availability and feeding patterns. Samples of H. melanochir were collected at different times of the day and night during autumn and winter, and assigned to 3-hourly intervals. The dietary composition and feeding patterns were determined from gut contents analysis. The main food items were seagrasses (Zosteraceae) and hyperbenthic crustaceans, mainly amphipods. Polychaetes and insects were also consumed. A clear diurnal trophic shift was evident: seagrass was consumed in large volumes during the day, whilst hyperbenthic invertebrates dominated the diet during the night. Plankton samples indicated that this trophic shift reflected the higher abundances of hyperbenthic invertebrates in the water column at night. Less time was spent consuming seagrass during winter, coinciding with shorter day-lengths. H. melanochir has a strong trophic association with seagrass beds, which may account for the high garfish abundance in northern Gulf St. Vincent where extensive Zosteracean seagrass beds occur. This may be useful fundamental information for future environmentally-based fishery management decisions to help ensure the sustainability of southern garfish population and the ecosystem of which it is a part.

Macrofaunal Patterns in and around du Couedic and Bonney Submarine Canyons, South Australia

Two South Australian canyons, one shelf-incising (du Couedic) and one slope-limited (Bonney) were compared for macrofaunal patterns on the shelf and slope that spanned three water masses. It was hypothesized that community structure would (H1) significantly differ by water mass, (H2) show significant regional differences and (H3) differ significantly between interior and exterior of each canyon. Five hundred and thirty-one species of macrofauna ≥1 mm were captured at 27 stations situated in depth stratified transects inside and outside the canyons from 100 to1500 m depth. The macrofauna showed a positive relationship to depth in abundance, biomass, species richness and community composition while taxonomic distinctness and evenness remained high at all depths. Biotic variation on the shelf was best defined by variation in bottom water primary production while sediment characteristics and bottom water oxygen, temperature and nutrients defined biotic variation at greater depth. Community structure differed significantly (p<0.01) among the three water masses (shelf-flowing South Australian current, upper slope Flinders current and lower slope Antarctic Intermediate Water) (H1). Although community differences between the du Couedic and Bonney regions were marginally above significance at p = 0.05 (H2), over half of the species captured were unique to each region. This supports the evidence from fish and megafaunal distributions that the du Couedic and Bonney areas are in different bioregions. Overall, the canyon interiors were not significantly different in community composition from the exterior (H3). However, both canyons had higher abundance and/or biomass, increased species dominance, different species composition and coarser sediments near the canyon heads compared to outside the canyons at the same depth (500 m), suggestive of heightened currents within the canyons that influence community composition there. At 1000–1500 m, the canyon interiors were depauperate, typical of V-shaped canyons elsewhere. The large number of species captured, given the relatively low sampling effort and focus on the larger macrofauna, support previous studies that identify the South Australian coast as a high biodiversity area.

Age validation, growth and population characteristics of greenback flounder (Rhombosolea tapirina) in a large temperate estuary

The greenback flounder (Rhombosolea tapirina) is an important fishery species in southern Australia and New Zealand, whose demographic processes are poorly understood. This study developed an ageing protocol based on otolith interpretation and provided estimates of age-based data for an exploited population in the estuary of Australias largest river system. The otoliths fulfilled three criteria that established their usefulness for ageing. Estimates of size and age were used to generate von Bertalanffy growth curves for male and female fish sampled from two areas within the estuary. There were no differences in growth between the sexes, despite the strong bias in number towards female fish. Spatial differences in growth were marginal. Populations involved only a few young age classes. Such truncation may relate to the fishery, to movement of older fish out of the system, or to recent poor environmental conditions limiting spawning and early life survivorship.

Low mortality rates of juvenile Pacific oysters in the German Wadden Sea are characteristic for invasive species: a reply to Beukema and Dekker

Many marine invertebrate species have decreasing mortality rates with age (Gosselin and Qian 1997), which raised a question (Beukema and Decker 2009) on the low mortality we reported for juvenile PaciWc oysters in the East Frisian Wadden Sea (Schmidt et al. 2008). High fecundity is traditionally interpreted as a life history strategy to counteract high mortalities during early life stages, when unfavourable environmental conditions, competition or high predation can lead to a low recruitment success (Olafsson et al. 1994; Hunt and Mullineaux 2002). Beukema and Decker (2009) may therefore not be alone in doubting our values and we welcome the opportunity to clarify that such low juvenile mortalities are indeed a trait observable in the early stages of the invasion of marine invertebrates. Furthermore, as Gosselin and Qian (1997) also highlight, survival rates vary throughout the Wrst year which may necessitate diVerentiation in future studies. Prior to any evaluation of mortality, correct assessment of population parameters has to be assured. This can be confounded by sampling design and sampling errors, in particular where investigations are primarily Weld based, early life stages diYcult to detect, or where high mobility of juveniles can lead to import and export of individuals (Gosselin and Qian 1997). In our study, the establishment of PaciWc oysters (Crassostrea gigas) was studied along a 160 km stretch of intertidal coastline, with a dense net of sampling stations and a high rate of replication, yielding about 1,500 quadrate counts at each annual sampling occasion. All shell material found on the quadrates was visually examined for oyster spat in the Weld, carefully lifting the material oV the sediment, cleaning it from attached sediment and checking all sides of the shells. Even oysters as small as 1 mm could be identiWed, as they diVer in their colouration and structure from other epibionts (e.g. barnacles) or byssus threat attachments. We are thus conWdent that we accurately estimated the small oysters in the Weld. Retrieving material for cross-checks in the laboratory would not have been feasible as the overall goal of the monitoring scheme was to establish a long-term/large-scale data series on the population development of PaciWc oysters in this region. Therefore, Wxed GPS positions marked with bamboo stakes in the Weld were used to relocate the study sites. Beukema and Decker (2009) are concerned that morphological changes in mussel beds could account for variation in settlement substrates between years. Only well-established mussel beds with a stable topography were used for our surveys, documented through mussel monitoring (Herlyn and Millat 2004) and own assessment of the perimeter in 2003 and 2006 (Schmidt 2009; Wehrmann et al. 2009). Beukema and Decker (2009) are further concerned about the allocation to cohorts based on size-frequency distributions, yet the program FISAT II allows for potential overlap, applying standard procedures to diVerentiate cohorts using Bhattacharya’s method (Gayanilo et al. 2005). To corroborate the mortality rate based on the annual monitoring, we calculated mortality for the Wrst year from spatfall collectors deployed in the Weld (Schmidt 2009). Communicated by H.-D. Franke.