An integrated study of the temporal and spatial variation in the supply of propagules, recruitment and assemblages of intertidal macroalgae on a wave-exposed rocky coast, Victoria, Australia

Recruitment is known to influence distributions and abundances of benthic marine organisms. It is therefore important to document patterns of variability in recruitment and how these relate to patterns in established assemblages. This study provides an integrated assessment of the temporal and spatial variation in supply and recruitment of propagules and established populations of several macroalgae. Propagules in water samples from two stages of the incoming tide, recruitment to artificial substrata and percentage cover of species established on the shore were recorded every 2 months from December 1994 to October 1995, in two zones of an intertidal, wave-exposed rocky shore. Variability in recruitment was measured at three spatial scales: 10s cm, 100s cm and 100s m. Availability and recruitment of most taxa were greatest between April and August, although many species had available propagules and recruited throughout the year. Temporal variation in the established assemblages was, however, more species-specific. Differences in established assemblages between zones were reflected in differences in availability and recruitment of propagules between zones. Recruitment could not be predicted directly from supply of propagules, but the two processes were linked. For most species, the greatest variation in recruitment occurred at the smallest spatial scale of 10s cm, although there was also considerable large-scale (between site) variation in recruitment of several species. Results indicate that while pre-and post-settlement mortality are likely to influence macroalgal distribution and abundance, the temporal and spatial variability in supply and recruitment of propagules can explain much of the patchiness in macroalgal assemblages.

Are intertidal invertebrates adequately protected in Victoria?

Intertidal invertebrates are under pressure from both direct and indirect threats, the most serious being from over-exploitation, habitat loss and alteration, decline in water quality, introduced species and pathogens, and global warming. This article explores the current protective mechanisms in place that provide some protection to intertidal invertebrates in Victoria and looks at a case study of Western Port, Victoria. The results of the case study indicated a general consensus that intertidal invertebrates are not adequately protected in Victoria. The recommended actions to ensure adequate protection in the future are an amendment to current legislation, increased education and greater law enforcement, and further implementation of the concept of integrated coastal zone management.

No outbreeding depression at a regional scale for a habitat-forming intertidal alga with limited dispersal

Hormosira banksii is distributed throughout southern Australasia, but dispersal of propagules is thought to be limited. In the present study, the hypothesis that outbreeding depression occurs in H. banksii was tested by assessing fertilisation success and early development of embryos in crosses between populations at local to regional spatial scales. Hierarchical experiments were conducted at three spatial scales with nesting present within each scale: small scale (within a rocky shore population), intermediate scale (regions separated by 70 km) and large scale (450-km separation between two states: Victoria and Tasmania). In each experiment, eggs and sperm were crossed within and between each population located in the spatial scale of interest. There were no consistent patterns of variable fertilisation success and subsequent development within a population or at different spatial scales. It was concluded that outbreeding depression is not detected in analyses of fertilisation success or early development processes in H. banksii. The results suggest one of the following to be likely: (1) H. banksii is capable of longer distance dispersal than previously considered, thus maintaining gene flow between distant populations, (2) gene flow is restricted by limited dispersal, but populations have not been isolated for a sufficient length of time to cause genetic divergence or (3) outbreeding depression is manifested as effects on later life-history stages.

Small-scale temporal variation in propagule supply of an intertidal red alga

Information on the variability in supply of algal propagules is scarce, hindered by the difficulty in identifying propagules, but this variability may affect the distribution and abundance of algal assemblages. This study examined the small-scale (½ hourly to hourly) temporal variation in propagule supply of Chondrus verrucosus (Gigartinaceae, Rhodophyta) over a dense, isolated bed in south-eastern Japan in summer and winter of 1999. Either 0.5 litre scoop samples or 5 litre pump samples were collected ½ hourly to hourly over 13, 22.5, and 30 h on three occasions in summer (June & July) and 32 h on one occasion in winter (December). Sampling was conducted around either the new moon (two occasions in summer) or full moon (one occasion in both summer and winter) and incorporated full tidal sequences including daytime (summer) and nighttime (winter) low-low (LL) tides. Chondrus verrucosus was the only red alga with spores within the size range of 15–20 μm that was fertile in the study area and surrounds at the time of sampling facilitating identification of spores. Spores in scoop samples were settled onto Petri dishes and identified on the basis of cell shape, colour and size. Pump samples were filtered onto transparent membrane filters and identified using epifluorescence microscopy: C. verrucosus spores fluoresced bright yellow and were easily distinguished from other micro-organisms of similar size, which fluoresced red or green. Results showed that while propagules could be found in the water column at most times, propagule supply of C. verrucosus was greatest during the 1–2 h period following LL tides. Variability in propagule supply was less than in previous studies examining surface or offshore waters. Spore release is thought to be stimulated by either desiccation or salinity changes associated with periods of emersion at low tide followed by re-immersion on incoming tides.

Passive flow through an unstalked intertidal ascidian: Orientation and morphology enhance suspension feeding in Pyura stolonifera

Passive flow is believed to increase the gains and reduce the costs of active suspension feeding. We used a mixture of field and laboratory experiments to evaluate whether the unstalked intertidal ascidian Pyura stolonifera exploits passive flow. We predicted that its orientation to prevailing currents and the arrangement of its siphons would induce passive flow due to dynamic pressure at the inhalant siphon, as well as by the Bernoulli effect or viscous entrainment associated with different fluid velocities at each siphon, or by both mechanisms. The orientation of P. stolonifera at several locations along the Sydney-Illawarra coast (Australia) covering a wide range of wave exposures was nonrandom and revealed that the ascidians were con- sistently oriented with their inhalant siphons directed into the waves or backwash. Flume experiments using wax mod- els demonstrated that the arrangement of the siphons could induce passive flow and that passive flow was greatest when the inhalant siphon was oriented into the flow. Field exper- iments using transplanted animals confirmed that such an orientation resulted in ascidians gaining food at greater rates, as measured by fecal production, than when oriented perpendicular to the wave direction. We conclude that P. stolonifera enhances suspension feeding by inducing pas- sive flow and is, therefore, a facultatively active suspension feeder. Furthermore, we argue that it is likely that many other active suspension feeders utilize passive flow and, therefore, measurements of their clearance rates should be made under appropriate conditions of flow to gain ecolog- ically relevant results.

Do reproductive tactics vary with habitat heterogeneity in the intertidal sea anemone Actinia tenebrosa?

Cnidarians display a diverse range of reproductive tactics including sexual and asexual modes of reproduction. Although few studies have looked for intraspecific variation in reproductive tactics, flexible expression of such life-history traits may be favoured in species that occupy a range of habitats. We tested this in the sea anemone Actinia tenebrosa by comparing cycles of reproductive activity and the mode of production of brooded larvae in local populations occupying boulder fields and stable rock platforms. We determined the mode of production of broods from eight rock platforms (separated by up to 1600 km) and two boulder shores on the south east coast of Australia using a combination of allozyme data and four newly characterised microsatellite markers.

We determined seasonal patterns of brooding and gonad development by monthly dissection of 15–30 adults from each of two boulder fields and two stable platforms. Previous genetic studies have shown that populations of A. tenebrosa on rock platforms can be highly clonal, whereas anemones on more heterogeneous boulder habitats display levels of genotypic diversity similar to that expected for sexual reproduction. We genotyped a total of 221 juveniles from 37 brooding adults including 11 broods and 80 juveniles from boulder shores. We did not detect any evidence of sexual production of broods. All brooded juveniles displayed identical multi-locus genotypes to their brood parent irrespective of habitat of origin or location, including 28 broods (200 juveniles) that were heterozygous at one or more locus. Similarly, we found that temporal patterns of gonad formation and brooding were consistent across habitats and locations. We detected 346 mature males, 234 non-reproductive or immature individuals, and no mature females within a total of 580 dissected individuals. These data suggest that the reproductive tactics of A. tenebrosa are essentially fixed and that variation in the genotypic diversity of populations may reflect variation in factors such as the input of sexually derived planktonically dispersed recruits or post-settlement processes. However, the apparent lack of females paradoxically implies that sexual reproduction, and hence recruitment, must be rare or no longer possible within some populations, and highlights the need for long-term studies of these populations.

Dislodgment and attachment strength of the intertidal macroalga Hormosira banksii (Fucales, Phaeophyceae)

Hormosira banksii is a dominant intertidal alga characterised by limited dispersal of propagules, yet it has a broad distribution throughout temperate Australasia. However, the high abundance of beach wrack of H. banksii on sandy beaches along southwest Victoria, the frequency of frond dislodgment and the force required to break it from the substratum remain unknown. Dislodgment of macroalgae has been shown to influence dispersal processes; therefore, we tested the model that long-distance dispersal of H. banksii is facilitated by weak attachment to the substratum and frequent dislodgment of fertile fronds. We monitored the dislodgment of H. banksii individuals and conducted in situ pull tests to determine the attachment strength of fronds of H. banksii. We further tested whether thallus size or pull direction influenced attachment strength of H. banksii. We found that breakage of vesicles and fronds was a regular event during the survey period and that the mean attachment strength of fronds of H. banksii was weak compared to other intertidal algae. Furthermore, we found that thallus size did not influence the force required to break fronds of H.banksii from the substratum but that the direction that fronds were pulled from the substratum did. We suggest that fronds of H. banksii do drift frequently and that long-distance dispersal is likely to be an important mechanism for the distributional success of this species.

Distributional overlap rather than habitat differentiation characterizes co-occurrence of bivalves in intertidal soft sediment systems

Diverse species assemblages are often associated with a diversity of habitat structures. Sedimentary systems seem to be no exception, as within sedimentary systems benthic species diversity within a sample point appears to correlate with sediment grain size complexity. However, it remains to be shown whether total benthic species diversity relates to a system’s sediment heterogeneity across multiple systems. In the present paper we examined whether bivalve diversity is associated with: (1) sediment heterogeneity across systems and (2) sediment grain size complexity within systems, at 9 temperate and tropical tidal flat systems. Although bivalve life-history strategies, like post-settlement habitat selection, might suggest that sediment heterogeneity should be important for bivalve species, bivalve diversity and sediment heterogeneity were not associated across systems. Interestingly, the association between total benthic diversity and sediment heterogeneity was also not significant, suggesting that changing species composition across systems does not account for the lack of a correlation between bivalve diversity and sediment heterogeneity. Instead of habitat differentiation, bivalve diversity within a sample point was highest in ‘complex’ fine-grained sediments and bivalve distributions showed a large degree of distributional overlap in all systems. The results of this study at both smaller and larger spatial scales suggest that coexistence between bivalve species in diverse tidal flats is not associated with increased sediment heterogeneity.

The Bassian Isthmus and the major ocean currents of southeast Australia influence the phylogeography and population structure of a southern Australian intertidal barnacle Catomerus polymerus (Darwin)

Southern Australia is currently divided into three marine biogeographical provinces based on faunal distributions and physical parameters. These regions indicate eastern and western distributions, with an overlap occurring in the Bass Strait in Victoria. However, studies indicate that the boundaries of these provinces vary depending on the species being examined, and in particular on the mode of development employed by that species, be they direct developers or planktonic larvae dispersers. Mitochondrial DNA sequence analysis of the surf barnacle Catomerus polymerus in southern Australia revealed an east–west phylogeographical split involving two highly divergent clades (cytochrome oxidase I 3.5 ± 0.76%, control region 6.7 ± 0.65%), with almost no geographical overlap. Spatial genetic structure was not detected within either clade, indicative of a relatively long-lived planktonic larval phase. Five microsatellite loci indicated that C. polymerus populations exhibit relatively high levels of genetic divergence, and fall into four subregions: eastern Australia, central Victoria, western Victoria and Tasmania, and South Australia. F_ST values between eastern Australia (from the eastern mitochondrial DNA clade) and the remaining three subregions ranged from 0.038 to 0.159, with other analyses indicating isolation by distance between the subregions of western mitochondrial origin. We suggest that the east–west division is indicative of allopatric divergence resulting from the emergence of the Bassian land-bridge during glacial maxima, preventing gene flow between these two lineages. Subsequently, contemporary ecological conditions, namely the East Australian, Leeuwin, and Zeehan currents and the geographical disjunctions at the Coorong and Ninety Mile Beach are most likely responsible for the four subregions indicated by the microsatellite data.