Katrin Berkenbusch

Factors influencing sediment turnover by the burrowing ghost shrimp Callianassa filholi (Decapoda : Thalassinidea)

Abstract Bioturbation by the burrowing thalassinidean shrimp Callianassa filholi (Milne-Edwards 1878) was studied at an intertidal sandflat in Otago Harbour, south-eastern New Zealand, over a period of 12 months. The amount of sediment expelled from shrimp burrows was measured each month (by direct entrapment over 24 h) and inhabiting shrimp were subsequently captured from the burrows. Rate of sediment expulsion was significantly related to seawater temperature, shore height of burrow and time, whilst the amount of sediment expelled from individual burrows was positively related to size of inhabiting shrimp but independent of sex. Annual sediment turnover for C. filholi was estimated at 96 kg (dry) m −2 year −1 which accounted for the variability imposed by monthly changes in seawater temperature and the spatial and temporal dynamics of the shrimp population. The results demonstrate the need to take account of physical and biological factors when constructing sediment turnover estimates for callianassid shrimps, and inferring their bioturbatory significance.

An examination of the spatial and temporal generality of the influence of ecosystem engineers on the composition of associated assemblages

The present study evaluated the generality of ecosystem engineering processes by examining the influence of sympatric burrowing shrimps (Callianassidae) and intertidal seagrasses (Zosteraceae) on benthic assemblage composition in two temperate regions, south-eastern New Zealand and north-western U.S.A. In each region, intertidal macrofauna assemblage composition was determined at sites of different burrowing shrimp/seagrass density and where both species co-occured, in three different size estuaries/tidal inlets, on two occasions. Results from both regions showed that the presence of shrimps and seagrasses consistently influenced the composition of the associated infaunal assemblages at all sites, in both summer and winter. Macrofauna assemblages at shrimp sites were significantly different to those at seagrass-only and mixed sites, whereas the composition of the latter sites was similar. The differences observed between sites were best explained by sediment variables. In New Zealand, % fines and seagrass debris showed the highest correlation to differences in assemblage composition, and in the U.S.A. % fines, % carbon and sediment turnover (by shrimp) appeared to be the most important environmental parameters measured. Four to six taxa exhibited the greatest discriminating significance (including corophiid amphipods, spionid polychaetes and oligochaetes) for dissimilarities in assemblage composition observed at the different sites, with generally lower abundances at shrimp than at seagrass sites. The present study highlights the functional importance of seagrasses and bioturbating shrimps as ecosystem engineers in soft-sediment environments, and reveals the generality of their influence on associated macro-invertebrate assemblages. The findings also allow for further development of a heuristic model for ecosystem engineering by shrimp and seagrass which indicate that numerical models that aim to explore the relationship between ecosystem engineer populations and habitat modification should be expanded to capture the interaction of co-occurring engineers and be both spatially and temporally explicit.

Intraspecific burrow plasticity of an intertidal population of Callianassa filholi (Crustacea: Decapoda: Thalassinidea) in relation to environmental conditions

Abstract Burrow morphology and intraspecific burrow plasticity in relation to environmental conditions were examined for an intertidal population of Callianassa filholi Milne‐Edwards, 1878 in southeastern New Zealand. Multivariate analysis of burrow features revealed no significant differences in burrow morphology between seasons. However, dissimilarities between burrows from different seasons could be identified and included differences in lateral extent, total length, and volume of burrows. These size parameters each showed low values in winter, coinciding with low seawater temperature and high organic content of the sediment. There was no significant relationship between any of the three size parameters and sediment grain size or shore height of the burrow, but lateral extent and total length of the burrow were significantly related to organic content of the sediment, decreasing with increasing organic content. Furthermore, there was a significant positive relationship between seawater temperature and total length of the burrow. Data from the present study imply that intraspecific variation in burrow morphology for callianassids is dependent on environmental variables, which has consequences for the construction of useful burrow models.

Population dynamics of the burrowing ghost shrimp Callianassa filholi on an intertidal sandflat in New Zealand (Decapoda: Thalassinidea)

Abstract Population samples of the burrowing ghost shrimp Callianassa filholi Milne-Edwards, 1878 were taken on an intertidal sandflat in Otago Harbour, south-eastern New Zealand, monthly over a period of 16 months. The spatial population distribution appeared to be contagious in autumn and early winter, and random from mid-winter to summer. The population density did not show any significant difference over the sampling period, with a mean density of 16 individuals m-2. Overall the sex ratio was significantly biased towards females (mean male:female ratio = 1:1.5). Visual inspection of size frequencies indicated the presence of a bimodal population. Large animals (7–14 mm carapace length [CL]) were present throughout the year and no significant difference in CL was observed between sexes. Post-larval shrimp (CL ≤ 3 mm) were present in spring and summer, which together with developing juveniles constituted a second group of smaller animals (< 7 mm CL). Regression analysis of allometric growth showed the s...

Lugworm (Abarenicola affinis) in seagrass and unvegetated habitats

Abstract In Otago, southern New Zealand, the lugworm Abarenicola affinis resides in neighbouring tidal inlets with and without seagrass (Zostera muelleri). A comparison of abundance, body size and biomass of A. affinis between seagrass habitat (Papanui Inlet) and unvegetated habitat (Hoopers Inlet) showed little seasonal variation of these parameters in each habitat and relatively similar abundances between both habitats. In contrast, lugworm biomass was considerably lower in the seagrass habitat due to the lack of large individuals compared with unvegetated habitat. In the seagrass habitat, there was a significant negative influence of Z. muelleri below-ground biomass on abundance and biomass of A. affinis, indicating that seagrass affected lugworm burrowing and/or feeding processes. In contrast to the unvegetated habitat, where lugworms spread relatively evenly across the intertidal area, lugworms were mostly restricted to the upper intertidal zone in the seagrass habitat. The findings suggest that the extensive seagrass bed in the mid and low intertidal zones of Papanui Inlet limited lugworm distribution in an otherwise suitable habitat. Whereas small lugworms colonised seagrass areas, the largest individuals occurred only in unvegetated sediment and seemed to be more hampered by the presence of seagrass than smaller individuals. The findings highlight negative feedback between antagonistic ecosystem engineers, with the potential of seagrass physical structures (autogenic engineering) to impact negatively on lugworm activity (allogenic engineering).

Distribution and abundance of Abarenicola affinis (Arenicolidae, Polychaeta) in tidal inlets of Otago, New Zealand

Lugworms are important members of the macrobenthos of sheltered intertidal sediments, and this study assessed the abundance and distribution of Abarenicola affinis in Otago, southern New Zealand. Using a faecal cast:lugworm ratio (1.00:1.09) revealed a patchy distribution across inlets, with an overall mean abundance of 11.1 individuals/m2 (range: 3.8 to 21.3 individuals/m2). Distribution patterns varied among inlets, with lugworms mostly confined to the high intertidal zone, but spread across all intertidal zones in one inlet. The lugworm habitat was characterised by muddy to medium sand (55–233 µm mean grain size) with low organic matter (< 2%) and chlorophyll a (1–12 µg/g dry sediment) contents. Depending on the inlet, lugworm abundance was related to sediment parameters, indicating that their distribution across Otago inlets is habitat dependent. The present findings also indicate that factors other than those measured contribute to A. affinis distribution and abundance patterns in this region.