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It has been postulated that mound-building callianassid shrimp and seagrass have mutually negative effects on each other. On reef flats in the Spermonde Archipelago, Indonesia, sediment mounds of callianassid shrimp are rare in wave exposed seagrass meadows, occur frequently in sheltered seagrass meadows, and reach their highest densities (4.2 mounds m− 2) in unvegetated subtidal areas. There sediment turnover is high at an estimated 3408 g DW m− 2 ∗ d− 1. Based on collected specimens and burrow casts, the most important bioturbators are Glypturus armatus within seagrass beds and Neocallichirus vigilax in unvegetated subtidal areas. Six shrimp exclusion, six control and six zero treatment plots were set up in an unvegetated subtidal N. vigilax bed. Half of the plots for each treatment were ca. 1.5 m (“shallow”) and the other half ca. 2 m or slightly more (“deep”) below spring low tide level. The survival of transplanted shoots of the six seagrass species Enhalus acoroides, Thalassia hemprichii, Cymodocea rotundata, Halodule uninervis, Syringodium isoetifolium and Halophila ovalis was monitored over 27 months. At the end, E. acoroides had survived in about half of the plots with no obvious relation to either treatment or water depth. C. rotundata and H. uninervis had successfully established themselves on the shallowest exclusion plot only, and had disappeared from all other plots. T. hemprichii and S. isoetifolium had disappeared from all plots. H. ovalis expanded rapidly in the dry seasons, but wet seasons caused a temporary decline in shoot numbers. At the end, the highest shoot numbers were counted in shallow exclusion plots, some shoots survived in deep exclusion plots and the species disappeared from all deep control and zero treatment plots. H. ovalis shoot numbers were significantly influenced by water depth (ANOVA p < 0.01) and, to a lesser extent, by treatment (ANOVA p < 0.05) but there was no interaction between the two factors. It is concluded that seagrasses are only controlled by shrimp bioturbation at the lower limit of their distribution in sheltered subtidal areas. Here they are already under considerable stress from a combination of factors such as light limitation and sedimentation. However, the distribution pattern of seagrasses vs. shrimp elsewhere on the islands suggests that seagrasses are equal competitors in sheltered intertidal areas, where they may even profit from the abundant small tidal pools between mounds created by the shrimp, and dominant competitors in exposed intertidal and subtidal areas.

Seagrass meadows support abundant and diverse fish assemblages, but there are very few studies on the relation between seagrass beds with distinctly different plant canopies and their associated fish fauna. In the present study, fish assemblages were investigated by underwater visual census at intertidal and subtidal sites with varying seagrass species composition, shoot density, biomass, and leaf area index (LAI) on two small coral islands in the Spermonde Archipelago, Indonesia. We investigated (1) whether fish assemblages in distinctly different seagrass beds differ regarding community parameters, and (2) whether seagrass parameters affect fish abundances. Overall, more than 120 fish taxa were found. Bray–Curtis cluster analysis and non-Metric Multidimensional Scaling ordination (nMDS) showed site-specific similarities for fish assemblage structure with a distinct separation into subtidal and intertidal sites. Species accumulation curves for gamma diversity, single study sites, and the two most diverse fish families (Labridae, Pomacentridae), suggesting that species numbers are likely to increase with diel sampling. Total fish abundance and abundance for six out of the nine most common species varied distinctly among the study sites. The study indicates that seagrass beds with differing canopy parameters support distinct fish assemblages that differ with respect to species richness, dominant species, and the abundance of total and most common fish species. These differences are likely due to different canopy structures and water depth.

The feeding habit of Austrolethops wardi (Gobiidae) in the seagrass beds of Barrang Lompo and Bone Batang Island in the Spermonde Archipelago, South West Sulawesi, Indonesia, was investigated through gut content analysis. The feeding preferences of this species are very similar on both islands: A. wardi, a burrow associate of Neaxius acanthus, was found to feed almost exclusively on seagrass (which was found in 100% of the investigated stomachs and made up >94% of food items). However, seagrass epiphytes (

Burrows of the thalassinidean shrimps Neaxius acanthus and Corallianassa coutierei are striking aspects in tropical seagrass beds of the Spermonde Archipelago, Indonesia. Burrow construction, behaviour, burrow type and associated commensal community were investigated to clarify the ecological role and food requirements of these shrimps and their commensals. Gut content analysis and stable-isotope data were used to unravel the food sources and the trophic interactions among the commensal community. Individuals of Neaxius acanthus were caught on Bone Batang Island. In narrow aquaria filled with sediment they constructed burrows resembling those found in the field. During burrow construction and maintenance only little sediment was brought to the surface, most was sorted and compacted to create a distinct lining. Maintenance work by single shrimps typically took about 5 min, after which the shrimp walked up to the entrance and rested for a similar period of time. There were no differences in behaviour between day and night. Intrasexual encounters inside the burrow were characterised by a high level of aggression and all resulted in one participant being driven out of the burrow. Intersexual encounters led to coexistence with both animals taking turns in burrow maintenance and guarding the entrance. Offered seagrass leaves were pulled underground, cut into pieces and eventually integrated into the lining. Burrows of Corallianassa coutierei resembled a deep U-shape. Chambers branching off halfway down and at the deepest point contained seagrass fragments. All steep parts of the burrow were lined similar to burrows of N. acanthus. No commensals were found associated with Corallianassa coutierei. However, burrows of Neaxius acanthus in the field typically contained a pair of shrimps, up to 8 individuals of the commensal bivalve Barrimysia cumingii and large numbers of gammarid amphipods. Other animals found associated with the burrow were the goby Austrolethops wardi, a palaemonid shrimp species and two species of tube-building polychaetes, one of which was also found as an epibiont on N. acanthus. Stable-isotope and gut content analyses indicate that the diet of Neaxius acanthus, its commensal Austrolethops wardi, and Corallianassa coutierei is mainly derived from detrital seagrass leaves, with a potential contribution of sediment organic matter and seagrass epiphytes. In contrast the isotopic signature of Barrimysia cumingii suggests the presence of symbiotic sulphide metabolism bacteria. This study underlines that, besides their interactions with the surrounding ecosystem, thalassinid shrimp burrows play an important role as a sub-habitat with a unique associated fauna.

The abundance, burrow characteristics, and in situ behaviour of the burrowing shrimps Neaxius acanthus (Decapoda: Strahlaxiidae) and Alpheus macellarius (Decapoda: Alpheidae) were studied to quantify the collection of seagrass material, to identify the fate of this collected material, and to determine the importance of these burrowing crustaceans in the nutrient (nitrogen and phosphorus) cycling of two tropical seagrass meadows on Bone Batang, South Sulawesi, Indonesia. Alpheus macellarius harvested 0.70 g dry weight (DW) burrow −1 d −1 seagrass material, dominantly by active cutting of fresh seagrass leaves. Neaxius acanthus collected 1.66 g DW burrow −1 d −1 , mainly detached leaves which floated past the burrow opening. The A. macellarius and N. acanthus communities together collected in their burrows an amount of seagrass leaf material corresponding to more than 50% of the leaf production in the meadows studied. The crustacean species studied might therefore fulfil an important function in the nutrient cycling of tropical meadows. In the burrow most of the collected material is shredded into pieces. The burrows of both species had special chambers which serve as a storage for seagrass leaf material. Neaxius acanthus incorporated most of the material into the burrow wall lining, which is made of small sediment particles and macerated seagrass leaves. Phosphate concentrations measured in N. acanthus burrows compared with pore-water and water-column concentrations suggests that a substantial amount of the seagrass material undergoes decomposition in the burrows. Oxygen levels measured in these water bodies are indicative for a possible exchange of water between the burrow and its surroundings, most likely supported by the shrimps irrigating their burrows. By collecting leaf material in their burrows, nutrients that are otherwise lost from the seagrass meadow associated with detached leaves and leaf fragments carried away in the water column, are maintained in the meadow and may form an important source of recycled nutrients.