Luc Brendonck

Species sorting in space and time—the impact of disturbance regime on community assembly in a temporary pool metacommunity

Abstract Ecosystems generally are not stable over long periods of time and are subject to disturbances of different frequencies and intensities. As a result, natural communities usually are not in equilibrium, and temporal snapshots can show different stages of community assembly. Nonequilibrium is especially common in temporary aquatic systems where desiccation continuously resets the development of communities and hydroperiod is an important determinant of species richness and community structure. We investigated effects of disturbance regime on community assembly of aquatic invertebrates in a cluster of 36 temporary rock pools in central South Africa. Pools were assigned to 4 categories based on their long-term disturbance regime and were sampled 4 times during 1 inundation. Disturbance modulated the relative importance of deterministic and stochastic processes driving community assembly and reduced both α and β diversity. Premature desiccation truncated community development. Initial communities were dominated by resident large branchiopods (fairy shrimp and clam shrimp). Arrival of flying colonists, including many predators, caused a drastic change in community composition and initiated a new successional phase dominated by cladocerans. For residents, the link between local environment and community structure was strongest early during inundation and decreased during community assembly. For flying colonists, variation in arrival time and habitat selection led to a better match between communities and environment later during inundation. We conclude that the combination of colonization–extinction dynamics and niche partitioning in space and time determines metacommunity structure in this extreme type of aquatic habitat.

The use of manipulated baker's yeast as an algal substitute for the laboratory culture of Anostraca

The production of unicellular algae is laborious and is a major constraint for the culturing of aquatic filter-feeders. Because of their small particle size and their high protein content yeasts are considered as a promising substitute for micro-algae. Furthermore, recent work has shown that bakers yeast can be converted into a digestible diet for Artemia by chemical treatment. The present study documents the use at laboratory scale of this manipulated yeast as an algal substitute for the culture of two anostracan species.The experiments were conducted with the brine shrimp artemia franciscana and the fairy shrimp Streptocephalus proboscideus. A similar experimental set-up was used for both species. The algal diet, consisting of Dunaliella tertiolecta for A. franciscana and Selenastrum capricornutum for S. proboscideus, was substituted at various levels by two types of treated bakers yeast: a fresh form and a dried product which was rich in highly unsaturated fatty acids (HUFA).The chemically-treated yeast offers promising possibilities as an algal substitute for Artemia; i.e. replacing 75% of the algae by the dried yeast resulted in similar survival and even higher growth rates in comparison with the reference algal diet; for the treated fresh yeast similar results could be achieved by up to 95% substitution. For S. proboscideus, a substitution of 75% by either of the yeast products resulted in good survival, though growth did not exceed 80% of the observed growth in the algal control. A diet consisting solely of yeast resulted in poor survival for larvae of both species. Experiments were run to investigate whether this was due to a sub-optimal feeding regime, nutritional deficiencies, or deterioration of the water quality.

Feeding in the Fairy Shrimp Streptocephalus Proboscideus (Frauenfeld) (Branchiopoda: Anostraca). I. Aspects of the Feeding Biology

ABSTRACT At 25°C and in a suspension of about 1,000,000 carmine particles per ml, the thoracopods of Streptocephalus proboscideus beat metachronally with a frequency of about 4.3 Hz. The phase difference between successive thoracopods is about one-eighth of an oscillation. For every particle concentration tested, retention time was lower in adult females (maximum 61 ± 19 min, minimum 30 ± 3 min) than in males (maximum 72 ± 23 min, minimum 47 ± 15 min) of equal size. The speed of transport through the gut was dependent on particle concentration; with increasing particle density, retention time decreased, but increased again at the highest densities tested. The shortest retention time of carmine particles was observed at a concentration of 1,000,000 particles per ml; for yeast and Selenastrum, this concentration was 100,000 cells per ml. At the latter concentration, maximum ingestion rate also was observed. After 5 min, ingested particles were seen in the abdominal region, where the speed of transport greatly decreased. In adults, no preference between algal species and yeast was observed. Instead, distinct particle size preferences were observed among different size classes. The maximum size of ingested glass spheres was 17.2 μm in larvae and 67.1 μm in adults, probably a function of the size of the mouth.

Feeding in the Fairy Shrimp Streptocephalus Proboscideus (Frauenfeld) (Branchiopoda: Anostraca). Ii. Influence of Environmental Conditions on Feeding Rate

ABSTRACT The relationships of filtration and ingestion rates at different food concentrations were analyzed in individuals of different sizes in the fairy shrimp Streptocephalus proboscideus and ingestion rate determined in relation to sex, animal density, light and darkness, culture medium, and water quality at one food concentration. Maximum ingestion rates of about 1,000,000 cells animal-1 h-1 in adult males (13-14 mm) were measured for food concentrations between 50,000 and 500,000 algal cells (Selenastrum capricornutum) ml-1. At higher food concentrations, ingestion rates dropped. For the largest size class tested (13.7 mm), maximum ingestion rate (about 820,000 cells animal-1 h-1) was more than 10 times higher than in the smallest size class (4.4 mm) and was reached at a lower cell concentration (about 80,000 cells ml-1) than in the smallest animals (about 280,000 cells ml 1). Significant sexual differences in ingestion rates were observed in SRW (P < 0.05) and EEC (P < 0.01) synthetic fresh-water media for the 12-h measurements and in SRW medium for the 3-h experiments (P < 0.05). These results, however, were not confirmed by a third series of 3-h experiments in SRW medium. Significant influences of culture medium on feeding rates were observed in females only. The ingestion rate in darkness (1,435,000 ± 1,118,000 cells animal-1 h-1) was not significantly different from that in light (1,120,000 ± 812,000 cells animal-1 h-1). Increasing the density to more than 10 animals in 500 ml of medium had a negative effect on the feeding rate. High NH3-N and NO2-N concentrations had an inhibitory effect on food uptake; interpolated EC50 values were approximately 4 mg 1-1 and 60 mg 1-1, respectively.

Faunistics of ephemeral rock pools in southeastern Botswana

Abstract: Rock pools on granite outcrops occur worldwide and are poorly studied,despite their intrinsic biological interest. In semi-arid Botswana, such habitats occurmainly on the granite outcrops in the southwestern Hardveld zone. To date, studies onthese systems have focused mainly on individual species or particular interactions. Bymeans of frequent sampling (every other day) during an entire wet phase (hydrocycle),we attempted to get a time integrated overview of invertebrate species composition ina set of 18 rock pools from two clusters (meta-communities).A faunal list is presentedand described. Rock pool species were separated in permanent and ephemeral inhabi-tants, based on their strategy to survive or escape the frequent dry phases of their habi-tat, respectively. A new chydorid species, four new turbellarian taxa and two new ost-racod species were discovered. All new taxa were permanent inhabitants, illustratingthe need for more intense and time-integrated studies of these ephemeral systems andespecially the permanent residents with specific adaptations to the vagaries of theirvariable habitat. The best sampling strategy to assess species richness in these rockpool systems is to randomly sample three to four pools in a cluster, each in the finalphase of their hydrocycle.Key words: rock pool, ephemeral, fauna, sampling.

On potentials and relevance of the use of copulatory structures in anostracan taxonomy

This paper reviews and extends the knowledge of anostracan penile morphology and its taxonomic significance. Since Linder‘s pioneering classification(based partly on male genital characters), the morphology of both basal and distal penile parts has been applied repeatedly to reorganize or establish higher anostracan taxa, or to extend their diagnoses. In general, the configuration of both basal and distal penile structures follows a constant pattern in well-defined genera. Size and shape of processes and spinulation on the basal (in Linderiella and Chirocephalidae) and the distal parts (in Parartemia), however, may show inter-specific differences. Variability in penile structures in the current genus Brachinella indicates the need for a taxonomic revision of this genus. Similar intra-generic penis morphology may contribute to hybridizability of congeners and validate Dubois‘genus concept for use in anostracans.

OCCURRENCE OF TETRAHEDRAL EGGS IN THE STREPTOCEPHALIDAE DADAY (BRANCHIOPODA: ANOSTRACA) WITH DESCRIPTIONS OF A NEW SUBGENUS, PARASTREPTOCEPHALUS, AND A NEW SPECIES, STREPTOCEPHALUS ( PARASTREPTOCEPHALUS ) ZULUENSIS BRENDONCK AND HAMER

ABSTRACT Streptocephalus (Parastreptocephalus), new subgenus, comprises *Streptocephalus (Parastreptocephalus) sudanicus, S. (P.) lamellifer. S. (P.) kaokoensis, and S. (P.) zuluensis, new species, Brendonck and Hamer. These species all produce tetrahedral eggs, a unique feature within the family Streptocephalidae, and share a similar male antennal morphology, which differs from that in all other streptocephalids. The designated type species, S. (P.) sudanicus, is extensively redescribed. In addition, the differentiating characters for S. (P.) zuluensis are presented. Both species can be distinguished using features of the morphology of their resting eggs and of the male antennae. A key to the species is also provided. Egg morphology and male antennal morphology appear to be consistent taxonomic criteria for the proposed subgenus. The presence of taxon-related differences in egg morphology raises the question whether or not differently shaped eggs confer any selective advantage. We suggest that selective pressure on the amount of energy allocated to reproduction has resulted in the fact that eggs share a combination of characteristics (hatching response, dispersal ability, drought, and mechanical resistance), which allows them to endure a specific set of environmental conditions.

A culture system for fairy shrimps (crustacea, anostraca)

Abstract A laboratory system for the semi-automatic culture of fairy shrimps is described and evaluated. The design is directed towards an automation of both the algal food culture and the feeding of the organisms themselves. The fairy shrimps are reared in conical tanks, equipped with a recirculation system consisting of a sand filter. Deposited cysts are collected in a separate compartment, which is easily removed for further processing of the harvest. Streptocephalus proboscideus, a subtropical species, was reared at an initial density of 56 animals/litre. Several physico-chemical and biological variables were monitored at regular intervals and compared to the results derived from a static/renewal system with an initial density of 30 individuals/litre and with discontinuous food supply. The present recirculation system proved to be a reliable technique in maintaining good rearing conditions, assuring low mortality rates (±5% per week) and a high cyst production (±200 cysts per brood cycle) in the experimental fairy shrimp population (sex ratio 1 1 ), opening possibilities for several types of applications in aquaculture and aquatic toxicology. Total production yielded an average of 18 000 cysts per day and per culture unit (volume: 61).

Anostracan monopolisation of early successional phases in temporary waters

The length of the hydrocycle restrains the development of communities in temporary aquatic habitats. Little information is available on the importance of biotic interactions in shaping the communities in these temporary habitats. A common observation in the freshwater rock pool community assembly process is the clearly separated density peaks over time of fairy shrimp (Anostraca) and cladocerans (Anomopoda). Fairy shrimp (Anostraca), the fl agship group and dominant fi lter feeders of temporary waters hatch early after inundation. This contrasts with the occurrence pattern of coexisting cladocerans, which despite higher chances of abortive hatching occur later during the hydroperiod. To study this apparent succession pattern and test the possible presence of a biotic interaction, we set up enclosure experiments in freshwater rock pools in Western Australia and monitored cladoceran (Macrothrix hardingii) densities in the presence of variable densities or absence of fairy shrimp (Branchinella longirostris). The anomopod cladoceran populations decreased with increasing anostracan densities revealing a negative biotic interaction. The competitive dominance of anostracans most probably was mainly due to exploitation competition, with possibly elements of interference competition or predation included. Early hatching and competitive superi- ority may help anostracans in temporary pools to monopolize the habitat with respect to small and/or other fi lter feeding invertebrates. As such they infl uence succession dynamics and invertebrate community structure, making anostracans keystone species of temporary pools.

Impact of hydroperiod on seasonal dynamics in temporary pool cladoceran communities.

We studied seasonal dynamics in cladoceran communities in relation to changing habitat characteristics and biotic interactions in 18 temporary freshwater pools in Kiskunsag (Hungary), with an inundation period varying from less than ten weeks (short) to 10-13 weeks (medium long) and more than 16 weeks (long). Pools were sampled for cladocerans and macroinvertebrates and habitat characteristics were monitored starting two weeks after snow melt (end of March 2005) until July. Analysis of early-season dynamics (March-April), when all three hydroperiod classes were represented, did not reveal differences in cladoceran communities recolonizing pools with a different hydroperiod. Species turnover later in the season (May - July) resulted in the association of some species with the medium and long hydroperiod class. Cumulative species richness increased with hydroperiod length from an average of ten species in short hydroperiod to 15 species in long hydroperiod pools. The longer time available most likely allowed more species to gradually occupy available niches in pools with a longer hydroperiod. With time, feeding and growth conditions became more favorable for predominantly small and/or plant-associated species, while the absence of large Daphnia may have been due to predation risk. A decline of pools with a long inundation cycle, through drainage and/or climate change, may adversely affect cladoceran communities and needs particular attention in the conservation of temporary pools and their inhabitants.