Fitting a predator–prey model to zooplankton time-series data in the Gironde estuary (France): Ecological significance of the parameters

The relationships between the seasonal fluctuations of the copepod Eurytemora affinis and the mysid Neomysis integer were studied from observed data and experimental results, using a predator–prey model in the oligo-mesohaline area of the Gironde estuary. Mean seasonal fluctuations of abundances were derived from long term data series collected from 1978 to 2003 for both species. In situ predator–prey experiments over a seasonal cycle were used to estimate the seasonal variation of the consumption rate of N. integer on E. affinis and to verify the order of magnitude of the biological parameters given by the model.Predator–prey experiments revealed a high seasonal variation in maximum consumption rates with a mean of 56 ± 9 ind. pred⁻¹ d⁻¹. Maximum consumption rates were always higher for adults than for juveniles of Neomysis integer. Recorded selectivities were higher on nauplii than on copepodids + adults of Eurytemora affinis, both for the juveniles and the adults of N. integer. Neomysis integer mainly fed on meroplanktonic larvae, when they were available in higher abundances, than E. affinis in their environment.Spring increases of abundance for Eurytemora affinis copepodids + adults seemed to be mainly controlled by temperature whereas its decreasing abundance in summer was more related to Neomysis integer predation, suggesting that summer fluctuations of E. affinis abundance are probably controlled by mysid predation at summer times. Using a Lotka–Volterra predator–prey model, the seasonal peak of abundance of the mysid N. integer was well reproduced considering a predation on copepodids + adults of E. affinis, and suggested a dependence between mysid and copepod seasonal variations. However, the seasonal peak amplitude could not be explained solely by a predation on copepodids + adults or on nauplii of the copepod. Thus, N. integer is probably dependent on the seasonal fluctuations of the copepod's abundance, complementing its diet with macrophytal detritus during periods of scarce food.     

Diel Activity Patterns and Foraging Dynamics of the Sea Urchin Tripneustes ventricosus in a Tropical Seagrass Community and a Reef Environment (Virgin Islands)

Abstract. Feeding and movement activity patterns and the foraging behavior of the sea urchin Tripneustes ventricosus were investigated in a Thalassia testudinum seagrass bed and on a patch reef at St. Croix, U.S. Virgin Islands.
Most of T. ventricosus in the seagrass bed fed during both the day and night. Consumption of seagrass blades amounted to 1.4 g dw · individual^-1· d^-1 (mean sea urchin size 12.1 cm ambitus diameter). The population consumed approximately 3.6% of the daily seagrass production.
The movement of T. ventricosus was diel with high nocturnal activity levels and rates of locomotion, and little activity during daylight hours. Foraging activity was reduced under turbulent conditions. Movement rates were not governed by food availability. T. ventricosus travelled 3.7 m · d^-1 on the patch reef. Individuals in the seagrass bed moved 8.8 m · d^-1, most of which was accomplished during the night. The area traversed contained more than one hundred times the amount of food consumed daily. It is assumed that this behavior requires high energy expenditures for locomotion, thus yielding a low net energy profit from feeding.
The high rate of movement in seagrass beds may represent an innate behavioral adaptation to predation by night-active helmet conchs ( Cassis spp.). Fitness optimization by T. ventricosus may have been achieved by minimizing the predation risk during night hours. In an evolutionary context, survival by means of an increased energy expenditure for locomotion may compensate for the lower net energy gain from foraging.     

Trade-off between safety and feeding in the sea anemone Anthopleura aureoradiata

After a physical disturbance, such as a predatory stimulus, several marine organisms withdraw into a refuge to ensure survival. Consequently, they cannot engage in other essential activities. Re-emergence reflects a trade-off between avoidance of disturbance and lost feeding opportunities. The New Zealand sea anemone (Anthopleura aureoradiata) avoids predators by withdrawing its tentacles into its upper body. In a field experiment, responses of 480 A. aureoradiata to a physical disturbance were studied at three locations to assess the influence of the following factors on hiding time: physical contact with a neighbour; density of nearby conspecifics; total density per pool; and body size. Withdrawal times were significantly shorter for anemones in contact with a conspecific, occurring in high-density patches or of larger size. These patterns are consistent with rapid re-emergence being stimulated by competition for food from immediate neighbours and/or a lower risk of predation when in groups (i.e. a dilution effect), with larger size also potentially reducing vulnerability to predation.     

Habitat use patterns of the invasive red lionfish Pterois volitans: a comparison between mangrove and reef systems in San Salvador,Bahamas

The Indo‐Pacific red lionfish Pterois volitans is widespread both in its native and its non‐native habitats. The rapid invasion of this top predator has had a marked negative effect on fish populations in the Western Atlantic and the Caribbean. It is now well documented that lionfish are invading many tropical and sub‐tropical habitats. However, there are fewer data available on the change in lionfish abundance over time and the variation of body size and diet across habitats. A recent study in San Salvador, Bahamas, found body size differences between individuals from mangrove and reef systems. That study further suggested that ontogenetic investigation of habitat use patterns could help clarify whether lionfish are using the mangrove areas of San Salvador as nurseries. The aim of the present study is to determine temporal trends in lionfish relative abundance in mangrove and reef systems in San Salvador, and to further assess whether there is evidence suggesting an ontogenetic shift from mangroves to reef areas. Accordingly, we collected lionfish from mangrove and reef habitats and calculated catch per unit effort (a proxy for relative abundance), compared body size distributions across these two systems, and employed a combination of stable isotope, stomach content, and genetic analyses of prey, to evaluate differences in lionfish trophic interactions and habitat use patterns. Our results show that populations may have increased in San Salvador during the last 4 years, and that there is a strong similarity in body size between habitats, stark differences in prey items, and no apparent overlap in the use of habitat and/or food resources. These results suggest that there is not evidence an for ontogenetic shift from mangroves to reefs, and support other studies that propose lionfish are opportunistic forages with little movement across habitats.     

Responses to Predation Cues and Food in Two Species of Sympatric, Tropical Sea Urchins

Abstract. We examined the responses of two tropical sea urchins, Lytechinus variegatus and Tripneustes ventricosus, to cues from predators, simulated predation events and food. Cues released from damaged conspecifics, heterospecifics and heteroclassics (holothurian) were used to simulate predation events in field experiments. Responses to the presence of seagrass, spiny lobster or both were tested in tanks. Findings were supplemented by natural history observations of dispersion patterns off Bermuda. In field experiments, neither species formed groupings in the presence of conspecific, heterospecific or heteroclassic cues. Flight responses were greatest in conspecific treatments; responses to heterospecific cues were intermediate to control and conspecific cues. Urchins in pre‐assembled associations remained in groups in control trials but dispersed when exposed to predation cues. Lytechinus exhibited greater sensitivity to predation cues than Tripneustes. Cues from a damaged sea cucumber invoked a response from Lytechinus but not Tripneustes. Both species employed a two phased response to cues from damaged conspecifics: initially a rapid, but ephemeral (2 min), alarm response followed by a slower (≈ 35 % lower) sustained flight phase for 6+ min, which in nature would disperse urchins downstream and away from a predator. In tank experiments, Lytechinus formed groupings only around food or food + predators. The presence of a predator reduced the aggregation response to food, suggesting that Lytechinus employed a risk aversion strategy. Tripneustes exhibited escape or refractory behavior in both control and experimental treatments in laboratory tanks.