Wasp-waist populations and marine ecosystem dynamics: Navigating the “predator pit” topographies

Many marine ecosystems exhibit a characteristic “wasp-waist” structure, where a single species, or at most several species, of small planktivorous fishes entirely dominate their trophic level. These species have complex life histories that result in radical variability that may propagate to both higher and lower trophic levels of the ecosystem. In addition, these populations have two key attributes: (1) they represent the lowest trophic level that is mobile, so they are capable of relocating their area of operation according to their own internal dynamics; (2) they may prey upon the early life stages of their predators, forming an unstable feedback loop in the trophic system that may, for example, precipitate abrupt regime shifts. Experience with the typical “boom-bust” dynamics of this type of population, and with populations that interact trophically with them, suggests a “predator pit” type of dynamics. This features a refuge from predation when abundance is very low, very destructive predation between an abundance level sufficient to attract interest from predators and an abundance level sufficient to satiate available predators, and, as abundance increases beyond this satiation point, decreasing specific predation mortality and population breakout. A simple formalism is developed to describe these dynamics. Examples of its application include (a) a hypothetical mechanism for progressive geographical habitat expansion at high biomass, (b) an explanation for the out-of-phase alternations of abundances of anchovies and sardines in many regional systems that appear to occur without substantial adverse interactions between the two species groups, and (c) an account of an interaction of environmental processes and fishery exploitation that caused a regime shift. The last is the example of the Baltic Sea, where the cod resource collapsed in concert with establishment of dominance of that ecosystem by the cod’s ‘wasp-waist” prey, herring and sprat.     

Epimenia arabica spec, nov., a Solenogaster (Mollusca) Feeding on the Alcyonacean Scleronephthya corymbosa (Cnidaria) from Shallow Waters of the Red Sea

Abstract. The body organization and some biological data of Epimenia arabica , a new species of Sotenogastres of the order Cavibelonia , are presented. The species is described based on four large-sized specimens (13–21 cm × 0.7-1 cm) from three different localities off the coast of the Sinai Peninsula (Red Sea). It feeds on Scleronephthya corymbosa V erseveldt & C ohen ( Octocorallia: Alcyonacea ), generally at depths of 2–5 meters.     

Gastric Evacuation in the Juveniles of two Flatfish from a Western Mediterranean Bay

Abstract. The gastric evacuation (at 11 ^oC and 19 ^oC) and the relation between prey size and fish size of juveniles of Buglossidium luteum and Arnoglossus thori (Pisces, Heterosomata) are studied. The fish were collected at 20 m depth in the Banyuls Bay (France, Western Mediterranean). B. luteum showed a very rapid evacuation rate at 19 ^oC (2h to 4h for a IgDW meal) and a slower one at 11^oC (9h 30min for a IgDW meal). A. thori showed similar rates at both temperatures (6h 20min to 7 h 30 min for a IgDW meal). The relationship between prey size and predator size showed that A. thori always took larger prey than B. luteum. The diet of the latter changes only slightly with the increasing size of the fish. In contrast, the diet of A. thori changes more rapidly towards larger prey. A hypothesis explaining the contradictory evidence concerning the effect of fish and meal size on gastric evacuation is proposed.     

Skin toxins in coral‐associated Gobiodon species (Teleostei: Gobiidae) affect predator preference and prey survival

Predation risk is high for the many small coral reef fishes, requiring successful sheltering or other predator defence mechanisms. Coral‐dwelling gobies of the genus Gobiodon live in close association with scleractinian corals of the genus Acropora. Earlier studies indicated that the low movement frequency of adult fishes and the development of skin toxins (crinotoxicity) are predation avoidance mechanisms. Although past experiments showed that predators refuse food prepared with goby skin mucus, direct predator–prey interactions have not been studied. The present study compares the toxicity levels of two crinotoxic coral gobies – Gobiodon histrio, representative of a conspicuously coloured species, and Gobiodon sp.3 with cryptic coloration – using a standard bioassay method. The results show that toxin levels of both species differ significantly shortly after mucus release but become similar over time. Predator preferences were tested experimentally in an aquarium in which the two gobies and a juvenile damselfish Chromis viridis were exposed to the small grouper Epinephelus fasciatus. Video‐analysis revealed that although coral gobies are potential prey, E. fasciatus clearly preferred the non‐toxic control fish (C. viridis) over Gobiodon. When targeting a goby, the predator did not prefer one species over the other. Contrary to our expectations that toxic gobies are generally avoided, gobies were often captured, but they were expelled quickly, repeatedly and alive. This unusual post‐capture avoidance confirms that these gobies have a very good chance of surviving attacks in the field due to their skin toxins. Nonetheless, some gobies were consumed: the coral shelter may therefore also provide additional protection, with toxins protecting them mainly during movement between corals. In summary, chemical deterrence by crinotoxic fishes seems to be far more efficient in predation avoidance than in physical deterrence involving body squamation and/or strong fin spines.     

基于Tweedie-GAM模型研究海州湾小黄鱼资源丰度与栖息环境的关系

本研究根据2011年及2013—2016年春季和秋季在海州湾及其邻近海域进行的底拖网调查数据,结合同步采集的底层海水温度、底层海水盐度、水深、底质类型,以及脊腹褐虾(Crangon affinis)、细螯虾(Leptochela gracilis)、鳀(Engraulis japonicus)、赤鼻棱鳀(Thrissa kammalensis)等小黄鱼(Larimichthys polyactis)主要饵料生物的资源丰度数据,采用条件数κ和方差膨胀因子(VIF)度量多重共线性的程度,选取关键环境因子,再应用基于Tweedie分布的广义可加模型(GAM)研究不同季节和不同生长阶段的小黄鱼资源丰度与环境因子的关系。多重共线性的检验表明,所有初始变量之间没有显著的多重共线性,均可作为解释变量代入模型。结果表明:不同季节和生长阶段,影响小黄鱼资源分布的主要因子及其偏差解释率各不相同,各变量所对应的适宜范围也不同。例如:影响春季小黄鱼幼体资源分布的主要因子有底层海水温度、底层海水盐度、水深和脊腹褐虾的分布,其中偏差解释率最大的因子为水深(16.09%);而影响春季成体资源分布的因子为底层海水温度、底层海水盐度、水深及脊腹褐虾和鳀的分布,其中偏差解释率最大的因子为底层海水盐度(13.56%)。本研究表明,海州湾及其邻近海域不同季节和不同生长阶段小黄鱼的资源分布与其自身的生态习性、海洋环境以及饵料生物的分布密切相关。