We modified Hamilton's (1971) selfish herd model by introducing directional movement to the prey groups and the predators. The consequences of this modification with regards to differential predation risks are compared to Hamilton's original model (using stationary prey groups) and tested against empirical data. In model 1, we replicated Hamilton's original predator-prey system. In models 2 and 3, prey groups were mobile and predators were mobile (model 2) or stationary (model 3). Our results indicate that additional to the positive risk gradient from centre to periphery predicted by Hamilton's model for stationary groups, there might be another positive risk gradient from the rear to the front part in moving groups. Furthermore, models 2 and 3 suggest that moving groups should generally exhibit an elongated shape (longer than wide along the axis of locomotion) if risk minimisation is the only factor concerned. Also smaller inter-individual distances are predicted for front individuals than individuals elsewhere in the group. Empirical data based on the three-dimensional structure of fish shoals (using roach, Rutilus rutilus) were consistent with the above two predictions. A second experiment which involved lake chub, Semotilus atromaculatus, as prey and rock bass, Ambloplites rupestris, as predators, provided direct support for the hypothesis that individuals in front positions of groups incurred a significantly higher predation risk than fish in rear positions. Finally, we discuss the differential risks of different group positions in the context of potential foraging gains which provides the basis for a dynamic model of position preferences in group-living animals.

Position preferences of well-fed and food-deprived juvenile roach were investigated in schools of 2 and 4 fish in the laboratory. Food-deprived fish appeared significantly more often in the front position than their well-fed conspecifics. For fish at the same hunger level, individuals at the front of the school had the highest feeding rate. These results represent the first evidence for a relationship between the nutritional state of individual fish and their positions in a school and suggest a functional advantage of the preference.

Fish shoals are usually seen as anonymous leaderless groups in which all individuals have the same influence on swimming velocity and direction. This hypothesis was tested by investigating swimming directions of shoals of roach (Rutilus rutilus) and three-spined stickleback (Gasterosteus aculeatus). In roach, the influence of front and rear fish on the shoal's swimming direction was compared by analysing video recordings. Front fish initiated new directions significantly more often and were followed by rear fish. In a second experiment two shoals of sticklebacks were released from two channels which were positioned at an angle relative to each other. The shoals usually appeared with a short time difference at the opening of the channels and then merged. Initially the two shoals faced in different directions based on the orientation of their respective channel and it was recorded which direction prevailed after the shoals had merged. The shoal that left the channel first, and therefore formed the front part of the merged shoal, clearly dominated the direction. Thus, both experiments gave evidence for front fish having a dominant influence on the direction of the shoal. In the context of sustained position preferences of individual fish, recently observed in roach, this suggests that fish shoals may have leaders over extended time periods.

The ctenophore (comb jelly) Mnemiopsis leidyi is a periodically abundant and voracious predator in U.S. coastal waters.Mnemiopsis leidyi is especially competitive at high prey concentrations because of its very efficient extracellular digestion. W e investigated the functional basis for these outstanding digestion capabilities. Extracellular digestion takes place in the pharynx and consists of three distinct and consecutive phases. The three phases take place in different regions of the pharynx so that various prey items can be treated simultaneously in each phase. The first phase is acidic, while the second and the third are alkaline. Extracellular digestion is completed by ciliary currents that mechanically disrupt the predigested food. Bulky indigestible food fragments are expelled through the mouth. Except for a small area, the paths for ingestion and egestion are separate. Hence, both ingestion and egestion can occur simultaneously. The flattened and elongated shape of the pharynx provides the morphological basis for this flow-through system with various regions for different digestive treatments of the food. This system is highly elaborated compared with those of other lower invertebrates and allows for an efficient, fast, and simultaneous digestion of many prey items, which accounts for the outstanding feeding capabilities of M. leidyi.

SUMMARYThe lobate ctenophore Mnemiopsis leidyi is a periodically abundant and voracious plankton predator in coastal waters along the east coast of the United States. In the 1980s it was accidentally introduced to the Black Sea where it caused a dramatic reduction in fisheries. We investigated how M. leidyi is affected by infestation with parasitic larvae of the sea anemone Edwardsia lineata. Infested M. leidyi contained 1–30 (median 7) E. lineata larvae. Within M. leidyi most larvae had their mouth in the gastrovascular system near the aboral end of the pharynx. Parasitic E. lineata ingested all food previously ingested and pre-digested by M. leidyi. Non-infested M. leidyi had higher growth rates than infested individuals, which had zero or negative growth rates. Egg production was similar for infested and non-infested M. leidyi of similar size. Simulation based on the empirical data suggests that growing, non-infested, M. leidyi are expected to have a larger life-time egg production than infested shrinking individuals. E. lineata could be at least partially responsible for the sharp decline of M. leidyi populations in fall in US coastal waters. Advantages and disadvantages of E. lineata as a potential candidate for the control of the artificially introduced M. leidyi population in the Black Sea are discussed.

Pigments destroyed during photoinhibition of water-splitting photosystem II core complexes from the green alga Chiamydomonas reinhardtii were studied. Under conditions of a transiently inactivated donor side, illumination leads to an irreversible inhibition of the electron transfer at the donor side that is paralleled by the destruction of chlorophylls a absorbing maximally around 674 and 682 nm. The observed stochiometry of 1 + 0.1 destroyed chlorophyll per inhibited photosystem II suggests that chlorophyll destruction could be the primary photodamage causing the inhibition of photosystem II under these conditions.In photosynthesis light energy is converted to chemical energy. However, side reactions can lead to considerable destruction of the photosynthetic apparatus and a concomitant loss of photosynthetic activity in a process called photoinhibition (1). In oxygenic photosynthesis the main target for photoinhibition is photosystem II (PSII) (2). In intact PSII the excited primary donor P680 reduces plastoquinone. The oxidized primary donor P680+ is then reduced by water via an redoxactive tyrosine "Z" (3). In case of a transient malfunction of the water-splitting reaction, P680+ has an extended lifetime during which it can degrade (4) or damage other PSII components, including carotenoids, chlorophylls (Chl), possibly Z, the manganese binding sites, and other amino acids of the PSII proteins (5-10). One or several of these damages result in an irreversible inhibition of the electron transfer from Z to P680+ and hence in an irreversible loss of the water-splitting activity (6,7,(10)(11)(12)(13)(14). Despite many investigations it is still not clear which of the various damages actually causes this inhibition. We investigated this problem by correlating pigment destruction with the loss of oxygen evolving activity.Quantitative observations of pigment destruction in vivo or in grana membranes are difficult due to the high number of pigments per PSII (200-700) (15, 16). PSII reaction center preparations (PSII-RC) contain only 4-6 Chl, 2 pheophytins (pheo), and 1-2 carotenoids (car) per PSII (17-21). Hence, destruction of less than one pigment per PSII can be easily observed in these particles (4,(22)(23)(24). However, since the electron transfer from Z to P680+ is already strongly impaired in freshly isolated PSII-RC (25-27), these particles cannot be used for the investigation of the first irreversibly inactivating reaction of photoinhibition. We therefore used PSII core complexes, which have an intermediate pigment content and are capable of water splitting (28). With these particles it is possible to observe the destruction of less than one Chl per PSII and to measure the loss of water-splitting activity (including the electron transfer from Z to P680+) in parallel.The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. Experimental ProceduresWater-s...

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Oxygen-evolving photosystem II complexes were isolated from the green alga Chlamydomonas reinhardtii by selective solubilization of thylakoid membranes with dodecyl maltoside followed by density gradient centrifugation and anion-exchange chromatography. In the presence of CaCl2 and K3[Fe(CN)6] the complexes evolved oxygen at rates exceeding 1000 mumol (mg of chl)-1 h-1. The particles contained 40 chlorophylls a and had properties very similar to those of PSII isolated from higher plants. Chlamydomonas reinhardtii is now the first organism which can be used for both site-directed mutagenesis and detailed biochemical and biophysical characterization of oxygen-evolving photosystem II. It seems therefore to be an ideal model organism for investigation of structure-function relationships in photosynthetic oxygen evolution.