A novel intramandibular joint facilitates feeding versatility in the sixbar distichodus

The intramandibular joint (IMJ) is a secondary point of movement between the two major bones of the lower jaw. It has independently evolved in several groups of teleost fishes, each time representing a departure from related species in which the mandible functions as a single structure rotating only at the quadratomandibular joint (QMJ). In this study, we examine kinematic consequences of the IMJ novelty in a freshwater characiform fish, the herbivorous Distichodus sexfasciatus. We combine traditional kinematic approaches with trajectory-based analysis of motion shapes to compare patterns of prey capture movements during substrate biting, the fish's native feeding mode, and suction of prey from the water column. We find that the IMJ enables complex jaw motions and contributes to feeding versatility by allowing the fish to modulate its kinematics in response to different prey and to various scenarios of jaw-substrate interaction. Implications of the IMJ include context-dependent movements of lower versus upper jaws, enhanced lower jaw protrusion, and the ability to maintain contact between the teeth and substrate throughout the jaw closing or biting phase of the motion. The IMJ in D. sexfasciatus appears to be an adaptation for removing attached benthic prey, consistent with its function in other groups that have evolved the joint. This study builds on our understanding of the role of the IMJ during prey capture and provides insights into broader implications of the innovative trait.

Diet of dominant demersal fish species in the Baltic Sea: Is flounder stealing benthic food from cod?

Knowledge about ecological interactions between species is of paramount importance in ecology and ecosystem-based fisheries management. To understand species interactions, studies of feeding habits are required. In the Baltic Sea, there is good knowledge of the diet of cod, but little is known about the diet of flounder, the second most abundant demersal fish in the region. In this study, we investigated the diets of cod and flounder for the first time using stomach content data collected simultaneously in 2015-2017 over a large offshore area of the southern Baltic Sea. The diet of flounder was relatively constant between sizes and seasons and was dominated by benthos, with a high proportion in weight of the benthic isopod Saduria entomon. The diet of cod differed between seasons and showed an ontogenetic shift with a relative decrease of benthic prey and an increase of fish prey with size. Historic diet data of cod were used to explore cod diet changes over time, revealing a shift from a specialized to generalist feeding mode paralleled by a large relative decline in benthic prey, especially S. entomon. Flounder populations have increased in the past 2 decades in the study area, and therefore we hypothesized that flounder have deprived cod of important benthic resources through competition. This competition could be exacerbated by the low benthic prey productivity due to increased hypoxia, which could contribute to explaining the current poor status of the Eastern Baltic cod. The results of this study point to the importance of including flounder in multispecies end ecosystem models.

Reduced growth in Baltic Sea cod may be due to mild hypoxia

Daily food consumption by small Baltic Sea cod has declined in recent decades, resulting in reduced growth and biomass. Declining oxygen may cause lower production of benthic prey for small cod, but an alternative explanation presented here is that the mildly hypoxic conditions that small cod experience reduces their rate of digestion and hence food consumption. Better information about the distribution and behaviour of small cod and their metabolic response to low oxygen levels is required.

The effects of resource subsidy duration on detritus-based stream ecosystem: a stream mesocosm experiment

SummaryMost of the resource subsidies are temporally variable, and studies have revealed that ecological processes can be mediated by the temporal attributes of subsidies, such as timing and frequency. Less studies have, however, examined the effects of the subsidy duration, an another major temporal attribute, on consumer populations, communities and ecosystem functions. Using an outdoor mesocosm experiment, we demonstrated that, even with the same total amounts, the prolonged subsidy let large-stage fish effectively monopolize the subsidy over small-stage fish, while the pulsed subsidy allowed small-stage fish to increase the ingestion rate of the subsidy. This effect resulted in causing weaker indirect positive effects on in-situ benthic prey and a leaf breakdown rate with the prolonged subsidy than with the pulsed-subsidy although it depended on dominant benthic prey species having different edibility. Increasing evidences have shown that global warming would not only advance, but also prolong the growing seasons, which may, in turn, make subsidies more prolonged. The ecological significance of the subsidy duration might be common in nature, and should be incorporated to better understand ecological processes in spatially and temporally coupled ecosystems.

Carotenoid-based skin ornaments reflect foraging propensity in a seabird, Sula leucogaster

Carotenoid-based ornaments are common signalling features in animals. It has long been proposed that such ornaments communicate information about foraging abilities to potential mates. However, evidence linking foraging with ornamentation is largely missing from unmanipulated, free-ranging populations. To investigate this relationship, we studied a coastal population of brown booby ( Sula leucogaster brewsteri ), a seabird with a carotenoid-based gular skin ornament. δ 13 C values from both feathers and blood plasma were negatively correlated with male gular colour, indicating birds that consumed more pelagic prey in offshore locations had more ornamented skin than those that fed on nearshore, benthic prey. This relationship was supported by our GPS tracking results, which revealed longer, more offshore foraging trips among highly ornamented males. Our data show that brown booby ornaments are honest indicators of foraging propensity; a link consistent with the rarity hypothesis and potentially driven by the concentration of carotenoids found in phytoplankton versus benthic algae. Carotenoid-based ornaments may reflect foraging tendencies in animals such as coastal predators that use food webs with distinct carotenoid profiles.

An improbable opportunistic predator: the functional morphology ofPinna nobilis(Bivalvia: Pterioida: Pinnidae)

The functional morphology ofPinna nobilisis described, with special reference to the uniquely pinnid pallial organ, the similarly unique buccal (formerly pallial) gland, the stomach and its contents. The pallial gland produces sulphuric acid which as well as functioning as a shell cleaning swab may be involved in prey capture. The buccal gland discharging into the oesophagus has proteolytic digestive functions while the stomach is adapted for the reception and digestion of captured, mucous-bound, mesozooplanktonic and epi- and endo-benthic, prey items.Pinna nobilisis thus not simply either an accidental or incidental predator of such species but is opportunistic. The buccal glands and stomachs of other Pinnidae are not so specialized as inP. nobilis,possibly indicating that in the particular, oligotrophic, environment of the Mediterranean and Adriatic Seas, and in which it is endemic,P. nobilishas, in addition to being a typical ctenidial suspension feeder, become an opportunistic predator. Although the deep sea representatives of the Septibranchia (Anomalodesmata) and Propeamussidae are obligate predators, this is the first record of any bivalve functioning as an opportunistic predator with unique morphological adaptations to facilitate this.