Size–frequency dynamics of NE Pacific abyssal ophiuroids (Echinodermata: Ophiuroidea)

The 17-year time-series study at Station M in the NE Pacific has provided one of the longest datasets on deep-sea ophiuroids to date. Station M is an abyssal site characterized by low topographical relief and seasonal and interannual variation in surface-derived food inputs. From 1989 to 2005, over 31,000 ophiuroid specimens were collected. Size–frequency distributions of the four dominant species, Ophiura bathybia, Amphilepis patens, Amphiura carchara and Ophiacantha cosmica, were examined for recruitment and the role of surface-derived food supplies on body size distributions. Juveniles were collected in sediment traps and used to investigate settlement patterns and seasonality. Trawl samples showed no indication of seasonal changes in recruitment to larger size classes; however, there was evidence of seasonal settling of juveniles. Interannual differences in median disk diameters and size distributions of trawl-collected adults are greater than those at the seasonal scale. Three of the four species, O. bathybia, A. patens and O. cosmica, had co-varying monthly median disk diameters, suggesting they may have a similar factor(s) controlling their growth and abundance. Interannual differences in monthly size distributions were generally greater than those between seasons. Cross-correlations between the particulate organic carbon (POC) flux (food supply) and size distribution indices for O. bathybia, A. patens and O. cosmica all were significant indicating that increases in food supply were followed by increases in the proportion of smaller size classes after approximately 17–22 months. These findings suggest that food inputs are indeed an important factor influencing deep-sea ophiuroid populations on interannual time scales, more generally supporting the long-hypothesized connection between food availability and population size structure in the deep sea. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Regression-type estimators for adaptive two-stage sequential sampling

Adaptive two-stage sequential sampling (ATSSS) design was developed to observe more rare units and gain higher efficiency, in the sense of having a smaller variance estimator, than conventional sampling designs with equal effort for rare and spatially cluster populations. For certain rare populations, incorporating auxiliary variables into a sampling design can further improve the observation of rare units and increase efficiency. In this article, we develop regression-type estimators for ATSSS so that auxiliary variables can be incorporated into the ATSSS design when warranted. Simulation studies on two populations show that the regression-type estimators can significantly increase the efficiency of ATSSS and the detection of more rare units as compared to conventional sampling counterparts. Simulation of sampling of desert shrubs in Inner Mongolia (one of the two populations studied) showed that by incorporating a GIS auxiliary variable into ATSSS with the regression estimators resulted in a gain in efficiency over ATSSS without the auxiliary variable. Further, we found that the use of the GIS auxiliary variable in a conventional two-stage design with a regression estimator did not show a gain in efficiency.     

The influence of resource specialization on the response of reef fish to coral disturbance

Ecological theory predicts that habitat generalists are less prone to decline or extinction in response to habitat disturbance than habitat specialists. One mechanism that may afford habitat generalists greater persistence is their ability to successfully emigrate from degrading environments. This study compared the response of habitat specialist and generalist reef fish species to live coral disturbance. In replicate coral colonies, live coral was experimentally degraded (low, medium and high coral loss). Species continued residence within the colonies was then surveyed over time. In addition, the ability of habitat generalist and specialist species to migrate between degraded (100% loss) and live coral colonies was compared. Habitat specialists exhibited a higher propensity to remain in colonies with low levels of coral loss. However, there was no significant difference between specialist and generalist species in continued residence in habitats with either medium or high levels of coral loss; both functional groups showed low levels of residence. In terms of migration success, generalists moved further than specialists and showed higher levels of successful migration over the majority of distances examined. The influence of habitat specialization on the behavioral response to coral loss may be a useful predictor of changes to coral reef fish communities in response to coral disturbance.     

Seasonal variation in the diversity and abundance of pelagic larvae of Antarctic marine invertebrates

Most marine benthic macroinvertebrate species reproduce via a larval phase but attempts to explain the occurrence of different larval strategies (feeding or non-feeding, pelagic or benthic) in different habitats have been largely inconclusive. There have been very few year-round surveys of meroplankton at any latitude and in consequence fundamental data on the diversity, abundance, and timings of larval life history phases are lacking. There has been considerable debate regarding the viability of pelagic larvae in cold waters with highly seasonal primary production but there has been only one year-round study of meroplankton in the Southern Ocean, and that was outside of the Antarctic Circle. We present data from the first year-round survey of meroplankton assemblages at a location within the Antarctic Circle. We surveyed abundances of meroplanktonic larvae over 1.5 year at Rothera Point, West Antarctic Peninsula (67°34′S, 68°07′W). Larvae were collected in monthly diver-towed net samples close to the seabed at 20 and 6 m total water depths at each of three locations and were identified and counted live immediately after sampling. A total of 99 operationally defined taxonomic types representing 11 phyla were recorded but this is likely to be an underestimate of true diversity because of inherent difficulties of identification. Larvae were present in all months of the year and although planktotrophic larvae were more abundant in summer, both feeding and non-feeding types were present in all months. Comparisons of seasonal larval abundances with data from a settlement study at the same sites and from the literature show that larvae of mobile adults settle in summer regardless of developmental type, whereas sessile taxa settle in all seasons. We suggest that this is a consequence of differences in the food requirements of mobile and sessile fauna and that the availability of food for post-larval juveniles is more critical for survival than factors affecting the larval stage itself.     

Quorum sensing, recruitment, and collective decision-making during colony emigration by the ant Leptothorax albipennis

When its nest is damaged, a colony of the ant Leptothorax albipennis skillfully emigrates to the best available new site. We investigated how this ability emerges from the behaviors used by ants to recruit nestmates to potential homes. We found that, in a given emigration, only one-third of the colony's workers ever recruit. At first, they summon fellow recruiters via tandem runs, in which a single follower is physically led all the way to the new site. They later switch to recruiting the passive majority of the colony via transports, in which nestmates are simply carried to the site. After this switch, tandem runs continue sporadically but now run in the opposite direction, leading recruiters back to the old nest. Recruitment accelerates with the start of transport, which proceeds at a rate 3 times greater than that of tandem runs. The recruitment switch is triggered by population increase at the new site, such that ants lead tandem runs when the site is relatively empty, but change to transport once a quorum of nestmates is present. A model shows that the quorum requirement can help a colony choose the best available site, even when few ants have the opportunity to compare sites directly, because recruiters to a given site launch the rapid transport of the bulk of the colony only if enough active ants have been "convinced" of the worth of the site. This exemplifies how insect societies can achieve adaptive colony-level behaviors from the decentralized interactions of relatively poorly informed insects, each combining her own limited direct information with indirect cues about the experience of her nestmates.