ALBERT

Notes: Abstract We investigated feeding by the hogchoker, Trinectes maculatus (Bloch and Schneider), in freshwater, oligohaline, mesohaline, and polyhaline regions of Chesapeake Bay, USA, and examined prey selection in relation to food availability. Otter trawling for fish and Van Veen grab sampling for benthic macrofauna occurred in July and August 1992 and August and September 1993. Hogchokers exhibited both opportunistic and selective feeding patterns along the estuarine salinity gradient in four tributaries (Potomac, Rappahannock, York, and James Rivers) and in the mainstem Chesapeake Bay. Major prey taxa included annelids, arthropods, and tellinid siphons. In polyhaline habitat, polychaetes dominated both the benthos and gut contents numerically and gravimetrically. On the other hand, oligochaetes were numerically dominant in freshwater/oligohaline areas but were rarely eaten, perhaps because of their burial depth. Arthropods (mostly amphipods) occurred at most salinities, were common in gut contents in low-salinity areas, and were replaced as prey by larger proportions of polychaetes in polyhaline regimes. Although hogchokers ate tellinid siphons, they rarely consumed whole bivalves or gastropods. These diet patterns (and especially the importance of siphon nipping) are similar to those of juvenile or small flatfish elsewhere in Europe, Africa, and North America. A size–salinity relationship for hogchokers occurred along the summer salinity gradient, with smaller fish predominating upstream and larger fish downstream. It was not clear from our data if variation in diet composition reflected changes in prey composition along the salinity gradient rather than changes in fish size.

Notes: Abstract The influence of temperature over 8 time intervals on survival of cleavage stages, trochophore larvae, and straighthinge veliger larvae of Mulinia lateralis (Say) was investigated using a thermal gradient apparatus. There was a direct relationship of mortality with increased period of exposure. As the clams aged, temperature tolerance increased, with cleavage stages being most sensitive to higher temperatures and straight-hinge larvae least sensitive. Multiple-regression equations were developed to allow prediction of percentage mortalities under different conditions of temperature and time exposure. Entrainment of M. lateralis embryos and larvae in the cooling-water systems of steam-electric power plants should be as short as possible to keep mortality toa minimum.

Notes: Abstract Distribution, abundance, and resistance adaptations to higher temperature and desiccation of three species of intertidal mussels (Mytilus edulis aoteanus, Perna canaliculus and Aulacomya maoriana) were studied in New Zealand. M. edulis aoteanus generally was more abundant upshore, with P. canaliculus dominating downshore. M. edulis aoteanus was more common than P. canaliculus on the outside of mixed-species clumps. Abundance of A. maoriana was variable, with individuals favouring damp habitats such as inside mussel clumps. In moving air at 75% relative humidity and at 20°C or 30°C, median lethal levels of water loss were similar for P. canaliculus and M. edulis aoteanus but lower for A. maoriana. Rates of desiccation varied inversely with size and were higher for P. canaliculus, due mainly to valve gaping with resultant loss of water from the mantle cavity. M. edulis aoteanus was more tolerant of higher water temperatures than were the other species. Success in colonizing upshore or more aerially exposed habitats seems to be related to ability of small mussels to tolerate desiccation, especially during hot, windy weather.

Notes: Abstract  Stocks of eastern oysters, Crassostrea virginica (Gmelin), in mesohaline Chesapeake Bay, USA, exhibit a high degree of inter-annual and spatial variability in recruitment. We found that cumulative oyster spatfall on off-bottom collector plates, measured throughout the summer in 14 years over a span of three decades, was highly positively correlated (r 2 = 0.8) with juvenile oyster recruitment on adjacent oyster bars. Total abundances of juvenile oysters on these bars were, however, generally 99.7% lower than predicted from cumulative seasonal larval settlement on collector plates. We propose that although the number of larvae metamorphosing was the key factor in determining the gross annual pattern of recruitment to these mesohaline oyster bars, the actual magnitude of recruitment was governed by post-settlement processes, such as competition for limited resources and predation. We tested the hypothesis that predation may be partly responsible for high post-settlement juvenile oyster mortality. We performed a series of 3-d field investigations over two summers (1989, 1990) at a mesohaline site, employing cages of various mesh sizes (400, 800, 1500 μm) to protect hatchery-reared spat of 0.5 to 4.0 mm shell height. Mortality rates for spat held for 3 d in the estuary (17.8%) were significantly higher (P = 0.0001) for the smallest spat (0.5 to 2.0 mm) compared with those of 2.01 to 4.0 mm (4.2%). In 1990, but not in 1989, enclosure within 400 and 800 μm mesh cages significantly (P = 0.004) increased survival during 3-d deployments (9.4 and 10.1%, respectively) compared with spat unprotected by mesh cages (21.9%). In a series of laboratory predation studies that used the entire community of invertebrates that could penetrate the cages, microscopic juvenile polyclad flatworms, Stylochus ellipticus, were the only organisms that we observed crawling into living oysters and feeding on oyster tissue. Large flatworms (50 to 200 mm2) are known to be important predators on oysters, but this ability of flatworms that were so small (〈ca. 5 mm2) and translucent as to be almost invisible without magnification to feed on immediate post-metamorphic oysters has not been documented previously. Our results suggest that the rate of mortality due to predation in mesohaline Chesapeake Bay is much reduced once spat survive for 2 to 3 weeks post-metamorphosis. Thus, it is likely that predation in the 1 to 2 week period immediately after settlement may be a crucial factor in the structuring of eastern oyster populations.

Notes: Abstract Thermal tolerance studies on three larval stages of five species of bivalve are compared: Crassostrea virginica, Mulinia lateralis, Argopecten irradians, Mercenaria mercenaria, and Spisula solidissima. Experiments were designed to simulate possible larval entrainment in the cooling systems of power plants and involved an 11 (temperature)x8(time-exposure) experimental matrix. Temperatures ranged from about 17.4° to 43°C at approximately 2C° intervals, and exposure times ranged from 1 min to 6 h (sometimes longer in the case of C. virginica). For all species, larval mortality generally increased with increasing exposure temperature and with increased time-exposure at any one temperature. In some species (M. mercenaria and A. irradians), a cold-shock was also apparent (i.e. increased mortality at temperatures below those at which the larvae were spawned). In the case of A. irradians, this effect was more noticeable at the trochophore stage than at the other two stages, although for all species investigated, there was a general trend of decreased thermal sensitivity with increasing age. Despite some interspecies variability from temperature to temperature and from stage to stage, S. solidissima was the most sensitive species and A. irradians was intermediate in thermal tolerance between this and the other three species. At temperatures as high as 40°–41°C, straight hinge M. mercenaria and C. virginica sustained low mortality for more than 2 h and on this basis were judged to be the most “temperature-shock” resistant of all larvae investigated. However, younger stages showed no significant differences between M. mercenaria, C. virginica or M. lateralis, and we conclude that these three species are generally very similar in their larval resistance to thermal increase. All three are more euryhaline than are the remaining two polyhaline or marine species.