首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 78 毫秒
1.
《Gondwana Research》2014,25(3):991-998
The juvenile morphology and ontogeny of the Cheiruroidid trilobite Hunanocephalus ovalis Lee, 1963 from the lower Cambrian Shuijingtuo Formation in Hubei Province, South China is presented. The new material comprises a relatively complete meraspid ontogenetic series (degree 0 to 10), which reveals more details on their morphological changes such as the contraction and disappearance of the pronounced posteromedial notch in the pygidium and the addition of the trunk segments, which are all documented for the first time and can also be used as developmental markers defining their ontogenetic phases. The trunk segmentation schedule of H. ovalis is also discussed, which is similar to the other early Cambrian oryctocephalid trilobites, i.e. as the boundary between the thorax and pygidium migrated posteriorly there is no change in the trunk segment number; the processes of liberation of the thoracic segment and segment insertion into the pygidium are separated from one another, implying that the control of trunk exoskeletal segment appearance and articulation might be decoupled in these trilobites.  相似文献   

2.
New thoracican cirripede material from the Kimmeridge Clay (Upper Jurassic, Tithonian) is described. This includes a log, encrusted on the lower surface with hundreds of perfectly preserved, articulated specimens of Etcheslepas durotrigensis Gale, 2014, and fewer specimens of Concinnalepas costata (Withers, 1928). Some individuals are preserved in life position, hanging from the underside of the wood, and the material provides new morphological information on both species. It appears that Martillepas ovalis (Withers, 1928), which occurs at the same level (Freshwater Steps Stone Band, pectinatus Zone) attached preferentially to ammonites, whereas E. durotrigensis and C. costata used wood as a substrate for their epiplanktonic lifestyle. Two regurgitates containing abundant barnacle valves, mostly broken, and some bivalve fragments, have been found in the Kimmeridge Clay. These were produced by a fish grazing on epiplanktonic species, and are only the second example of regurgitates containing barnacle valves known from the fossil record. The evolution of modern barnacle groups is discussed in the light of the new Jurassic material as well as recently published molecular phylogenies. New clades defined herein are called the Phosphatothoracica, the Calamida and the Unilatera.  相似文献   

3.
Predation is often the largest source of mortality for juvenile fish and the risk of predation can influence growth rates by either forcing young fish into suboptimal foraging habitats or reducing the amount of time spent foraging. We used field experiments to test effects of predation risk by gulf flounder (Paralichthys albigutta) on juvenile pinfish (Lagodon rhomboides) growth rates by measuring changes in length and weight in three habitats (sand, low density, and high density shoalgrass,Halodule wrightii) in Perdido Key, Florida. Benthic cores, seagrass samples, and stomach contents were also analyzed to examine differences in pinfish prey densities, grass densities and epiphyte coverage, and diet, respectively, among habitat and predator treatments. Both length and weight growth rates were determined and showed similar results. We found that pinfish inhabiting seagrass habitats, particularly low densityHalodule displayed the fastest growth rates in the beginning of the growing season (June) and those in sand had the fastest growth rates later in the season (October). These differences in growth rates did not appear to be influenced by densities of pinfish prey items since the treatment having the highest density of prey was not that in which growth rates were the greatest. This seasonal shift may be attributed to increasing pinfish size. Larger pinfish in October may have been inhibited by high density grass, reducing foraging efficiency. These results demonstrate how occupying a suboptimal foraging habitat can affect juvenile pinfish growth rates. Predation risk significantly reduced length and weight growth rates of pinfish in June, but not October. This suggests that smaller pinfish early in the season traded time spent foraging for predator avoidance, while larger pinfish were likely to have reached a size refuge from predation. This study demonstrates that nonlethal effects from predation are also important influences on juvenile pinfish.  相似文献   

4.
Average summer densities of the xanthid crab,Eurytium limosum, in an intertidal salt marsh on Sapelo Island, Georgia were in the range of 7.5 to 80.0 individuals m?2. Crab densities were lowest in wet, lowlying marsh and highest in well-drained creekbank and mussel mound habitats. An analysis of crab stomach contents indicated that feeding occurred mostly around high tide, especially at night. Although the diet included some plant material,Eurytium limosum is primarily predatory and consumed crabs, polychaetes, ostracods, bivalves, and snails. In the laboratory, under simulated low-tide field conditions, both small (11–15 mm carapace width) and large (20–37 mm CW)Eurytium could capture and consume young killifish (Fundulus heteroclitus). Large crabs consumed the entire size range (7–19 mm total length) of larval/juvenile fish offered, but small crabs did not prey upon fish >11.5 mm TL. The potential importance ofE. limosum as a predator on young killifish may not be realized in the field because alternative prey are available and the crabs feed primarily at high tide, when young killifish are dispersed in the water column and are less vulnerable to benthic predators.  相似文献   

5.
Seven species of mojarras (Gerreidae) were collected from three stations in the Indian River lagoon near the Sebastian Inlet, Florida over an eighteen month period to study spatial and temporal variations in occurrence and feeding habits. Two major habitats common to this area were represented in the study; a sandy beach and a seagrass flat. Of the 2,899 gerreids collected,Eucinostomus gula represented 67% and was found primarily in theHalodule seagrass site. The next most abundant species,E. argenteus (11%),E. jonesi andE. lefroyi (each 10%), were found most frequently at the inlet site.Diapterus auratus, E. melanopterus, andGerres cinereus were captured infrequently and were considered incidental. Prey items found included amphipods, bivalves, copepods, fish eggs, foraminifera, isopods, nematodes, ostracods, polychaetes and crustacean parts. Significant amounts of sand and unidentifiable amorphous debris were also included in the gut content analysis. Little interspecific variation in diet was found among the four major species examined. The general shift seen from a diet composed primarily of copepods and other crustaceans to one of polychaetes suggests an intraspecific ontogenetic progression. Spatial variations in diet composition were also indicated between the two major habitats. Feeding analyses showed gerreids to be diurnal feeders with the greatest consumption (total gut content weight) occurring at dusk.  相似文献   

6.
English sole (Pleuronectes vetulus) is one of a few commercially important marine fishes on the Pacific coast of North America that use estuarine areas as nurseries for juvenile stages. Trawl surveys of four United States Pacific Northwest estuaries were conducted to determine spatial patterns of juvenile English sole residence in estuaries during 1998–2000. Additional data from 1983–1988 were also analyzed. Two size classes of juvenile English sole were identified during surveys, with densities of small (Total length [TL] <50 mm) sole ranging from 0 to 11,300 fish ha?1 across all sites, and densities of large (TL 50–150 mm) sole ranging from 0 to 33,000 fish ha?1 across all sites. Principal components analysis of static habitat data collected at each trawl survey site was used to define habitat types within each estuary, and discriminant function analysis was used to test the resulting classification scheme. Both small and large cohort English sole used lower side channel locations at significantly higher densities than other estuarine areas. Small English sole also showed significant relationships with both bottom temperature and depth. These patterns in habitat use were consistent across all estuaries and indicate that English sole used shallow depth areas surrounded by extensive tidal flats, where temperatures were optimal for growth. The analysis also suggested a carrying capacity may exist for large English sole in nursery estuaries.  相似文献   

7.
During August and September 2007, the lower Chesapeake Bay and its tributaries experienced a massive bloom of Cochlodinium polykrikoides Margalef (>104 cells per milliliter) that persisted for over a month, was extremely patchy, and at times penetrated into the Atlantic Ocean. The onset of the bloom coincided with a period of intense rainfall and stormwater runoff after a protracted summer drought period. Genetic evidence showed this species to be distinct from many Asian strains but similar to other North American strains. Populations dominated by C. polykrikoides took up a variety of nitrogenous compounds to support their growth and were able to mobilize additional sources of organic nutrients through peptide hydrolysis. Bioassays determined that C. polykrikoides exerted a lethal affect on juvenile fish and shellfish, causing 100% mortality of juvenile fish (Cyprinodon variegates) in less than 24 h and 20% mortality in juvenile American oysters (~21 mm; Crassostrea virginica) within 72 h.  相似文献   

8.
We provide an example of the type of bias assessment that should, but often is not, used in ecological studies using techniques such as caging, tethering, and trawl sampling. Growth rates of summer flounder (Paralichthys dentatus) from cage enclosures were compared to those from mark-recapture trials, and prey types (identified through stomach content analysis) compared between caged fish and wild fish collected from nursery sites over a 2-yr period. Site-specific growth rates estimated from the caging method were similar (± 15%) to those estimated from the mark-recapture trials. Prey types were nearly identical between caged and wild fish, although selectivity may have varied quantitatively. Caging summer flounder will generally be an appropriate tool with which to measure growth rates in the wild, but comparisons with an independent measurement method are necessary for validation. In tethering trials, predation was significantly greater on tethered than on untethered fish, indicating that tethering is not an appropriate tool with which to measure absolute rates of predation on juvenile summer flounder. The lack of effects of substrate (sand versus mud) and fish origin (hatchery-reared versus wild) on predation of tethered versus untethered fish indicates that tethering trials will not indicate treatment-specific differences when none exist. Tethering may be an acceptable method for comparing relative rates of predation on different substrates and between hatchery-reared and wild juvenile summer flounder in the field, although true differences in treatment levels could be masked by tethering. Beam trawl efficiency estimates for juvenile summer flounder were similar between beach and marsh habitats, but differed significantly between marsh sites, indicating that site-specific trawl efficiency estimates may be critical to accurately assess juvenile flounder the appropriateness of comparisons of size-frequency information between the sites and habitats used in this study. Caging, tethering, and beam trawl sampling are appropriate tools for measuring ecological parameters of juvenile summer flounder, but only if possible biases of each method are identified and compensated for when interpreting data collected using these methods.  相似文献   

9.
Estuarine nursery areas are critical for successful recruitment of tautog (Tautoga onitis), yet they have not been studied over most of this species' range. Distribution, abundance and habitat characteristics of young-of-the-year (YOY, age 0) and age 1+juvenile tautog were evaluated during 1988–1992 in the Narragansett Bay estuary, Rhode Island, using a 16-station, beach-seine survey. Estuary-wide abundance was similar among years. Greatest numbers of juveniles were collected at northern Narragansett Bay stations between July and September. Juvenile abundances varied with density of macroalgal and eelgrass cover; abundances ranged from 0.03 fish per 100 m2 to 8.1 fish per 100 m2. Although juveniles use eelgrass, macroalgae is the dominant vegetative cover in Narragansett Bay. Macroalgal habitats play a previously unrealized, important role and contribute to successful recruitment of juvenile tautog in Narragansett Bay. Juvenile abundances did not vary with sediment type or salinity, but were correlated with surface water temperature. Fish collected in June were age 1+ juveniles from the previous year-class (50–167 mm TL) and these declined in number after July or August. The appearance of YOY (25–30 mm TL) in July and August was coincident with the period of their greatest abundances. A precipitous decline in abundance occurred by October because of the individual or combined effects of mortality and movement to alternative habitats. Based on juvenile abundance, a previously unidentified spawning area was noted in Mount Hope Bay, a smaller embayment attached to the northeastern portion of Narragansett Bay. In August 1991, Hurricane Bob disrupted juvenile sise distribution and abundance, resulting in reduced numbers of YOY collected after the storm and few 1+ juveniles in 1992.  相似文献   

10.
Salinity fluctuation has been proposed as an important determinant of estuarine fish distribution. To test this idea, we compared distribution, behavioral preference and physiological sensitivity of two juvenile estuarine fishes, spot (Leiostomus xanthurus) and croaker (Micropogonias undulatus), with respect to salinity change. In field collections, spot: croaker ratios were positively correlated with salinity variation. Subsequent behavioral observations revealed that croaker tend to cross a 10‰ salinity gradient less often than spot. We proposed that energetic costs of salinity adaptation may be higher for croaker, resulting in the observed avoidance behavior. Oxygen consumption rates over rapid salinity fluctuations showed no significant differences in metabolic response between species, although there was some indication that sensitivity changes with fish size. Apparently, juvenile spot and croaker are well-equipped to withstand extreme changes in salinity. We conclude that environmental factors correlated with salinity change may be responsible for distribution differences between these two abundant species.  相似文献   

11.
The temporal variability in abundance and population structure of the gastropodChilina ovalis Sowerby was studied in the upper intertidal zone of Queule River estuary, south-central Chile (c. 40°S). Snails were collected monthly (September 1995–December 1997) from haphazardly-located quadrats (50×50 cm, n=5 each time), and counted and measured (shell height) in the laboratory. Water and sediment samples were collected at the same time to study the snail's habitat characteristics. Overall mean abundance was 115 individuals m?2 (SD=55). Monthly abundance estimates indicated a clear decrease during 1997. This decrease appeared to be related to the annual recruitment success of the species and at least partially to water temperature and sedimentological variability. Overall size range ofC. ovalis was 1.5–27.5 mm shell height. Growth varied seasonally with highest growth rates observed after recruitment (November–February). Slower growth continued throughout the austral winter months. Despite changes in abundance between 1996 and 1997, no differences were detected when population growth estimates were compared between years. A maximum longevity of approximately 4 yr was estimated from the growth curves of the cohorts, and a life cycle with more than one reproductive period is suggested.  相似文献   

12.
Although Iguanodon is one of the most abundant and well-known of Europe's dinosaur genera, fossils of young specimens are very rare. Indeed, the fossil record contains very few examples of the young of any non-hadrosaurid iguanodontian. Here we report the discovery of 13 Iguanodon perinates from the Lower Cretaceous of Galve (Teruel, Spain). The characteristics of an adult and juvenile found nearby show these perinates to belong to a new species: Iguanodon galvensis sp. nov. The histological and osteological features of these young animals indicate them to have been in their first year of life. The taphonomic features of their remains, plus the finding of clearly embryonic vertebrae alongside them, suggest the perinates of this species remained in the vicinity of their nests for some time, possibly congregating in nursery areas.  相似文献   

13.
Juvenile spot,Leiostomus xanthurus (Pisces), were collected at 2-h intervals over two 48-h periods in a tidal creek in North Inlet, South Carolina, during July 1986. Gut fullness, dry gut contents weight as a percentage of fish wet weight, was measured to test the hypothesis that feeding intensity is randomly distributed through time. Spot were also collected and serially sacrificed from a holding tank to estimate their gut evacuation rate. Gut fullness was greatest during periods of ebb and high tides and was little affected by the amount of light available. Gut contents were evacuated at a rate of about 15% per hour in the laboratory, in close agreement with field emptying times of about 6 h from peak fullness to empty guts. A conservative estimate of daily ration for juvenile spot (19–42 mm standard length) was 4.5% of their live body weight per day. Studies of spot feeding ecology can be greatly affected by when fish are collected during the tidal cycle.  相似文献   

14.
The feeding habits of several fish species in a water reservoir of the Ria Formosa, Portugal, that has similar ecological characteristics to the outside tidal channels, were studied and compared with food availability. The gilthead seabream (Sparus aurata), the most abundant fish species, mainly selected gastropods and bivalves, although occasionally fish and small crustaceans such as tanaids, ostracods, and cumaceans were also selected. Polychaetes, although abundant in the environment, were not particularly selected by any of the fish species studied. The diets of all the species studied were characterized by a large variety of prey, allowing them to survive in environments of low diversity and poor stability, such as coastal lagoons. These fish are largely benthic feeders, essentially eating the epimacroinvertebrates and endomacroinvertebrates and, occasionally, fish.Diplodus vulgaris andDiplodus annularis preferentially selected gastropods and small crustaceans.Spondyliosoma cantharus generally preyed on crustaceans, including the highly mobile epifauma, the mysids, and decapods.Halobatrachus didactylus andAnguilla anguilla, had very diversified diets that included fish.Mullus barbatus were found to have selected all groups of crustaceans and also bivalves. Wrasses, gobies, andDiplodus sargus, all small-sized fish, singled out small crustaceans, gastropods, and bivalves. The Sparids were the least specialized predators, with broader niches than the other species. They preferentially selected molluscs, which were abundant in the environment. A large overlap of diets was observed and competition may be important when fish biomass is high.  相似文献   

15.
Prionotus carolinus and Prionotus evolans were collected from many locations within Long Island Sound in 1971–1973, and in 1976–1977. Data from earlier collections in Block Island Sound (1943–1945) were also included. A total of 1751 specimens, 960 P. carolinus and 791 P. evolans was examined within these two time periods. Both species entered the Sound in April and spawned during June and July. P. evolans appeared to spawn slightly earlier in summer than P. carolinus. Adults began to leave the Sound after spawning and were usually absent after November. Young-of-the-year were taken regularly from August to November and, occasionally in water over 20 m deep, into February when the bottom water temperature was 1.4°C. At the end of the first growing seasons both species exhibited large variations in standard lengths. Back-calculations from scale annuli measurements indicated that linear growth rates during the juvenile years were similar in both species. However, P. evolans was considerably heavier than P. carolinus. During adulthood P. evolans was not only longer and heavier than P. carolinus, but lived longer. Growth rates are described by the following equations: P. carolinus Lt+1=9.60+0.68 (Lt), and p. evolans Lt+1=7.70+0.80 (Lt). Both species were opportunistic feeders, and crustaceans were clearly the dominant group of prey. Young-of-the-year, between 3–6 cm, ate copepods. As they grew they ate larger prey, such as Neomysis americana, Diastylis quadrispinosus, various species of amphipods of small sizes, and juvenile Crangon septemspinosus. Older fish ate larger sizes of these same prey, a number of species of crabs, juvenile Homarus americanus, and Squilla empusa. Occasionally they ate polychaetes, molluscs, and juvenile fish. Partitioning of the resources of Long Island Sound by these two species appeared to be by prey size. P. evolans ate prey that, on the average, were slightly larger than those eaten by P. carolinus. Furthermore, P. evolans ate a greater amount of nektonic species than P. carolinus, which appeared to prefer benthonic invertebrates.  相似文献   

16.
Extensive new material of scalpelliform cirripedes, including articulated individuals, from the Late Jurassic (Tithonian) Kimmeridge Clay of Dorset, southern England, is described and used as a basis for taxonomic revision of the family Zeugmatolepadidae. Two subfamilies, the Zeugmatolepadinae and Martillepadinae nov., are established and two new genera (Martillepas, Concinnalepas) in the latter subfamily are described from the Upper Jurassic and one (Icenilepas) from the Upper Cretaceous Chalk. Material from the Kimmeridge Clay of Dorset also includes the oldest fossil representative of the family Pollicipedidae for which a new genus and species, Etcheslepas durotrigensis, are erected, and the oldest calanticid, Cretiscalpellum sp. nov. Jurassic taxa established by T.H. Withers in 1928, on the basis of sparse material, are redescribed, and referred to the new genera as Martillepas ovalis, M. costata, M. hollisi, Concinnalepas concinna and Etcheslepas fragilis.  相似文献   

17.
Young zircons from crystal-poor volcanic rocks provide the best samples for the investigations of pre-eruption magmatic processes and for testing a possible relationship between zircon Eu anomalies and crustal thickness. We report trace element chemistry and Hf-O isotope compositions of young zircons from 3 Holocene volcanoes in the Tengchong volcanic field, SE Tibet, in order to provide insights into magma evolution processes and conditions for high-K calc-alkaline volcanic rocks in a post-collisional setting. As decreasing zircon Ti content and falling temperature, zircon Hf content and Yb/Sm increase whereas zircon Eu anomaly and Th/U decrease, indicating fractional crystallization of plagioclase and zircon during magma cooling. More importantly, zircon Hf isotope ratio (εHf values) increases with decreasing zircon Ti content and falling temperature (T), suggesting gradually increasing incorporation of relatively high εHf juvenile materials in the crystallizing zircons during magma evolution. Negative correlations between zircon εHf and zircon δ18O also support open-system magma evolution. Our data suggest fractional crystallization of a magma with simultaneous contamination by high εHf and low δ 18O juvenile (immature) crustal materials during monotonic cooling after zircon saturation. The low-T, high-εHf and low- δ 18O zircons may indicate the involvement of the early Cretaceous juvenile granitic country rocks during shallow magma evolution. Average Eu anomalies in zircons from young Tengchong lavas yield crustal thickness of 40.7 ± 6.8 km, consistent with present crustal thickness (42.5 km) determined by geophysical methods.  相似文献   

18.
Mean daily consumption rates on Mytilus spp. were compared among juveniles of the non-indigenous Carcinus maenas, juveniles of the indigenous Cancer irroratus, and adults of the indigenous Dyspanopeus sayi between June and August 2005 to assess the relative impact of juvenile C. maenas in field (Benacadie Channel (45°54′ N, 60°53′ E), Bras d’Or Lakes, Nova Scotia, Canada) and laboratory experiments. This study examined: (1) whether consumption rates in a field setting vary among species; (2) the effect of laboratory and field settings on species-specific consumption rates, and whether rates vary between settings for each species; and (3) the effects of temperature and salinity on the consumption rates of these species. In field experiments, there was no significant difference in consumption among C. maenas, C. irroratus, and D. sayi (0.100?±?0.067, 0.450?±?0.189, and 0.800?±?0.423 mussels crab?1 d?1, respectively). However, both C. maenas and C. irroratus consumed two to four times more prey in the laboratory than in the field. D. sayi prey consumption was also greater (although not significant) in the laboratory than in the field. In the laboratory, consumption rate was greater for C. irroratus in salinities of 26 than 17 (2.75 and 1.69–1.81 mussels crab?1 d?1, respectively), and in 17°C than 13°C (1.10–1.21 and 0.56–0.64 mussels crab?1 d?1, respectively) for C. maenas. In all experiments, consumption rates of juvenile C. maenas were lower than or similar to those of the juvenile and adult indigenous species, suggesting that the potential predatory impact of juvenile C. maenas on Mytilus spp. may not be as significant as that of the adults of this non-indigenous species.  相似文献   

19.
This is the first report of juvenile (age 0+) Atlantic menhadens, Brevoortia tyrannus, in the Annapolis River, Nova Scotia. Juvenile Atlantic menhaden were incidentally collected during a 3-yr study of the effects of the STRAFLO hydro-power turbine at Annapolis Royal on alosine fishes. Length distribution and time of capture of collected fish suggest that Atlantic menhaden may spawn in the Bay of Fundy.  相似文献   

20.
During the year 1978, juvenile salmonids were collected from coastal streams running through China Poot Marsh and the stomach contents analyzed. Stomach contents of threespine stickleback (Gasterosteus aculeatus) and staghorn sculpin (Leptocottus armatus) from China Poot and of threespine stickleback from Potter Marsh were also analyzed; these two species were generally caught in tidal pools on the marshes. The juvenile coho salmon (Oncorhynchus kisutch) had the most varied diet; 37 different prey items were identified in the stomachs. By comparison, 25, 26, and 33 prey taxa were identified in the stomach contents of Dolly Varden char (Salvelinus malma), threespine stickleback, and staghorn sculpin, respectively. Amphipods were the dominant prey of all fish collected from China Poot Marsh; chironomidae larvae were the most common item in the stomach contents of threespine stickleback from Potter Marsh. The diets of all species changed over the course of the study period; the change was most dramatic for juvenile salmonids and sculpins.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号