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1.
Muscular-hydrostats, muscular organs which lack typical systems of skeletal support, include the tongues of mammals and lizards, the arms and tentacles of cephalopod molluscs and the trunks of elephants. In this paper the means by which such organs produce elongation, shortening, bending and torsion are discussed. The most important biomechanical feature of muscular-hydrostats is that their volume is constant, so that any decrease in one dimension will cause a compensatory increase in at least one other dimension. Elongation of a muscular-hydrostat is produced by contraction of transverse, circular or radial muscles which decrease the cross-section. Shortening is produced by contraction of longitudinal muscles. The relation between length and width of a constant volume structure allows amplification of muscle force or displacement in muscular-hydrostats and other hydrostatic systems. Bending requires simultaneous contraction of longitudinal and antagonistic circular, transverse or radial muscles. In bending, one muscle mass acts as an effector of movement while the alternate muscle mass provides support for that movement. Torsion is produced by contraction of muscles which wrap the muscular-hydrostat in a helical fashion.  相似文献
2.
Cephalopod fisheries are among the few still with some local potential for expansion; in fact, as groundfish landings have declined globally, cephalopod landings have increased. We propose the hypothesis that, although increased cephalopod landings may partly reflect increased market demand, overfishing groundfish stocks has positively affected cephalopod populations. Data from 15 key FAO areas reveal that, with the exception of the north- east Atlantic, cephalopod landings have increased significantly over the last 25 years while groundfish have risen more slowly, remained stable, or declined. In terms of volume, cephalopods have not replaced groundfish. This is hypothesized as owing to the shorter life cycle of cephalopods, and rapid turnover and lower standing stocks than for longer-lived finfish species. Under high fishing pressure, groundfish are probably poor competitors, having less opportunity for spawning and replacement. In West Africa, the Gulf of Thailand and Adriatic there is strong circumstantial evidence that fishing pressure has changed ecological conditions and cephalopod stocks have increased as predatory fish have declined. We recommend that this hypothesis be tested thoroughly in other areas where suitable data exist. Most coastal and shelf cephalopod fisheries are likely to be fully exploited or overexploited, and current annual fluctuations in cephalopod landings are probably largely environmentally-driven.  相似文献
3.
Evolutionary expansion of the Monogenea   总被引:4,自引:0,他引:4  
The evolutionary expansion of the monogeneans has taken place in parallel with the diversification of the fish-like vertebrates. In this article the main trends in monogenean evolution are traced from a hypothetical skin-parasitic ancestor on early vertebrates. Special consideration is given to the following topics: early divergence between skin feeders and blood feeders; diversification and specialization of the haptor for attachment to skin; transfer from host to host, viviparity and the success of the gyrodactylids; predation on skin parasites and camouflage; colonization of the buccal and branchial cavities; diversification and specialization of the haptor for attachment to the gills; phoresy in gill parasites; the development of endoparasitism and the origin of the cestodes; the success of dactylogyroidean gill parasites; the uniqueness of the polyopisthocotyleans; ovoviviparity and the colonization of the tetrapods. Host specificity has been the guiding force of coevolution between monogeneans and their vertebrate hosts, but the establishment of monogeneans on unrelated hosts sharing the same environment (host-switching) may have been underestimated. Host-switching has provided significant opportunities for evolutionary change of direction and is probably responsible for the establishment of monogeneans on cephalopod molluscs, on the hippopotamus and possibly on chelonians. There are indications that host-switching may be more common in monogeneans that spread by direct transfer of adults/juveniles from host to host. A limitation on the further expansion of monogeneans is the need for water for the dispersal of the infective larva (oncomiracidium).  相似文献
4.
头足类早期生长阶段的比较研究   总被引:4,自引:0,他引:4       下载免费PDF全文
董正之 《动物学研究》1992,13(3):209-216
本文首次系统描述了乌贼科、枪乌贼科、微鳍乌贼科和蛸科(章鱼科)浅海性种类早期生长阶段的形态特征,并与成体的形态特征相比较。对比了浅海性头足类与大洋性头足类的早期生长阶段。发现浅海性头足类在早期生长阶段中的形态变化较小,而大洋性的头足类在早期生长阶段中的形态变化较大。表明在系统演化中,浅海性头足类已走在大洋性头足类的前面。  相似文献
5.
1. Monthly series of abundance indexes for the English Channel squid stock, based on fishery statistics of the United Kingdom (1980–93) and France (1986–96), were compared with water temperature data. The two objectives of the study were to test empirical predictive models and to analyse the stock–environment relationship at various time scales; both correlation and time-series statistical techniques were applied. Sea surface temperature (SST) showed inter-annual fluctuations and month-to-month auto-correlation in addition to the annual cycle.
2. Trends in squid landings and temperature at the annual scale were found to be related, whatever the statistical method used (moving averages, cumulative functions or regression using averaged data).
3. Variable selection applied in a 'multi-month' model suggested that fishing season indexes could be predicted from temperatures observed in the previous winter. The link between mild winter conditions and cohort success in winter/spring spawning species suggested that early life survival (and/or growth) was involved. This empirical model is a first step in the development of environment-predicted recruitment indexes useful for management advice.
4. Seasonal decomposition was performed on both the squid resource data and SST data in search of short-term relationships. In spite of the flexibility of the loliginid life-cycle, no significant relationship was found between squid seasonally adjusted indexes and temperature anomalies in the previous months. This underlined the conclusion that temperature effect on cohort success was not constant throughout the year.  相似文献
6.
头足类若干结构的形态比较   总被引:3,自引:0,他引:3  
董正之 《动物学报》1993,39(4):348-354
本文首次系统观察分析比较了中国近海头足类的肉鳍、平衡器、齿舌和漏斗器,并参考世界代表性种类和古生头足类的文献和图,全面研究了这些结构形态的式型和亚型,探讨了它们与生活方式和演化的相互关系。  相似文献
7.
Cuttlefish camouflage: a quantitative study of patterning   总被引:2,自引:0,他引:2  
To investigate camouflage design, we compared the responses of two species of cuttlefish ( Sepia officinalis and Sepia pharaonis ) with controlled but naturalistic backgrounds, consisting of mixtures of 1-mm and 9-mm diameter coloured pebbles. Quantitative analysis of image data using methods adapted from functional imaging research found differences in how the two species camouflage themselves. Whereas S. officinalis switches from background resemblance to a disruptive pattern as it moves from a fine to a coarsely patterned background particle, S. pharaonis blends the two types of pattern. We suggest that the differences may arise because S. pharaonis needs to produce camouflage that is effective when viewed over a relatively wide range of distances.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 335–345.  相似文献
8.
Squid are important components of many marine ecosystems from the poles to the equator, serving as both important predators and prey. Novel aspects of their growth and reproduction mean that they are likely to play an important role in the changing oceans due to climate change. Virtually every facet of squid life-history examined thus far has revealed an incredible capacity in this group for life-history plasticity. The extremely fast growth rates of individuals and rapid rates of turnover at the population level mean that squid can respond quickly to environmental or ecosystem change. Their ‘life-in-the-fast-lane’ life-style allows them to rapidly exploit ‘vacuums’ created in the ecosystem when predators or competitors are removed. In this way, they function as ‘weeds of the sea’. Elevated temperatures accelerate the life-histories of squid, increasing their growth rates and shortening their life-spans. At first glance, it would be logical to suggest that rising water temperatures associated with climate change (if food supply remains adequate) would be beneficial to inshore squid populations and fisheries—growth rates would increase, life spans would shorten and population turnover would accelerate. However, the response of inshore squid populations to climate change is likely to be extremely complex. The size of hatchlings emerging from the eggs becomes smaller as temperatures increase and hatchling size may have a critical influence on the size-at-age that may be achieved as adults and subsequently, population structure. The influence of higher temperatures on the egg and adult stages may thus be opposing forces on the life-history. The process of climate change will likely result in squids that hatch out smaller and earlier, undergo faster growth over shorter life-spans and mature younger and at a smaller size. Individual squid will require more food per unit body size, require more oxygen for faster metabolisms and have a reduced capacity to cope without food. It is therefore likely that biological, physiological and behavioural changes in squid due to climate change will have far reaching effects.  相似文献
9.
An exceptionally well-preserved fauna from a new exposure in the Peterborough Member (Oxford Clay Formation) of southern England, equivalent to the famous and now inaccessible Lagerstätte at Christian Malford, Wiltshire, is described. It comes from a single bed and includes coleoid cephalopods with phosphatized soft tissues, and fully articulated fish. The level is unusual in that it lacks permanent benthos and is dominated by 'rain-out' from the upper water column. It was deposited on a sea floor that experienced prolonged periods of anoxia and which was overlain by a water column that was at least intermittently stratified. It is postulated that the coleoids formed large shoals that were killed en masse , together with other elements of the associated fauna, in one or more catastrophic mass mortality events that affected a significant area of the Peterborough Member sea. During the event(s), many of the coleoids preyed upon moribund fish and other coleoids, sometimes of the same species, before becoming overcome themselves. Crucially for the phosphatization of their soft tissues, the substrate surface was 'soupy' and allowed them to sink to a zone of rapid diagenesis where levels of dissolved phosphorus were greatly augmented by the large number of associated decaying carcasses.  相似文献
10.
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