SHAPE AND FUNCTION OF THE SHELL: A COMPARISON OF SOME LIVING AND FOSSIL BIVALVE MOLLUSCS

• 1 The review is mainly concerned with Carboniferous non-marine Anthracosiidae and Myalinidae, of which only the shells are known, and with certain unspecialized non-byssate suspension-feeding bivalves which had smooth shells and burrowed shallowly.
• 2 Limited experimental evidence and observation of living bivalves suggest that in certain Recent siphonate species and in some members of the non-siphonate Anthracosiidae the shape of the shell was functionally related to movement through the sediment in the same way. Predicted optimum shapes of shells for downward burrowing and for upward near-vertical movement in sands and silts were apparently realized in the Anthracosiidae, which constituted a series of highly variable opportunistic assemblages. It is stressed, however, that the shape of the shell always appears to be a compromise between several functional requirements.
• 3 In both the early Anthracosiidae and in several analogous Recent marine genera, orientation of the long axis of the shell was the same for downward burrowing and for upward pushing, that is near the vertical, with posterior end upward.
• 4 Invasion of the Pennine late-Namurian delta took place when marine bivalves pushed upward, thereby avoiding sedimentation from delta lobes moving seaward relatively swiftly. The evolution of Carbonicola occurred at about this time (the Marsdenian Age) when the bivalves acquired a smooth elongate shell of ‘streamlined’ form, having a hinge plate with swellings and depressions on it (later to evolve into teeth). All these features tend to characterize the active shallow burrowers of today.
• 5 Entry into soft-bottom eutrophic conditions of fresh water is characterized in several unionids by increase in height/length (H/L or w/m) ratio of shell, in anterior end/length (A/L) and in obesity (T/L) (see Fig. 2, centre). These changes also took place in established faunas of Carbonicola characteristic of richly carbonaceous shales, in faunas of supposed Anthraconaia in more carbonaceous sediments of mid-and late-Carboniferous times in the U.S.A. and in Anthraconauta of the British late Carboniferous (Westphalian C and D). The genus Anthracosphaerium epitomizes the culmination of these trends in the Anthracosiidae, and species of the genus were probably epifaunal or shallowly infaunal active burrowers on soft bottoms in Westphalian upper A and B time.
• 6 Two contrasting patterns of growth characterize the shells of the widely variable unionid Margaritifera margaritifera. In the first, dorsal arching of the shell, with straightening and reflexion of the ventral margin, provides increased weight but decreased ligamental strength. In the second, in which the ‘hinge line’ tends to remain straight while the dorsal margin becomes more rounded and obesity increases, there is increased metabolic efficiency for active surface movement. The maintenance of these trends within the species, which may be regarded as secondarily opportunistic, affords a means of insurance for survival within the highly variable environments of fresh water. The same trends are recognizable in established faunas of Carbonicola, where it is likely that they performed the same function, as well as in Mesozoic and Cenozoic Unionidae.
• 7 The functional explanation outlined in paragraph (6) may be extended to provide an ecological meaning for Ortmann's ‘Law of Stream Distribution’, which states that obesity of unionid shells increases downstream. This applies broadly, within a fairly wide range of variation, a fact which again suggests ‘insurance’ of faunas against the variable hazards of fresh-water habitats.
• 8 In bivalves having considerable thickness of shell in relation to their size, and having strong umbonal development, specific gravity of the living bivalve is correlated with H/L and T/L ratios of the shell, as in the venerid Venerupis rhomboides. In this species, differences in the specific gravities of the bivalves, as well as their shape, appear to be functionally related to shallowly infaunal burrowing in different substrates.
• 9 The conclusions of paragraphs 6 and 8 provide a functional explanation, in terms of selection, for the palaeoecological ‘law’ of Eagar (1973), which is applicable to established faunas of Carbonicola in mid-Carboniferous time, and relates variational trends in two main groups of shells primarily to increases in the relative water velocities of the palaeoenvironments.
• 10 Where the growth of relatively unspecialized bivalve shells has been measured, allometric relationships have usually been found in H-L and T-L scatters. Logarithmic lines have two inflexions and linear scatters a sigmoidal form. A similar pattern of allometric growth has been found in both Carbonicola (H-L) and Anthraconauta (m-w). These patterns appear to be related to the optimum requirements of water-borne larvae, the initial byssal phase of settlement, when ability to burrow quickly is essential, and the main period of growth and activity. It is herein suggested that the final second inflexion, which indicates a falling off of gain in H/L and T/L ratios, may be a genetically controlled modification of the growth pattern which counteracts the operation of the ‘cube-square rule’ (of Thompson, 1942) and prolongs productive life.
• 11 Patterns of relative growth of the shell may be significantly modified by conditions of the habitat; both T/L and H/L ratios may be increased, with general reduction in size, in the less ‘favourable’ habitats. Both these ratios have been similarly modified, the one in the ‘natural laboratory’ of a lake formed by the damming up of streams, and the other in transplant experiments with living Venerupis. In both these latter cases, phenotypic changes took place in the same direction as those expected on the basis of natural selection. Direct response to environmental factors cannot therefore be ruled out as an agent in similar changes noted in Carbonicola and supposed Anthraconaia in paragraphs (5) and (9) and may have been operative in those of paragraphs (7) and (8).

     

VASCULAR BUNDLES IN PETIOLES OF SOME HERBACEOUS AND WOODY DICOTYLEDONS

Structure and distribution of vascular bundles (VB) have been investigated in petioles of 26 herbaceous and woody dicotyledons. No single pattern of vascular bundle distribution could be found for both groups of plants. Both groups of plants had VB patterns ranging from a cylinder to separate bundles arranged in a variety of patterns, e.g., distorted cylinder, various configurations of a crescent, and other types. The total number of xylem vessels in herbaceous and woody plant petioles varied between 110 and 1756.     

UNUSUAL DARK-GROWTH AND ANTHERIDIAL DIFFERENTIATION IN SOME GAMETOPHYTES OF THE FERN ONOCLEA SENSIBILIS

A fertile frond of O. sensibitis was found which yielded spores with unusual growth characteristics. About 25% of the gametophytes derived from the spores were able to undergo 2-dimensional development in darkness, in contrast to normal plants which are filamentous in darkness. When the aberrant spores were cultured in darkness under conditions of reduced ethylene concentration, the proportion of 2-dimensional plants rose to 75%, and, moreover, up to 50% of the gametophytes produced antheridia within 2 weeks. Under comparable conditions normal gametophytes produced no antheridia. The medium from antheridial cultures of the aberrant spores failed to induce the formation of antheridia in other plants.     

暖温带若干落叶阔叶林群落物种多样性及其与群落动态的关系

 分析比较了暖温带中部至西南部20个落叶阔叶林群落类型的物种多样性,结果表明：1)一般情况下,乔、灌、草3个层次的物种丰富度(Sp)和多样性指数(D和H′)多为乔木层<灌木层<草本层,但均匀度指数(Ea)则多以灌木层居高;2)处于亚顶极阶段的森林群落各层次多样性指数(D和H′)特征是乔木层<草本层<灌木层;3)当群落处于顶极亚顶极阶段时,群落各层次的均匀度指数均在0.5以上;4)以群落总体重要值为测度指标来测度群落的总体物种多样性指数和均匀度指数时,发现群落总体Simpson多样性指数(D)可用于判断群落稳定性的高低,即总体多样性指数D低于或接近0.5的群落其稳定性较差或为特殊生境下的群落类型;当总体多样性指数D高于0.5时有两种情况：群落的均匀度指数Ea大于D的群落为地带性群落类型,它们有很高的稳定性,相当于顶极群落,而Ea小于或与D接近的群落具有较高的物种丰富度,在群落的演替动态方面处于顶极的前期,相当于亚顶极群落。     

FLIGHTLESSNESS IN STEAMER-DUCKS (ANATIDAE: TACHYERES): ITS MORPHOLOGICAL BASES AND PROBABLE EVOLUTION

Flightlessness in Tachyeres is caused by wing-loadings in excess of 2.5 g·cm^–2, which result from the large body size and small wing areas of the flightless species. Reduced wing areas of flightless species are related to absolutely shorter remiges, and to relatively or absolutely shortened wing bones, although these reductions differ among species. Reduced lengths of the ulna, radius, and carpometacarpus are associated most strongly with flightlessness. Pectoral muscles and the associated sternal keel are well developed in all species of Tachyeres, largely because of the use of wings in “steaming,” an important locomotor behavior. Relative size of these muscles was greatest in largely flighted T. patachonicus; however, sexual dimorphism in wing-loadings results in flightlessness in some males of this species. Proportions in the wing skeleton, intraspecific allometry, and limited data on growth indicate that the relatively short wing bones and remiges of flightless Tachyeres are produced developmentally by a delay in the growth of wing components, and that this heterochrony may underlie, in part, skeletal sexual dimorphism. Increased body size in flightless steamer-ducks is advantageous in territorial defense of food resources and young, and perhaps diving in cold, turbulent water; reductions in wing area probably reflect refinements for wing-assisted locomotion and combat. Flightlessness in steamer-ducks is not related to relaxed predation pressure, but instead was permitted selectively by the year-round habitability of the southern South American coasts. These conditions not only permitted the success of the three flightless species of Tachyeres, but at present may be moving marine populations of T. patachonicus toward flightlessness.     

高等植物性别分化研究的某些进展

高等植物性别分化研究的某些进展邵宏波（四平师范学院生物工程研究室吉林四平１３６０００）关键词高等植物,性别分化,基因表达ＳＯＭＥＡＤＶＡＮＣＥＳＩＮＴＨＥＳＥＸＵＡＬＤＩＦＦＥＲＥＮＴＩＡＴＩＯＮＲＥＳＥＡＲＣＨＯＦＨＩＧＨＥＲＰＬＡＮＴＳ￥Ｓｈａｏ...     

SHELL GROWTH IN SOME NILE BIVALVES

The growth of three species of mussels in the White Nile nearKhartoum was studied from caged specimens and natural populations.True annual rings develop as a result of a silting-induced restingstate during the annual flood from July to October. False ringsalso develop, possibly as a result of unfavourable factors likethe change of habitat, inadequate food, handling of the mussels. Growth is inhibited during November-January as a result of thelower water temperature and reproduction. However, there wasevidence that during this period juveniles grew at a higherrate than the adults. It could be concluded that there is atrade-off between reproductive effort and growth in the adults;growth is inhabited until the eggs are released from the gonadsinto the demibranchs. The period from February to July is the growth season; it ischaracterised by high water temperature and high transparency.The mussels had already spawned by February. Significant incrementswere observed on the shells of caged specimens especially afterMay. These increments were, however, smaller than comparablegrowth in natural populations. This indicated that the cageshad suppressed the growth of the mussels. Growth rates are slow. Estimation of maximum sizes for the unionidsusing Ford-Walford plots was almost in full agreement with theactual maximum sizes. The poor agreement in the case of M. dubiais related to the fact that this species is not well establishedin the habitat. ¹Present Address: Biology Department, Royal Holloway and BedfordNew College, Egham, Surrey TW20 0EX (Received 4 April 1989; accepted 20 July 1989)     

鲁北的贝沙岗与贝沙植被类型

有关鲁北贝沙岗的植被,已往对它缺乏系统研究。本文概述了在鲁北渤海湾浅海这一特异环境条件下,植被的组成成分、生态特性、外貌结构和系统类型。     

THE GOLGI ZONE OF THE RAT SPERMATID AND ITS ROLE IN THE FORMATION OF CYTOPLASMIC VESICLES

Electron microscopic study of the Golgi zone of the rat spermatids revealed two main types of vesicular structures: flat vesicles generally accumulated in stacks and spheroidal vesicles of various sizes. The Golgi zone of young spermatids showed a predominance of flat vesicles and small spherical vesicles, while the "residual" Golgi zone of the maturing spermatids displayed an increased number of large spherical vesicles and thus became increasingly vacuolated in appearance. The images presented were suggestive of a transformation of the flat vesicles into spherical vesicles by fragmentation and dilatation and also of a passage of the spherical vesicles from the Golgi zone to the surrounding cytoplasm.