Epibiont habitation patterns and their implications for life habits and orientation among trigoniid bivalves

The bivalve superfamily Trigoniacea has persisted from the Late Paleozoic to the Recent. Late Jurassic and terminal Cretaceous mass extinctions decimated this once-dominant group in shallow marine facies; only a single genus with seven species survives today in the Austral Province. Trigoniacea retain a vestigial byssus and primitive but efficient schizodont dentition. They have been widely considered as infaunal bivalves, burrowing with a very large foot to shallow depths, with inhalant and exhalant apertures at or slightly below the sediment-water interface (SWI). Yet the Trigoniacea are poorly adapted for this life habit. The mantle in living species is unfused and non-siphonate, and some fossil Trigoniacea have permanent shell gapes over these apertures, enhancing the probability of sediment fouling of feeding and respiratory structures. Some living Neotrigonia , e.g., N. margaritacea , solve this problem by having a semi-infaunal life habit, with the inhalant and exhalant apertures elevated above the SWI and the zone of active sediment transport. Semi-infaunal species commonly have epibionts cohabiting the exposed posterior-posteroventral portion of the shell. Numerous well-preserved species of South American Mesozoic Trigoniacea have phototropically and geotropically oriented epibionts on co-attached valves, strongly suggesting a semi-infaunal life mode for at least some members of these taxa. These shell symbionts allow orientation of extinct trigoniid shells relative to the SWI during life, as well as analysis of their depth of burial. Careful analyses of the kinds, size classes, orientation, and dispersion of various epibionts on fossil Trigoniacea thus yield important new information on their life habits, and demonstrate that semi-infaunal life modes were far more common than previously supposed.     

Burrowing mechanisms and sculptures in Recent gastropods

Savazzi, Enrico 1989 01 15: Burrowing mechanisms and sculptures in Recent gastropods. Lethaia , Vol. 22, pp. 31–48. Oslo. ISSN 0024–1164.
Burrowing was observed in 32 gastropod species, belonging to 8 families, from Italy and the Philippines. Most species burrow by repeating a three-phase sequence: (1) digging with the foot, (2) dragging the shell forward and downward, and (3) rocking the shell around its longitudinal axis. Minor specific differences in the burrowing dynamics are common, and totally different mechanisms also occur. Burrowing sculptures consisting of terraces or asymmetrical tubercles are observed in the majority of the studied species. Characteristics of the burrowing process explain some cases of apparent divergence of burrowing sculptures from the paradigm. Burrowing sculptures in the Gastropoda should be expected to occur mostly among medium-slender, rather than markedly high-spired, shell morphologies. □ Mollusca, Gastropoda, Cerithiidae, Nassariidae, Mitridae, Costellariidae, Conidae, Terebridae, Turridae, Hydro-biidae, shell, sculpture, burrowing, functional morphology, ecology, behaviour, Holocene, Indo-Pacific. Philippines, Mediterranean, Italy .     

Constructional morphology of strombid gastropods

The extreme diversity in shell shape of strombid gastropods is interpreted as the result of three independent factors: (1) The terminal growth pattern of the Strombidae allows the circumvention of geometric constraints on shell morphology found in gastropods with continuous or periodic growth patterns. (2) Shell morphology in the Strombidac is adaptive in epifaunal locomotion, burrowing. infaunal or semi-infaunal habits, and passive protection from predators. Specialization for one of these functions often conflicted with the others. thus bringing about a forced 'choice' among mutually exclusive morphological characters. (3) Conservatism in life habits and anatomy of the soft parts has allowed the multiple evolution of extreme shell morphologies, as well as the secondary return to relativcly unspecialized morphologies. □ Constructional morphology, functional morphology. growth. behaviour. evolution, locomotion, burrowing, predation, exoskeleton. shell. Mollusca. Gastropoda. Strombacea. Strombidae.     

Some morphological adaptations in freshwater bivalves

Savazzi, E. & Yao, P. 1992 04 15: Some morphological adaptations in freshwater bivalves. Lethaia , Vol. 25, pp. 195–209. Oslo. ISSN 0024–1164.
Several freshwater bivalves possess peculiar shell morphologies. An extension of the postero-dorsal shell margins above the hinge line evolved convergently in several unionids. This extension supplements the opening momentum of the ligament, but must be broken off periodically in order to allow further shell growth. Arconaia and Cuneopsis have evolved twisted commissure planes, comparable to those found in unrelated marine bivalves. In marine forms, byssus is believed to have played a fundamental role in the evolution of shell torsion. However, the twisted Unionidae do not possess a byssus in the adult stage, thus forcing us to re-evaluate our ideas on the adaptive value and evolution of shell torsion. Solenaia oleivora is apparently incapable of reburrowing and of retracting its foot within the shell. The foot may be functional as an anchor, and is perhaps involved in chemosynthesis by storing sulphur extracted as sulphide from the surrounding sediment. Other adaptations of freshwater bivalves include selective thickening of portions of the shell that enhance its stability, permanent anterior and posterior gapes, and oyster-like morphologies and shell structures. * Functional morphology, constructional morphology, burrowing, shell torsion, Mollusca, Bivalvia, Unionacea, Recent, Quaternary, People's Republic of China .     

Functional inference from gastropod shell morphology - some caveats

The dangers of making broad paleobiological inferences from shell morphology, based on limited observations of a few taxa, are indicated using as examples the secondary protoconchs of Colina species (Cerithiidae), and the ratchet sculpture of some surface dwelling species of Cerithium. Secondary protoconchs are not indicative of types of development and ratchet sculpture does not always indicate burrowing. □ Protoconch, shell morphology, functional morphology, burrowing, Cerithiidae.     

Ratchet riposte: more on gastropod burrowing sculpture

Recent warnings concerning paleobiological inferences based upon gastropod shell morphology (Houbrick 1991) merit serious consideration, although the dangers have been overstated. Ratchet sculpture, an asymmetrical sculpture that assists marine invertebrates in burrowing, is not qualitatively different from sculptures that apparently do not aid in burrowing. Therefore, the interpretation of such sculpture might be problematical. Nevertheless, the large body of empirical evidence demonstrating the function of ratchet sculpture in burrowing by bivalves, gastropods, carpoid echinoderms, brachiopods, and arthropods and the lack of evidence supporting alternative functions in the Gastropoda warrant the continued, although cautious, association of ratchet sculpture with burrowing in marine gastropods. □ Functional morphology, Gastropoda, ratchet sculpture, burrowing.     

The burrowing process of Dentalium (Scaphopoda)

Investigations of the burrowing activity of Dentalium , using cine film and electronic recording techniques, have shown it to penetrate the sand in a series of steps, each termed a "digging cycle". Cycles involve first, pedal dilation, second, retraction followed by extension and probing of the foot. The epipodial lobes are elevated during pedal dilation and form a secure pedal anchor so that at retraction the shell is drawn down over the foot.
A comparison of the burrowing process in the Scaphopoda with that of the Bivalvia indicates that essentially the same mechanisms and sequence of activities are involved, for in both digging consists of the integration of pedal protraction and retraction with the application of shell and pedal anchors. The principal differences, such as the absence in Dentalium of water jets to loosen the sand and high pressures in the pedal haemocoele, are related to the form of their shell. The strength of the pedal anchor was determined and, relative to the weight of Dentalium , is comparable to that of bivalves. In contrast the probing force was relatively weak since the shell anchor of Dentalium , which holds the shell still during probing, is largely limited to its own weight, whereas that attained by the Bivalvia is principally due to the valves being pressed against the substrate by the opening moment of the ligament.     

The chondrichthyan fauna from the Middle-Late Triassic of Guanling (Guizhou province,SW China)

The Recent volutids Cymbiola and Melo burrow in a forward direction. This agrees well with the spiral terrace pattern observed in Eocene Volutidae, thus strengthening the idea that they were forward burrowers as well. The presence of collabral terraces near the aperture in the Cassidae suggests an oblique burrowing direction, by convergence with the Nassariidae. This is confirmed by observations on living cassids. The sutural canals and associated structures of the Olividae and Seraphidae are functional in detecting whether the posterior region of the shell is buried. This confirms the idea that fossil Strombidae possessing similar features were burrowers.

In contrast with these “predictable”; observations, burrowing was observed in a few gastropods the shell morphology of which would seem to exclude such behaviour, such as Architectonica, Cymatium, Volema and Bolinus. Thus, a few morphologic criteria appear to be reliable and of general applicability in inferring burrowing habits in gastropods. At the same time, the broad variety of burrowing mechanisms and life habits of gastropods makes it unlikely that general criteria will ever be found to exclude burrowing habits on the basis of shell morphology.     

Effects of shell shape,size, and sculpture in burrowing and anchoring abilities in the freshwater mussel Potamilus alatus (Unionidae)

Because of the sedentary lifestyle of freshwater mussels, studies examining their movement capabilities are scarce. However, the ability to burrow into the substrate and the ability to remain stationary are likely crucial components of their behavioural repertoire. The performance of these different tasks is likely to be affected by the presence of the shell ornamentation characteristic of many mussel species. Previous studies have suggested that shell ornamentation results in a trade‐off between burrowing ability and remaining stationary when an extrinsic force attempts to dislodge it from the substrate once buried. We examined the effect of morphology and shell ornamentation on burrowing performance and anchoring ability by artificially creating shell ornamentation on a relatively smooth‐shelled species (Potamilus alatus). Burrowing behaviours and performance and the force required to dislodge mussels (anchoring ability) were quantified with and without ornamentation. Interestingly, we found that the artificial shell ornamentation had no significant effect on burrowing behaviours and performance or dislodgement force. Burrowing and dislodgement, however, were both highly influenced by shell size and shape. All of the available information suggests that shell size, shape, and sculpture influence burrowing and anchoring in complex ways that needs further examination. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 136–144.     

Life habit and ontogeny of the unusual arcid bivalve Ambrogia mytiloides (Brocchi, 1814)

The unusual Plio-Pleistocene arcid Ambrogia mytiloides (Brocchi, 1814) has a large, elongate, smooth and streamlined shell. On the basis of these characters and the occurrence of moderate shell torsion, the mode of life of this species was formerly thought to be semi-infaunal endobyssate, obliquely oriented like the twisted arcid Trisidos. The discovery of shells in life position suggests that this species lived in a subvertical position. Rather than a recliner, this arcid was then a sticker, whose stability was provided by the byssus, which also was used to aid the bivalve in burrowing, and by its large size. The morphology of juvenile valves, smaller than 4 mm, suggests an epibyssate mode of life in its early growth stages. Ambrogia represents a remarkable pathway in the secondary return of arcids to soft bottoms: with this genus, they reached their deepest burrowing level. However, this strategy was not very successful, probably because of evolutionary constraints on the Arcoida.     

A biomechanical hypothesis explaining upstream movements by the freshwater snail Elimia

1. Many taxa of freshwater invertebrates show active upstream movements, particularly the snails. Hypotheses explaining this behaviour invoke the search for food or space, compensation for drift, avoidance of predation and hydrodynamic effects. The pervasiveness of upstream movements among remote lineages of snails (two subclasses, three orders, 10 families), however, suggests that snails may move upstream for mechanical rather than adaptive reasons.
2. It is proposed that upstream movements by snails are a function of torque on the snail's foot generated by hydrodynamic drag on the shell. When subject to high broadside drag-forces on their shells, snails are able to reduce torque and stabilize orientation only by directing their anterior aspect upstream.
3. Movements of the freshwater pleurocerid snail Elimia were studied by following marked free-ranging individuals in six streams in Alabama, USA (four species, eight populations).
4. Populations showed either no net movement (two streams) or significant upstream movements ranging to a mean of ≈40 m over a 3-month period (four streams). Movement patterns were stream specific rather than species or population specific. Within populations showing significant upstream movements, snails with shell lengths ≤10 mm showed little net movement. Larger snails showed movements from 0 to >200 m upstream.
5. A torque-constrained random walk model was used to perform a post hoc test of the hypothesis that upstream movements were a function of torque on the snail's foot generated by hydrodynamic drag on the shell. The model predicted upstream and size-dependent movement patterns that approximated those observed for snails in the field.     

Adaptive modifications of carapace outlines in the Cytheroidea (Ostracoda: Crustacea)

Shell outlines of 202 extant cytheroidean ostracods were analysed in dorsal, lateral, and posterior views by elliptic Fourier analysis. The results obtained confirm that the exterior morphology is related to ecological factors as well as phylogenetic constraints. Phytal species living on tall seagrass and benthic species burrowing in sediments are characterized and differentiated from the species crawling on sediments by the presence of slender shells with tapered venters. With reference to reconstruction of ancestral state of outline traits on the molecular phylogeny, the hypothetical ancestor of cytheroidean ostracods is presumed to have had an average-shaped shell. Morphological plasticity of the shell outline was observed in many families. The phytal species living on tall seagrass appear to have evolved convergently with species from other habitats, acquiring slim shell outlines during the Cenozoic period. The present analysis also reveals the phylogenetic constraints on the morphological evolution of the Trachyleberididae in their adaptation to a burrowing habit.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 810–821.     

The mechanism of shell elevation in Haliotis (Mollusca: Gastropoda) and a consideration of the evolution of the hydrostatic skeleton in Mollusca

Early in molluscan evolution, the development of a conical shell with shell or pedal retractor muscles led to the need of a mechanism for the extension of the foot or the raising of the shell. The forces generated during pedal retraction and extension have been studied in Haliotis midae , an easily obtainable and conveniently large archaeogastropod. In the mantle cavity, cephalopedal venous sinus and ventricle pressure pulses were observed during pedal retraction elicited by the shadow withdrawal reflex, but were never present during extension. However, pressure pulses were recorded in the proximal region of the columellar (or shell) muscle, both during retraction and pedal extension. Sections of this region of the muscle show a three dimensional network of muscle fibres, consisting of retractor fibres passing down to the foot and circumferential and radial fibres. Contraction of the two latter sets of fibres would bring about extension of the retractors, without the use of a discrete hydrostatic skeleton, and appears to be the principal mechanism of pedal extension. Similar muscular structures, here termed the muscular antagonistic system, have been observed in the columellar muscle of other gastropods and in the cephalopod mantle. In contrast, this system has not been observed in the proximal region of the pedal retractors of bivalves or scaphopods, for the pedal haemocoel, which allows muscular antagonism in the manner of a classical hydrostatic skeleton, has developed in association with the burrowing habit. The significance of the muscular antagonistic system in molluscan evolution is discussed.     

Spikey bivalves: intra-periostracal crystal growth in anomalodesmatans

The external shell surfaces of most anomalodesmatan bivalves are studded with small spikes, particularly at the posterior end. We have studied the morphology, mode of growth, and distribution among taxa of these spikes. In this study we found that spikes vary widely in morphology, from acute spikes to flat plaques. Optical and electron microscopy has revealed that the periostraca of Laternula, Myadora, and Thraciopsis consist of an outer dense layer and an inner translucent layer. The dense layer grows at the expense of the inner layer as it progresses toward the shell edge. The spikes begin to grow in the free periostracum, within the translucent periostracal layer, immediately below the dense layer. With growth, they push the dense periostracal layer upward but without penetrating it. Those parts of the spike in contact with this layer cease to grow, which explains the typical conical shape of spikes. When fully grown, spikes reach the base of the translucent layer, becoming incorporated into the outer shell layer. Scanning electron microscopy and electron backscatter diffraction analysis reveal that the spikes of Lyonsia norwegica and Lyonsiella abyssicola are prisms of aragonite composed of twinned crystals, with the c-axis vertical. A survey of the occurrence of spikes within the anomalodesmatans shows that they are present in all but a few families. Elsewhere within the closely related palaeoheterodonts, intra-periostracal calcification is also known in Neotrigonia and unionids, which indicates that this character may be plesiomorphic for these bivalves. The present data do not support the homology of spikes in other bivalve groups (e.g., veneroids) or in the aplacophorans or polyplacophorans.     

Silurozodus,new genus,the oldest known member of the Trigonioida (Bivalvia,Mollusca)

The trigonioid bivalveSilurozodus gotlandicus n. gen. et n. sp. is described in terms of functional morphology, ecology, and life habit. The excellently preserved material has made it possible to reconstruct the nature of the foot and its muscles and the position of the gills.Silurozodus gotlandicus is interpreted to have been a nonsiphonate, actively burrowing, infaunal suspension-feeder that lived in a high-energy and unstable environment.Silurozodus gotlandicus is of late Wenlockian (Homerian) age and is the oldest known representative of the Trigonioida. It is probably ancestral to all younger members of this order.     

合浦珠母贝幼虫变态中的形态、器官变化及运动与摄食观察

为揭示合浦珠母贝幼虫至稚贝生长发育过程中其外部形态变化及内部器官改变的内在规律, 掌握其形态和器官与运动和摄食行为之间的关联。在光学显微镜下对整个幼虫生长发育及变态过程中的外部形态、内部器官特征进行了系列观察和性状测量; 利用非线性回归参数拟合, 描述各形态性状生长特点及不同属性之间的联系; 观察不同发育阶段其运动与摄食过程。结果显示, 幼虫在正常生长过程中, 其壳长生长方式为加速正增长、壳高为减速正增长、绞合线长为加速负增长, 壳高相对于壳长的生长为快速生长、绞合线长相对于壳长为慢速生长。幼虫生长至壳长为(209.26±9.22) μm时, 内部器官发生改变, 面盘开始逐渐退化从而发育成鳃, 斧足逐渐形成; 壳长生长至(234.30±14.00) μm时, 次生壳开始长出, 外部形态逐渐向稚贝转变。稚贝阶段, 其鳃丝长、鳃丝间距和鳃丝数量相对于壳长的生长均表现为慢速生长。幼虫在水中的运动和摄食过程主要依靠面盘外周纤毛的摆动来完成, 俯视观幼虫绕不规则圆沿顺时针方向运动, 垂直观幼虫螺旋上升或下降。稚贝阶段, 依靠斧足的往复伸缩来完成爬行, 依靠鳃的过滤完成摄食。在幼虫变态过程中, 面盘退化至鳃具备滤食功能期间, 变态幼虫运动功能降低, 摄食能力丧失, 依靠自身能量储备来完成生长和器官发育, 这一过程是苗种培育中的重要关键点。     

Spatial distribution of Trichoptera larvae in the sediments of an Austrian mountain brook

SUMMARY 1. A set of thirty-six permanently installed standpipc traps was used over 2 years to sample caddis larvae at various depths in the gravel bed of an Austrian brook. From a total of 805 specimens caught, the most abundant species were Sericostoma sp. (Sericostomatidae), and the three limnephilids Potamophylax cingulatus Steph., Allgamus auricollis Pictet and Allogamus uncatus Brauer.
2. Sericostoma sp. and Ecclisopteryx guttulata Pictet were collected down to a seditnent depth of l m. In Sericostoma sp., a burrowing species, tiny larvae were found in sediments at 20–60 em, where most of the life cycle is spent; fully grown larvae were mostly collected at a depth of 0–20 em. All instars of Potamophylax cingulatus, Allogamus auricollis and Allogamus uncatus larvae were most abundant at the sediment surface.
3. The horizontal distribution of the most abundant species was studied at the 20 cm depth stratum. Larvae were most abundant in midstream areas.
4. Factors probably responsible for the observed spatial distribution pattern are briefly diseussed.     

The skull of the rare Malaysian snake Anomochilus leonardi Smith, based on high-resolution X-ray computed tomography

The skull of the rare Malaysian snake Anomochilus leonardi is described in detail on the basis of a high-resolution X-ray computed tomographic scan of a mature specimen. Its skull anatomy is compared with that of Anomochilus weberi , as well as with that of scolecophidians and basal alethinophidians such as Anilius , Cylindrophis , uropeltines and selected Booidea. Anomochilus leonardi is found to be more paedomorphic than Anomochilus weberi . The genus Anomochilus most closely resembles uropeltines in skull anatomy. Both Anomochilus and uropeltines develop a 'central rod design' of skull morphology, which requires the presence of medial frontal pillars, in adaptation to burrowing habits. These pillars are an alethinophidian characteristic, absent in the skull of scolecophidians, which develop an 'outer shell design' in adaptation to burrowing. These results are discussed in the light of the hypothesis that scolecophidians and basal (i.e. non-macrostomatan) alethinophidians are 'regressed macrostomatans'.  © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society , 2007, 149 , 671–685.     

Muscle development in Antalis entalis (Mollusca,Scaphopoda) and its significance for scaphopod relationships

We applied fluorescence staining of F-actin, confocal laser scanning microscopy, as well as bright-field light microscopy, SEM, and TEM to examine myogenesis in larval and early juvenile stages of the tusk-shell, Antalis entalis. Myogenesis follows a strict bilaterally symmetrical pattern without special larval muscle systems. The paired cephalic and foot retractors appear synchronously in the early trochophore-like larva. In late larvae, both retractors form additional fibers that project into the anterior region, thus enabling retraction of the larval prototroch. These fibers, together with the prototroch, disappear during metamorphosis. The anlagen of the putative foot musculature, mantle retractors, and buccal musculature are formed in late larval stages. The cephalic captacula and their musculature are of postmetamorphic origin. Development of the foot musculature is dramatically pronounced after metamorphosis and results in a dense muscular grid consisting of outer ring, intermediate diagonal, and inner longitudinal fibers. This is in accordance with the proposed function of the foot as a burrowing organ based on muscle-antagonistic activity. The existence of a distinct pair of cephalic retractors, which is also found in basal gastropods and cephalopods, as well as new data on scaphopod shell morphogenesis and recent cladistic analyses, indicate that the Scaphopoda may be more closely related to the Gastropoda and Cephalopoda than to the Bivalvia.     

The Middle Jurassic Bivalve 'Cuspidaria'Ibbetsoni: A Corbulid Not A Septibranch

A distinctive shallow–burrowing bivalve with an elongate rostrum, common in the British Middle Jurassic, is frequently referred to as Cuspidaria ibbetsoni (Morris, 1853) and as such is recognised as one of the earliest members of the anomalodesmatan family Cuspidariidae. Since modern cuspidariids are highly specialised bivalves and are voracious carnivores in the deep sea, whereas C. ibbetsoni is prevalent in brackish–water facies, questions have been raised over its true affinities. We have investigated new and pre–existing material of C. ibbetsoni , and have concentrated on shell microstructure, morphology, and musculature. Our findings suggest that the inequivalve shell is composed of crossed–lamellar aragonite rather than either homogeneous or prismato–nacreous structures. It also lacks the complex musculature associated with the raptorial siphon of the predatory septibranchs. These characters suggest that C. ibbetsoni should more properly be placed within the myoid family Corbulidae and a new genus, Rostrocorbula , has been erected to accommodate it. It has also been necessary to establish a neotype. The significance of this familial redesignation to our understanding of the evolution of carnivory within the Bivalvia is discussed.