Outer Demibranchs Research Articles

Комплексний вплив нітратів і біотичних чинників на активність in vitro війок фронтального миготливого епітелію зябрового апарату Sinanodonta woodiana (Mollusca, Bivalvia, Unionidae)

Effects of MPC, 2 MPC, and 3MPC water solutions of potassium nitrate and biotic factors (trematode infection and “gill pregnancy”) were studied on the duration of locomotion and beat frequency of epithelial cilia of outer demibranchs of Sinanodonta woodiаna (Lea, 1834). The species has recently invaded the system of water bodies of Ukraine. It has occupied numerous water bodies in the Kiliya distributary of Danube Delta, establishing populations of density and biomass. Statistically significant differences by the studied parameters of ciliary epithelium (duration of locomotion and beat frequency of cilia) were not observed in the control group of non-infected males and latent females of S. woodiana. In females with “gill pregnancy”, the duration of ciliary locomotion decreased by 1.5 times, and beat frequency by 1.4 times (P>99.9 %). It should be noted that the outer surface of demibranchs of female mollusks was completely covered by the filled to the brim marsupial pockets. Thus, the functional load of gills is high, leading to the development of compensatory processes in experimental animals. The processes induce increased effectiveness of protective factors that counteract them. Another biotic factor negatively affecting the experimental pond mussels was the high rates of trematode infection in the hosts’ reproductive glands which contained sporocysts and cercariae of Rhipidocotyle campanula Dujardin, 1845. Prevalence of that infection ranged from 50 to 100 %. These rates of infection were associated with sharply declining functions of cilia of the gill epithelium in studied animals. The compound pathogenic influence of parasite and “gill pregnancy” factors even more affected the functional parameters of outer gill ciliary epithelium. The MPC levels of toxicant in the water medium were accompanied with the first symptom of intoxication of the Chinese pond mussel, the stage of stimulation. At that point, the functional parameters of epithelial cilia of “pregnant” females and in moderately and weakly infected mollusks of both sexes increased in response to exposure to the toxic factor. With the growing concentrations of toxicant to 2 MPC and 3 MPC, the experimental pond mussels developed the pathological process of intoxication, which progressed from the stage of depression to sublethal and lethal stages. Accordingly, the locomotion activity of ciliary epithelium increasingly declined and, in the end, stopped. The “gill pregnancy” and high rates of trematode infection exacerbated the consequences of the process.

Morphology and distribution of the freshwater mussel Diplodon granosus, a rare and poorly understood species

ABSTRACT Diplodon granosus was one of the first freshwater mussels to be described for South America. However, the status of the species was confusing for a long time, receiving different taxonomic treatments. In this paper, we redescribe the shell, with new data on the soft parts and information on the distribution and conservation of D. granosus, a rarely recorded species. The shell is thin, not inflated; the macrosculpture is composed only by granules that cover the whole shell, not forming bars; the microsculpture comprises short spikes. In the soft parts we highlight the few, weak and irregularly distributed lamellar connections of the outer demibranch and some features of the stomach, such as the distally enlarged minor typhlosole. There are records of D. granosus over a large area of South America, from Guiana to Argentina. However, most of these records are related to other species and the distribution of D. granosus is restricted to the north of South America in the basins of the Amazonas and Orinoco rivers, and coastal rivers in between. Despite this wide distribution, the species occurs in specific habitats, mainly streams (igarapés), resulting in an extremely fragmented occurrence. Thus, any disturbance to these habitats can threat this freshwater mussel.

Worm-riding clam: description of Montacutona sigalionidcola sp. nov. (Bivalvia:Heterodonta: Galeommatidae) from Japan and its phylogenetic position.

A new galeommatid bivalve, Montacutona sigalionidcola sp. nov., is described from an intertidal flat in the southern end of the Kii Peninsula, Honshu Island, Japan. Unlike other members of the genus, this species is a commensal with the burrowing scale worm Pelogenia zeylanica (Willey) (Annelida: Sigalionidae) that lives in fine sand sediments. Specimens were always found attached to the dorsal surface of the anterior end of the host body. This species has a ligament lithodesma between diverging hinge teeth, which is characteristic of Montacutona Yamamoto Habe. However, it is morphologically distinguished from the other members of this genus in having elongate-oval shells with small gape at the posteroventral margin and lacking an outer demibranch. Molecular phylogenetic analysis based on the four-gene combined dataset (18S + 28S + H3 + COI) indicated that this species is monophyletic with Montacutona, Nipponomontacuta Yamamoto Habe and Koreamya Lützen, Hong Yamashita, which are commensals with sea anemones or Lingula brachiopods. This result suggests that host shifting across different phyla occurred at least twice in this clade.

Ontogeny of juvenile freshwater pearl mussels, Margaritifera margaritifera (Bivalvia: Margaritiferidae).

The gills of juvenile freshwater bivalves undergo a complex morphogenesis that may correlate with changes in feeding ecology, but ontogenic studies on juvenile mussels are rare. Scanning electron microscopy was used to examine the ultrastructure and ontogeny of 117 juvenile freshwater pearl mussels (Margaritifera margaritifera) ranging in age from 1–44 months and length from 0.49–8.90 mm. Three stages of gill development are described. In Stage 1 (5–9 inner demibranch filaments), only unreflected inner demibranch filaments were present. In Stage 2 (9–17 inner demibranch filaments), inner demibranch filaments began to reflect when shell length exceeded 1.13 mm, at 13–16 months old. Reflection began in medial filaments and then proceeded anterior and posterior. In Stage 3 (28–94 inner demibranch filaments), outer demibranch filaments began developing at shell length > 3.1 mm and about 34 months of age. The oral groove on the inner demibranch was first observed in 34 month old specimens > 2.66 mm but was never observed on the outer demibranch. Shell length (R2 = 0.99) was a better predictor of developmental stage compared to age (R2 = 0.84). The full suite of gill ciliation was present on filaments in all stages. Interfilamentary distance averaged 31.3 μm and did not change with age (4–44 months) or with size (0.75–8.9 mm). Distance between laterofrontal cirri couplets averaged 1.54 μm and did not change significantly with size or age. Labial palp primordia were present in even the youngest individuals but ciliature became more diverse in more developed individuals. Information presented here is valuable to captive rearing programmes as it provides insight in to when juveniles may be particularly vulnerable to stressors due to specific ontogenic changes. The data are compared with two other recent studies of Margaritifera development.

Continuous reproduction of Sinanodonta woodiana (Lea, 1824) females: an invasive mussel species in a female-biased population

The reproductive activity of females of the Chinese pond mussel (Sinanodonta woodiana) was investigated in a European population inhabiting cooling water. The study used histological and stereological methods to determine gonad structure, changes in reproductive follicles during gametogenesis, oocyte development, glochidia structure, and brooding periods. Water in the channel of the “Dolna Odra” power plant did not freeze during the winter, and its mean annual temperature was 18.4°C. The population sex ratio was female biased (χ2 = 25.70, df = 1, P < 0.0001). Ovaries in mussels were formed by reproductive follicles. Previtellogenic and vitellogenic oocytes were attached to the follicle wall via the cytoplasmic stalk, and mature ovulated oocytes were present in the follicle lumen. Individuals with mature oocytes in gonads were present over the whole two-year study period, which indicates the continuous activity of gonads. Hooked glochidia had a larval thread. Females incubated the offspring in gill marsupia of outer demibranchs and were characterized by multiple tachytictic brooding periods. All gravid individuals presented mature and spent stages of gonadal development. The study provides the first direct information about the reproductive dynamics of this invasive species outside its original Asiatic range.

Functional morphology, biology and sexual strategy of the circumboreal, adventitious crypt-building,Crenella decussata(Bivalvia: Mytiloidea: Crenellidae)

The anatomy ofCrenella decussata(Mytiloidea) is described. Individuals of this circumboreal species occupy granular crypts composed of sand grains held in place by mucus. The swollen basal region of the tubule is occupied by an individual, which connects to the sediment surface by two posterior tubes accommodating the inhalant and exhalant streams. There is reduction in musculature and, most importantly, anterior foreshortening of the outer ctenidial demibranchs and loss of the labial palps. This creates an anterior space in the mantle for the initial brooding of fertilized ova by females to the prodissoconch stage. Subsequently, these larvae are transferred to the exhalant tube of the crypt wherein they attach by a single fine byssal thread and are further brooded until the crawl-away juvenile stage is attained. Experimental studies of larval behaviour suggest that parental pheromones sustain the female/offspring bond. Newly hatched individuals responded to parental exhalant water by actively attaching themselves using a byssal thread. This response persisted for 28 days, but not after 55 days when, we suggest, the pheromonal response ceases and offspring are developed sufficiently to take up life in their own nests. Offspring retrieved from parental crypts and fed continuously reached an average shell length of 500 μm after 7.5 months. Brooded offspring thus appear to rely on embryonal energy resources until post-metamorphosis, after which suspension feeding becomes essential for further growth and development before the parental crypt is vacated.

The strange case of the tetragenous Anodonta anatina.

Unionoid freshwater mussels have a unique life cycle with a form of parental care where the larvae are developed and kept inside the gills until release, followed by an obligate parasitic stage on fish. The size and location of the marsupium have been used as important phylogenetic characters in unionoids and in Anodontini its location was described exclusively on the outer demibranchs. Two recent surveys in a lake in the North of Portugal revealed large anodontine mussels morphological identical to Anodonta anatina but with glochidia in both demibranchs and with an unusual large size. In order to establish the identity of these mussels, a barcoding approach was used and an anatomical description of the gills and glochidia was performed. These mussels were identified as A. anatina and presented an inner demibranch pair with tripartite tubes. The glochidial sizes were much higher than previously reported for the species reaching maximum (length × height) values of 566 × 552 μm. This species reveals a high ecological plasticity being able to change its size and anatomy to increase its fertility as well as infestation performance. J. Exp. Zool. 325A:52-56, 2016. © 2015 Wiley Periodicals, Inc.

The early life history of two sympatric New Zealand octopuses: eggs and paralarvae of Octopus huttoni and Pinnoctopus cordiformis

This study combined morphological and morphometric information on egg clutches, egg capsules and paralarvae of two sympatric coastal octopuses from New Zealand waters, Octopus huttoni and Pinnoctopus cordiformis, to provide species-specific traits to identify their early life stages obtained from field surveys. Eggs of O. huttoni (2.5 mm length; 1 mm width) were entwined with one another forming strings that ranged from 11 to 25.8 mm in length. Eggs of P. cordiformis (6.4 mm length; 1.5 mm width) were significantly bigger than those of O. huttoni and were grouped in small clusters of about seven eggs. Paralarvae O. huttoni and P. cordiformis differed in hatching size (1.4 mm versus 3.1 mm mantle length), number of suckers per arm (four versus eight), number of lamellae per outer demibranch (five versus ten) and arrangements of chromatophores in the body surface (29 to 59 versus 91 to 179), respectively. The morphological traits described in hatchlings from the laboratory allowed comparisons with field-collected paralarvae, suggesting that such characters were reliable species-specific patterns to enable a consistent differentiation between the early life stages of these two sympatric species, even in the absence of the brooding female.

일본재첩, Corbicula japonica 아가미의 정상구조와 고염분 급성노출에 의한 조직병리학적 변화

96시간 동안 고염분에 노출시킨 일본재첩, Corbicula japonica의 <TEX>$LC_{50}$</TEX>은 19.550 psu였다. 0, 5, 10, 20 psu에 7일 동안 노출시킨 실험개체들은 실험종료시기에 각각 95%, 80%, 35%, 10%의 개체들이 생존하였다. 일반적인 일본재첩의 아가미는 좌 우 한 쌍으로서 내부판의 면적은 외부판보다1.37배 넓었다 (p < 0.001). 아가미의 새엽에는 그 위치에 따라 상부에 정단섬모상피세포 (<TEX>$7{\mu}m$</TEX>), 정단측면섬모상피세포 (<TEX>$5{\mu}m$</TEX>), 후정단측면섬모상피세포 (<TEX>$3{\times}8{\mu}m$</TEX>), 측면섬모상피세포 (<TEX>$5{\mu}m$</TEX>) 가 존재하고, 새엽의 중간부분에는 혈림프동을 둘러싸고 있는 혈관상피세포가 존재하며, 하부에는 새엽하부상피세포가 존재하고 있었다. 새엽의 하부에 주로 존재하는 분비세포들은 전자밀도가 낮은 섬유성의 분비과립을 가지고 있었다. 5 psu에 7일 동안 노출된 일본재첩의 아가미는 부분적인 섬모의 탈락과 glycogen 과립이 다수 관찰되었다. 10 psu에 노출된 개체들은 일부 새엽의 상피세포가 파괴되었으며, 미토콘드리아를 포함한 세포소기관 또한 파괴되었다. 섬모들은 원형질막이 팽창되었고 미세융모를 연결시키는 당질층의 파괴도 관찰되었다. 20 psu에 노출된 일본재첩의 아가미는 새엽섬모상피세포 핵비대, 세포소기관의 파괴, 세포질내 glycogen 과립의 침적과 공포형성이 관찰되었고, 50% 이상의 새엽은 새엽상피층의 탈락으로 인하여 키틴질 기둥이 모두 노출되었다. 따라서 이러한 섬모와 상피세포의 파괴는 생리활동의 장애를 유발시키고, 개체 사망의 직접적인 원인으로 작용할 것이다. This study was performed to observe ultrastructure of the gill and to ascertain the effect of salinity on histopathological and ultrastructural changes in the gill of the Japanese clam, Corbicula japonica. Experimental period was 7 days. Experimental groups consisted of control, 5, 10, 20 psu. <TEX>$LC_{50}$</TEX> (96 h.) by the probit was 19.55 psu. Mortality was significantly different from the control (p < 0.05). Inner demibranch of the gill of C. japonica was wider 1.37 times than outer demibranch (p < 0.001). The filament zone on the plica can be distinguished by the six epithelial celll cell; frontal ciliated epithelium (<TEX>$7{\mu}m$</TEX>), latero-frontal ciliated epithelium (<TEX>$5{\mu}m$</TEX>), postlatero-frontal epithelim (<TEX>$3{\times}8{\mu}m$</TEX>), and lateral ciliated epithelium (<TEX>$5{\mu}m$</TEX>) in the frontal zone, endothelial cellin the intermediate zone, and abfrontal cell in the abfrontal zone. It had one type of secretory cell that was filled with fibrous substances of low electron density. The gill of C. japonica exposed to 5 psu for 7 days was observed partially disappearance of the cilia, and glycogen granule in the filament. In the 10 psu, gill appeared partially modification of epithelial cell and destruction of the glycocalyx. Gill exposed to 20 psu was extended nuclus of the ciliated epithelial cell, destruction of the organelles, and observed glycogen granules infiltration and numerous vacuoles. Moreover, more than 50% filaments were observed that come out chitinous rod from disappearance of epithelial cell in the filament. Therefore, the destruction of the cilia and epithelial cell induce physiological activity and it may be leading directly to death.

The unionids of Tunisia: taxonomy and phylogenetic relationships, with redescription of Unio ravoisieri Deshayes, 1847 and U. durieui Deshayes, 1847

Using an approach based on molecular, reproductive and morphological characters, four independent evolutionary lineages represented by the following species were identified in Tunisia: Unio ravoisieri Deshayes, U. gibbus Spengler, U. durieui Deshayes and Potomida littoralis (Cuvier). The species U. ravoisieri and U. durieui are redescribed, including details of their distribution, biology and glochidium morphology. The puzzling distribution of the former (Algeria, Tunisia and two localities in Northeast Spain) could reflect introduction or natural dispersal. In Tunisia, the release of the glochidia of U. ravoisieri starts in March, but in Spain this is delayed until August or September. The entire interlamellar space of the outer demibranchs acts as a marsupium (ectobranchy). Glochidia are rounded-triangular, globose, and either hooked or hookless. Unio durieui is distributed across Algeria and Tunisia, where it releases glochidia in December and in March. Glochidia are always hooked, but the elaboration of the hook is variable. The molecular phylogenetic analysis supports two clusters within Unio, each including both European and North African species. Strong bootstrap support was obtained for the two clades within U. ravoisieri, of which one includes specimens from the Iberian Peninsula as well as Tunisia. Unio durieui appeared as the sister species of U. gibbus. The Tunisian populations of P. littoralis were not segregated from those in Europe, and U. fellmanni should be considered a synonym.

Form and functional morphology of Raetellops pulchella (Bivalvia: Mactridae): an example of convergent evolution with anomalodesmatans

Abstract. The bivalve Raetellops pulchella is a highly specialized, deposit‐feeding member of the Mactridae. Studies of its form and function provide an example of how the bivalve body plan can be modified to facilitate the exploitation of mud as a food resource, and help in understanding how this lifestyle has evolved. Adaptations to this lifestyle include an overall reduction in ctenidial size and loss of the descending lamellae of both outer demibranchs. This reduction is associated with the enlargement of the labial palps to process inhaled sediment. In the mantle cavity, a waste canal below the posterior mantle flaps facilitates pseudofeces removal. The midgut is long and capacious, presumably to cope with the large amounts of ingested organic material. In addition, individuals of R. pulchella have unusually thin, brittle, and rostrate shells, with narrow siphonal gapes. They possess a shell buttress in each valve extending from the hinge plate to above the posterior adductor muscle. This buttress functions to prevent the brittle shell valves from fracturing when adduction occurs. A buttress is also seen in some representatives of the Anomalodesmata; in particular, the situation in R. pulchella is most like that seen in individuals of the similarly deposit‐feeding species Offadesma angasi (Anomalodesmata: Periplomatidae). I interpret the similar shell form of these deposit‐feeding clams as an example of convergent evolution.

Gill Development and Its Functional and Evolutionary Implications in the Blue MusselMytilus edulis(Bivalvia: Mytilidae)

Study of gill development in bivalve larvae and postlarvae provides information on the evolution of this organ and feeding mechanisms of early stages. Scanning electron microscopy was used to document the development of the filibranch homorhabdic gill in hatchery-reared larval, postlarval, and juvenile Mytilus edulis. Four key stages were identified during gill development: (1) transfer of the particle collection function from velum to gill at metamorphosis, with subsequent elongation of the gill filaments to form a gill basket, with complete frontal ciliation; (2) reflection of the inner demibranchs, and transition to a V-shaped gill; (3) delayed development of the outer demibranchs, occuring simultaneously along the gill axis, with transition to the adult final W-shape; and (4) formation of the ventral particle grooves and concomitant acquisition of dense abfrontal ciliation. These key stages signal shifts in the mechanisms of particle processing during the early development of M. edulis. Gill development in the homorhabdic filibranch M. edulis was similar to that of the early homorhabdic stages of the heterorhabdic filibranchs studied to date (Pectinidae), but different from that of the pseudolamellibranchs (Ostreidae), suggesting divergent evolution of this character. Similarly, the systems responsible for gill cohesion and structural integrity are common to both the homorhabdic and heterorhabdic filibranchs, suggesting evolutionary proximity, but they are patently different from those of the eulamellibranchs and pseudolamellibranchs, suggesting evolutionary divergence.

Unusual modes of oogenesis and brooding in bivalves: the case of Gaimardia trapesina (Mollusca: Gaimardiidae)

Abstract. I describe an unusual case of follicular oogenesis in a bivalve, Gaimardia trapesina, a common marine bivalve from the Magellan Region and adjacent Sub‐Antarctic waters, whose members brood their developing larvae. The gonad in G. trapesina is an acinus organ that infiltrates the perivisceral connective tissue; the walls of the acini are formed by tall, slender cells with distal nuclei, supported by a thin conjunctive tissue layer. At the onset of vitellogenesis, each developing oocyte becomes surrounded by a one‐cell‐thick layer of follicle cells, which may originate from the wall of the acinus. The cells form a follicle that completely encompasses single oocytes, except at the basal zone, where oocytes are in contact with the acinus wall. The follicle persists beyond the end of vitellogenesis and spawning. After gamete release, the persistent follicle participates in the attachment of ova and developing embryos to the interfilamental junctions of the inner and outer demibranchs of the gill, where embryos are incubated until hatching as late‐stage pediveliger larvae. Ripe eggs are large (∼250 μm diameter), suggesting that development is entirely lecithotrophic. The follicle cells that mediate connections between developing embryos and the maternal individual probably have a mechanical role only, providing support and possibly facilitating the accommodation of a large number of embryos to maximize the branchial space available for brooding.

Worth a second look: gill structure in Hemipecten forbesianus (Adams &amp; Reeve, 1849) and taxonomic implications for the Pectinidae

The gill of the reef-dwelling Hemipecten forbesianus was examined using histology and scanning electron microscopy, in order to verify its reported homorhabdic state, and to provide the first detailed information on tropical pectinid gill structure. Due to the difficulty of obtaining live individuals, museum reference specimens were used for this study. The gill is heterorhabdic, and presents several notable differences compared to the larger, temperate species studied to date: (1) the small number of ordinary filaments per plica (probably associated with the very small size of these specimens), (2) the outer demibranch is approximately 15% shorter than the inner demibranch, (3) several characteristics which confer gill cohesion in other pectinid gills are absent and (4) the principal filaments are devoid of mucocytes, such that particle transport of positively-selected particles must be entirely effected in water. The apparent universality of the heterorhabdic condition within the Pectinidae leads us to recommend its use as a reliable soft-body taxonomic character at the family level. The anatomical differences between the gill of H. forbesianus and that of the larger, temperate species call for further comparison with both large and small, temperate and tropical species, in order to elucidate the effects of size and habitat on gill structure in this family.

Morphological and genetic description of Octopus insularis, a new cryptic species in the Octopus vulgaris complex (Cephalopoda: Octopodidae) from the tropical southwestern Atlantic

A medium-sized Octopus species is described based on material collected in shallow equatorial waters around the oceanic islands of Fernando de Noronha Archipelago, Rocas Atoll, St Peter and St Paul Archipelago and the mainland of northeastern Brazil. The new species, Octopus insularis, is described morphologically, and also characterized by the large mitochondrial subunit ribosomal RNA gene (mt 16S rDNA). The new species has relatively short and stout arms, rugose reddish brown skin in preserved specimens, 8 to 11 gill lamellae on the outer demibranchs, small ligula, characteristic symmetrical radula, spermatophore and beak, small eggs and high fecundity (213,000 oocytes under 1.5 mm diameter). The habitats and skin patterns of living animals are briefly described. The new species differs both morphologically and genetically from Octopus vulgaris in the Mediterranean, Venezuela and southern Brazil.

Colonisation of the Channels of Międzyodrze (North-Western Poland) by &lt;i&gt;Sinanodonta Woodiana&lt;/i&gt; (Lea, 1834) (Bivalvia: Unionidae)

Sinanodonta woodiana (Lea, 1834) is an alien species in the fauna of Poland. It comes from the catchment area of the rivers Amur and Yangtze. In Poland it was found in the early 1980s in the system of heated lakes near Konin (Kujawy Lake District, C-Poland). It appeared there together with the silver carp Hypophthalmichthys molitrix (Valenciennes, 1884) and big-heated carp Aristichthys nobilis (Richardson, 1845) introduced from Hungary. In 2003 it was found in the release channel of cooling water from the Dolna Odra power plant. In the years 2004-2005 an inventory of the Miedzyodrze (N-W Poland) channels was made in order to determine the current range of occurrence of S. woodiana in the Western Pomerania Region. The mussels were found at five localities below the release of the cooling water. The channels are partly frozen in winter, which indicated the adaptation abilities of this species to waters typical of temperate zone. In the outer demibranch marsupia of females the presence of glochidia was detected, which pointed to the readiness of the mussels for reproduction.

Morphological development of the juvenile through to the adult in the freshwater pearl mussel, Hyriopsis (Limnoscapha) myersiana, under artificial culture

Summary The morphological development of the freshwater mussel, Hyriopsis (Limnoscapha) myersiana (Lea, 1856), was observed using light and scanning electron microscopes, from the newly transformed (0 days old) juvenile to the onset of the adult stage (360 days old). As in the glochidium, the early juvenile has a semi-oval and equivalve shell with an equilateral valve. After day 1 the shell develops a larger anterior than posterior region until day 40, after which the posterior region grows larger than the anterior region. The form of the juvenile at 260-day-old resembles that of a fully grown adult. The shell microstructure of 0–20-day-old juveniles shows two differentiated layers, the periostracum and the prismatic layer. By day 30 the prismatic layer lies under a clear columnar structure that has formed a third layer, the nacreous layer. The mantle develops incurrent and excurrent siphons when juveniles are 60 days old. The development of juvenile gills initiates from a pair of gill bars at 0 days old, and formation of the inner demibranch starts from 10 days old and the outer demibranch from 90 days old. From this stage, numerous cilia form the latero-frontal cirri of the inner demibranchs. Additionally, longitudinal and transversal interfilamentous junctions of the inner and outer demibranchs begin to develop when juveniles are 200 and 240 days old and are complete at 230 and 260 days of age, respectively. Interlamellar septa join the inner surface of descending and ascending gill filaments to form water chambers when juveniles are 250 and 280 days old, respectively, and the development of inner and outer demibranchs is complete.

Anatomy and reproduction of viviparous Pisidium ( Parapisidium) reticulatum Kuiper, 1966: Implications for the phylogeny of Sphaeriidae (Mollusca: Bivalvia: Heterodonta)

The pea clams Sphaeriidae represent a major molluscan freshwater radiation with cosmopolitan distribution in all kinds of lotic and lentic habitats. Their phylogenetic relationships are still controversial, with comprehensive taxonomic sampling and examination of morphological characters still challenging. Here, based on rare and rediscovered original material, we study in detail the anatomy and aspects of brood protection of the African Pisidium reticulatum Kuiper, 1966. Representing the monotypic subgenus Parapisidium Kuiper, 1966, this species is characterized by its peculiar combination of shell and anatomical features of potentially high phylogenetic relevance. While similar to other congeners in several anatomical characters (e.g. reduction of inhalant siphon and descending lamella of outer demibranch, simplified structure of intestine coil and nephridium), P. reticulatum differs from other Pisidium species in retaining both pairs of retractor muscles of the inhalant siphon, and particularly in its peculiar mode of brooding. The yolky eggs are relatively large (160–170 μm in diameter) and are incubated in the gill, albeit in the absence of the formation of brood pouches. During later stages of incubation the larvae are surrounded by large cells similar to nourishing cells in other sphaeriids and probably with similar function. This unique combination of reproductive features is hypothesized to represent an intermediate stage between the typical ovoviviparity of Euperidae and euviviparity (i.e. nourishment by the parent animal) as found exclusively in Sphaeriidae, the latter being characterized by the possession of closed brood pouches. Phylogenetic analyses based on a comprehensive set of morphological characters reveal Parapisidium as the most basal lineage within a clade Pisidium. Evaluating the phylogenetic reconstructions based also on available molecular data for Sphaeriidae, we discuss alternative scenaria of (parallel) evolution of brood pouches and viviparity in this group.

Reproduction and sperm structure in Galeommatidae (Bivalvia, Galeommatoidea)

Reproduction and/or sperm structure was studied in 18 species belonging to five genera of Galeommatidae from Phuket Island, Andaman Sea, Thailand, and Hong Kong. Incubation of the ova occurs in the inner and outer demibranchs of both gills, and sperm is most probably transferred to the ctenidial brood chamber as masses of agglutinated and non-encapsulated spermatozoa. The smallest specimens are males, but change into the female sex and there is a strong indication that some of the species are alternate hermaphrodites. Dwarf males occur in one species (Galeomma layardi). The sperm of all 18 species studied are of the ent-aquasperm type with a more or less hemispherical acrosome that is tilted with respect to the long axis of the moderately elongated nucleus. It is suggested that this type of sperm affords the best autapomorphy for the family Galeommatidae.

Gill development, functional and evolutionary implications in the Pacific oyster Crassostrea gigas (Bivalvia: Ostreidae)

Development of the Crassostrea gigas gill was studied in order to better understand the feeding biology of early life stages, identify potentially critical developmental stages which may influence rearing success or recruitment to wild populations, and shed light on the evolution of the basic bivalve gill types. Larvae and juveniles were reared in an experimental hatchery, and larger specimens were obtained from a commercial hatchery. Specimens were relaxed, fixed, dried, and observed using scanning electron microscopy (SEM). The right and left gills developed symmetrically, via a “cavitation–extension” process from the gill buds. The inner demibranchs developed first (V-stage, 0.29–2.70 mm), in a sequential postero-anterior series of homorhabdic filaments. The outer demibranchs developed later (W-stage, from 2.70 mm), also as homorhabdic filaments, synchronously along the gill axis. The principal filaments (PF) developed from the progressive fusion of three ordinary filaments (OF), at a size of 7.50 mm, and the consequent plication was accentuated by the formation of extensive tissue junctions. Effective filament number (number of descending and ascending filaments) showed a marked discontinuity at the transition from the V- to the W- stage of the gill. Filament ciliation showed several important changes: establishment of OF ciliation in the homorhabdic condition (2.70 mm), ciliary de-differentiation of the PF in the heterorhabdic condition (7.50 mm), and establishment of a latero-frontal cirri length gradient from the plical crest to the PF base. Reversal of direction of ciliary beat is also necessary prior to adult functioning of the PF. Three major transitions were identified in C. gigas gill development, each potentially important in rearing success or wild population recruitment: (1) transition from velum to gill at settlement, (2) transition from a V- to a W-shaped gill (2.70 mm), and (3) transition from the homorhabdic to the heterorhabdic condition (7.50 mm). Complete gill development was much more prolonged than in species previously studied. The major ontogenetic differences between the C. gigas heterorhabdic pseudolamellibranch gill and the pectinid heterorhabdic filibranch gill suggest that the heterorhabdic condition evolved independently in these two bivalve families.