Larval Types Research Articles

Transformation of skin from larval to adult types in normally metamorphosing and metamorphosis-arrested salamander, Hynobius retardatus

Transformation of skin from larval to adult types in a salamander, Hynobius retardatus, which had been reported to show neotenic reproduction in a specific environment, was examined morphologically in normally metamorphosing, precociously metamorphosing and metamorphosis-arrested larvae. Typical larval skin was composed of an epidermis constituted by three types of cells such as apical, Leydig, and basal cells. The Leydig cells were larval specific, and thus disappeared and were replaced by adult epidermal cells during the metamorphosis. Disappearance of the Leydig cells was accomplished by apoptosis as confirmed by the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end-labeling method and electron microscopy during the normal metamorphosis and precocious metamorphosis induced by exogenously applied triiodothyronine. Typical adult skin was composed of epidermis constituted by stratified squamous cells and of dermis mainly occupied with two types of dermal glands, mucous and serous glands. When the metamorphosis was arrested by different procedures (thyroidectomy, hypophysectomy, goitrogen treatment, and rearing at low temperature), the larval-specific Leydig cells fully remained in the epidermis, suggesting that the disappearance of these depended on the thyroid activity. Contrary to this, dermal glands behaved differently from the Leydig cells, though they developed and differentiated from epidermal basal cells and constituted the same skin. Those in the metamorphosis-arrested (thyroidectomized, hypophysectomized, or goitrogen-treated) larvae, except in the larvae reared at 4° C, appeared a little later than in the controls. Thus, the aged, metamorphosis-arrested larvae had skin which consisted of larval type epidermis (Leydig cells) and adult type dermis (mucous and serous glands).

Prevalence of anthelmintic resistant nematodes in sheep and goats in Western France

Twenty-three sheep farms and 15 dairy goat farms, located in western France and randomly selected, were surveyed for gastrointestinal nematodes resistant to anthelmintics. On each of the 23 sheep farms, 60 lambs were randomly distributed into four groups of 15: untreated control group, fenbendazole (benzimidazole) group (5 mg/kg BW), ivermectin group (0.2 mg/kg BW) and levamisole hydrochloride group (6.8 mg/kg BW). On each of the 15 goat farms, 30 adult goats were allocated into two groups of 15: untreated control group and fenbendazole group (10 mg/kg BW). Faecal egg counts and larval cultures were done 10 to 14 days after anthelmintic treatment. Furthermore, egg hatch assays conducted with thiabendazole were performed on pooled faeces of fenbendazole treated goats, results being expressed as Logarithmic Concentration 50 (LC 50). Faecal egg count reductions (FECR) less than 95% are indicative of anthelmintic resistance and LC 50 values above 0.1 μg/ml suggest benzimidazole (BZ) resistance. On this basis, in the sheep survey, anthelmintic resistance occurred on 83% of farms for benzimidazole and on 50% of farms for levamisole. Post-treatment larval cultures indicated Teladorsagia/ Trichostrongylus and Cooperia were the predominant resistant genera. A negative relationship was found between FECR with fenbendazole and the average number of annual BZ treatments given to ewe-lambs ( P<0.01). In contrast, no case of ivermectin resistant nematodes was seen in the 23 sheep farms. In goat survey, virtually all the 15 surveyed farms showed the occurrence of benzimidazole resistant nematodes according to the results of both FECR test and egg hatch assay. In post-treatment coprocultures, Teladorsagia/ Trichostrongylus was the predominant larval type on seven farms, and Haemonchus on two farms, while on the remaining farms there was a well-balanced mixture of all the genera including Oesophagostomum/ Chabertia. Although no relationship could be established between drenching practices and results of in vivo and in vitro tests, the frequent and sometimes exclusive use of BZ compounds associated with a permanent underdosing of these drugs achieved by using sheep dose rates are probably the main causes of the very high frequency of BZ resistance in nematodes on goats. The consequences of anthelmintic resistance on sheep and goat parasitism management are discussed.

Origin and evolution of animal life cycles

ABSTRACTThe ‘origin of larvae’ has been widely discussed over the years, almost invariably with the tacit understanding that larvae are secondary specializations of early stages in a holobenthic life cycle.Considerations of the origin and early radiation of the metazoan phyla have led to the conclusion that the ancestral animal (= metazoan) was a holopelagic organism, and that pelago‐benthic life cycles evolved when adult stages of holopelagic ancestors became benthic, thereby changing their life style, including their feeding biology.The literature on the larval development and phylogeny of animal phyla is reviewed in an attempt to infer the ancestral life cycles of the major animal groups. The quite detailed understanding of larval evolution in some echinoderms indicates that ciliary filter‐feeding was ancestral within the phylum, and that planktotrophy has been lost in many clades. Similarly, recent studies of the developmental biology of ascidians have demonstrated that a larval structure, such as the tail of the tadpole larva, can easily be lost, viz. through a change in only one gene. Conversely, the evolution of complex structures, such as the ciliary bands of trochophore larvae, must involve numerous genes and numerous adaptations.The following steps of early metazoan evolution have been inferred from the review.The holopelagic ancestor, blastaea, probably consisted mainly of choanocytes, which were the feeding organs of the organism. Sponges may have evolved when blastaea‐like organisms settled and became reorganized with the choanocytes in collar chambers.The eumetazoan ancestor was probably the gastraea, as suggested previously by Haeckel. It was holopelagic and digestion of captured particles took place in the archenteron. Cnidarians and ctenophores are living representatives of this type of organization. The cnidarians have become pelago‐benthic with the addition of a sessile, adult polyp stage; the pelagic gastraea‐like planula larva is retained in almost all major groups, but only anthozoans have feeding larvae.Within the Bilateria, two major lines of evolution can be recognized: Protostomia and Deuterostomia. In protostomes, trochophores or similar types are found in most spiralian phyla; trochophore‐like ciliary bands are found in some rotifers, whereas all other aschelminths lack ciliated larvae. It seems probable that the trochophore was the larval type of the ancestral, pelago‐benthic spiralian and possible that it was ancestral in all protostomes. Most of the non‐chordate deuterostome phyla have ciliary filter‐feeding larvae of the dipleurula type, and this strongly indicates that the ancestral deuterostome had this type of larva.

In vivoTreatment of Bullfrog Tadpoles with Aldosterone Potentiates ACh-Receptor Channels, but not Amiloride-Blockable Na+Channels in the Skin

Amiloride-blockable Na(+) channels participate in active Na(+) transport across adult, but not larval, bullfrog skin. Their development is induced in vitro by culturing the tadpole skin with aldosterone. When tadpoles were raised in aldosterone (5 x 10(-7) M) for 2 weeks, however, neither development of such channels nor localization of antigen A, a marker of adult-type epidermis, was seen, the skin still being of the larval type. In contrast, aldosterone treatment did potentiate (by a factor of two) the activity of the acetylcholine receptor (ACh-receptor) channel, a functional marker of larval-type skin. The short-circuit current (SCC) across the skin, far from being inhibited by amiloride, was stimulated by both amiloride and ACh. The nystatin-stimulated SCC was about twice its control amplitude, suggesting that the aldosterone treatment also potentiated the activity of the Na(+) pump.

Reproductive Strategies of Marine Benthic Invertebrates Revisited: Facultative Feeding by Planktotrophic Larvae

Fecundity-time models of reproductive strategies in marine invertebrates all predict that reproductive success is maximized only at the extreme levels of investment. Selection should drive egg sizes toward small eggs and planktotrophy or large eggs and lecithotrophy. The existence of two distinct larval types, feeding and nonfeeding, has been taken as confirmation of this prediction and has established the current paradigm for larval ecology. However, comparative and experimental evidence does not support the prediction that egg size is minimized in species with planktotrophic larvae. Recent discoveries have documented the existence of planktotrophs that have intermediate egg sizes, differing degrees of dependence on exogenous food, and differing capacities for facultative feeding. A fecundity-time model is presented that includes facultative larval feeding by dissociating the onset of feeding capability from the need for exogenous food. The facultative feeding model shows that reproductive success can be maximized at intermediate levels of investment per offspring between the minimum for development and the threshold for lecithotrophy, depending on the amount of food available to larvae and the intensity of planktonic mortality. A continuum of larval strategies is predicted.

Effects of contrasting modes of larval development on the genetic structures of populations of three species of prosobranch gastropods

In south-eastern Australia, the prosobranch gastropods Morula marginalba (Blainville), Cominella lineolata (Lamarck) and Bedeva hanleyi (Angas) have similar fine-scale distributions, but appear to possess very different dispersal capabilities due to contrasting modes of larval development. M.marginalba produce planktonic larvae, whereas C. lineolata and B. hanleyi undergo direct development in benthic egg capsules and emerge as crawling juveniles. To test for possible effects of contrasting life histories on levels of genetic variation within and among populations, a survey was conducted of allozyme variation at six polymorphic loci in 8 to 9 local populations of each species. Collections of snails were made between June 1992 and November 1993. Sampling ranges spanned between 162 and 180 km of coast. Regardless of larval type, proportions of single-locus genotypes in each collection were consistent with the recruitment of offspring which had been generated through random mating. However, genotypic diversity was lower in those species that undergo direct development. Loci surveyed in C. lineolata and B. hanleyi were polymorphic (i.e. frequency of most common allele <95%) in fewer populations than those examined for M.␣marginalba (P <0.001) and, where polymorphisms occurred, also possessed significantly fewer alleles (P <0.001). Consequently, average levels of expected heterozygosity were greater in populations of M. marginalba than in those of either of the other species (P <0.001). Genetic variation among populations, expressed as the standardised variance in allele frequencies (FST), was inversely related to expected larval dispersal capability. The nine collections of M. marginalba showed little overall differentiation (FST = 0.017; P <0.001), reflecting the ability of planktonic larvae to interconnect local populations, and so limit divergence due to drift and natural selection. In contrast, there were high levels of allelic heterogeneity among the nine collections of C. lineolata (FST = 0.523; P <0.001) and eight collections of B. hanleyi (FST = 0.140; P <0.001). These data imply that for species which undergo direct development, local populations are effectively closed and evolve largely independent of one another.

Reproduction in Zoanthidea (Anthozoa: Hexacorallia)

Summary The state of knowledge concerning the reproductive biology of zoanthids is reviewed. Colonies in the suborder Macrocnemina, with few exceptions, are gonochoric; those in Brachycnemina are probably basically hermaphroditic but accurate sampling in colonies that fragment is difficult and exceptions by species or populations occur. Two deepsea species of Epizoanthus breed continuously but temperate shallow water zoanthids, and those in the tropics outside ∼15° latitude are iteroparous with spawning in summer or when temperatures are rising. Oogenesis occurs over a few months but the spawning event may be very brief. Parazoanthus parasiticusin Bermuda spawns mainly on 1–2 nights in late August or September, apparently at full moon;Protopalythoa sp. 1, and probably Palythoa caesia in the Great Barrier Reef spawn 3–5 nights after full moon in November (early summer). These species with short, precisely defined spawning seasons may undergo a second, smaller-scale spawning event one month after the first. Some intertidal populations seem rarely to reproduce sexually, and there have been no studies on the annual cycles of zoanthids in near equatorial waters. With few exceptions gametogenesis occurs only in non-directive macrosepta; in hermaphroditic colonies oocytes and sperm cysts may be intermingled on the same septa or be separate. Germ cells in Isozoanthus giganteus originate only from the endocoelic face of the septa, but this has not been confirmed in other species. Nutritive structures (trophonemata) are present both in oocytes and sperm cysts, in the latter functioning also for the egress of sperm. Trophonemata may be conspicuous, as in Par-parasiticus, or poorly developed, as in Protopalythoa sp. 1. Live eggs vary from −300 µm (most zoanthids) to ∼500 µm diameter in Palythoa spp.; in the two mass spawning species from the Great Barrier Reef they are shed as compact ovoid egg (or egg/sperm) bundles. Fecundity in zoanthids, from few studies, is variable and dependent on polyp size; estimates of 100–300×103 ova 100 cm−2 of colony have been made for Caribbean reef species. Internal brooding is known definitely only in Isozoanthus giganteus. Amongst zooxanthellate zoanthids, only Protopalythoa sp. 1 is known to transmit zooxanthellae via its oocytes. Several zoanthids are known to produce palytoxin but it is unclear to what extent the toxin accumulates in the ova. Larval development is summarized. Zoanthella-larvae are known to be produced in Palythoa, Protopalythoa and Sphenopus, and zoanthina-larvae in Isaurus and Zoanthus, all Brachycnemina. Larval type in Epizoanthus and Parazoanthus, both Macrocnemina, is unknown. Early metamorphosis of zoanthella and zoanthina larvae has been reported but not studied; older larvae, taken from the plankton, may have lost competency to settle. All zoanthids, except Sphenopus, form colonies, clones, or mixtures of the two, and propagate by budding. Attention is drawn to unusual methods of budding or cloning in Acrozoanthus, Isaurus, Palythoa, Sphenopus, and Parazoanthus axinellae. In reefal Isaurus, Protopalythoa, and Zoanthus actual colony size remains small due to fragmentation, but genet size may be substantial. Population structure depends on the interplay of recruitment, fragmentation, and local dispersal.

Description of the Life Stages of Carina forda Westwood (Lepidoptera: Saturniidae)

All the development stages of Cirina forda Westwood, i.e. egg; 1 to 6 instar larvae; pupa and the adult were described and illustrated with particular emphasis on the mature, edible, commercially utilized 6th larval instar. Eggs that are in clusters are small, oval, 1.68mm long and 1.38mm wide. An average of 50 eggs is about 0.065g. C. forda larva has a dark-brown head and anal shields and greenish-black trunk with yellow blotches. The pupa is obtect, adecticous, highly sclerotised, smooth and robust. Two distinct slits occurring on segments A8 and A9 of the female pupa represent apertures of the bursa corpulatrix and the oviduct. The adult is generally brown and has alar expanse of 10cm with very small eye spots located on both hind wings. This detailed description is to enhance recognition of the larvae and to avoid the inedible closely related larval types

Comparative fertilization biology of gymnolaemate bryozoans

Locations and oogenic stages of eggs at the time of sperm-egg fusion were determined for nine gymnolaemate species exhibiting different reproductive strategies with respect to site of embryonic development (e.g., water column, introvert, embryo sac, or ovicell) larval type, and zooid sexuality. Ovarian, coelomic, and recently spawned oocytes obtained from freshly collected colonies between 1988 and 1995 were stained with aceto-orcein and examined for the presence of a sperm nucleus. All nine species are shown to have a similar fertilization biology in which sperm fuse with eggs in or near ovaries and egg activation is delayed until eggs are spawned. In species such as the etenostomeAlcyonidium sp. and the cheilostomeElectra pilosa (Linnaeus, 1767), which produce marierons oocytes that are spawned into the water column and have cyphonautes larvae, sperm fuse with late growth stage ovarian oocytes after germinal vesicle breakdown (GVBD) at or near ovulation. In species that produce few oocytes, brood embryos, and release coronate larvae, sperm fuse with late growth stage ovarian primary oocytes before GVBD, as in the ctenostomeBowerbankia gracilis Leidy, 1855, or with very early growth stage ovarian primary oocytes, as in the cheilostomesCribrilina corbicula (O'Donoghue, 1923),Dendrobeania lichenoides (Robertson, 1900),Hippodiplosia insculpta (Hincks, 1882),Schizoporella serialis (Heller, 1867),T ricellaria gracilis (Smitt, 1867), andWatersipora arcuata Banta, 1969. Fertilization success was high in all nine species, with 83 to 100% of oocytes at an appropriate oogenic stage containing a sperm nucleus or early male pronucleus. Gymnolaemate bryozoans may maintain high levels of fertilization success by: (1) concentrating spawned sperm from the water column using colony feeding currents; (2) localizing sperm-egg encounters in or near the ovary, (3) using sperm when available by permitting sperm to fuse with early growth stage oocytes; and (4) selfing.

Chironomus pilicornis fabricius, 1787 and C. heteropilicornis sp.n. (diptera: Chironomidae) in fennoscandian reservoirs: Karyosystematic and morphological results

In Fennoscandia, two Chironomus species show the characteristic features of C. pilicornis Fabricius, but are different in karyotypes, larval types, pupal characters and size. I associated the smaller species with C. pilicornis and described the larger species as C. heteropilicornis sp.n. Both species occur sympatrically in Kyrkosjarvi/ Western Finland and seem to prefer very shallow areas. Exact data on ecology and distribution are scarce because the many older notes on “C. pilicornis” cannot be attributed unambigously to either species.

Palatability of invertebrate larvae to corals and sea anemones

Risk of larval mortality is an underlying theme in debates and models concerning the ecology and evolution of the differing reproductive characteristics among marine benthic invertebrates. In these discussions, predation is often assumed to be a major source of larval mortality. Previous studies, focused primarily on planktotrophic larvae, suggested that marine larvae generally were susceptible to, and poorly defended against, planktivorous fishes and invertebrates. Larval-planktivore interactions involving larger and more conspicuous lecithotrophic larvae that are typical of many brooding sessile invertebrates have not been well studied. This lack of data for diverse larval types has hindered testing broad generalities about marine larvae and planktivore prey-preferences. This study demonstrates that lecithotrophic larvae of many Caribbean and temperate western Atlantic invertebrates are distasteful to co-occurring corals and anemones. These larval predators frequently rejected larvae of sponges (6 of 9 species), gorgonians (7 of 9 species), corals (3 of 3 species), hydroids (2 of 2 species) and a bryozoan. Larvae of three temperate colonial ascidians were readily consumed. Frequencies of survivorship for larvae captured but rejected by corals and anemones were generally high and, in 20 of 24 assays, were not statistically different from those of unattacked control larvae. Levels of metamorphosis (when it occurred) of rejected larvae also rarely differed significantly from those of unattacked controls. These results provide further evidence that larval palatability to predators may not be as high as once thought, particularly for brooded larvae of sessile colonial invertebrates. The means by which larvae may avoid or deter predators, and the demographic consequences for marine invertebrates and for the evolution of invertebrate life-history patterns, need to be assessed.

Adult-type splenocytes of Xenopus induce apoptosis of histocompatible larval tail cells in vitro

Larval cells of the anuran tadpole are replaced by adult cells in the corresponding tissues of the frog during metamorphosis; tissues of the tail, which have no counterpart in the adult, are completely eliminated during metamorphosis. We have previously demonstrated that young adults of the major histocompatibility complex (MHC) homozygous inbred J strain of Xenopus laevis reject skin grafts from larvae of the same strain, showing that there is histoincompatibility between larval and adult skin tissues [19]. Thus, we postulated that some immunological recognition might be involved in this specific elimination of the tail tissue and set out to test the idea. Using in vitro assays, we detected a significant increase in proliferation of splenocytes derived from adult and metamorphic climax animals co-cultured with larval tail tissue. This response was shown to be thymus-dependent. However, the degeneration of larval tissues was not observed in such co-cultures under our standard culture conditions. To detect the possible cytotoxicity of splenocytes, the culture conditions were modified by supplementing with 10% heat-inactivated adult Xenopus serum instead of 10% fetal calf serum (FCS). After this modification, the degeneration of larval tissues was observable macroscopically and microscopically with co-cultured adult splenocytes, but not tadpole ones. The nuclear fragmentation of the epidermal cells was seen by light and electron microscopy. Apoptosis was evidenced by the demonstration of the “ladder pattern” upon electrophoresis of genomic DNA obtained from the degenerating larval tissues. Surprisingly, this response was thymus independent. Moreover, it was shown that this response was not observed when the larval tissues were cultured with adult thymocytes or adult epidermal cells. In vivo, migration of T cells into the epidermis of tail tissues at the late climax of metamorphosis was demonstrated immunohistochemically using a monoclonal antibody against Xenopus T cells, even in the early thymectomized tadpoles. Considering these results, we propose that populations of adult-type non-T leukocytes might participate in the specific detection and elimination of larval type cells during metamorphosis.

Heterochronic Expression of Several Adult Phenotypes in Normally Metamorphosing and Metamorphosis-Arrested Larvae of a Salamander Hynobius retardatus

Abstract Heterochronic expressions of several larval and adult phenotypes in Hynobius retardatus, which had been reported to show neotenic reproduction, were described in normally metamorphosing animals (controls) and goitrogen-induced, metamorphosis-arrested larvae. External gills, dorsal tailfins and Leydig's cells in the epidermis were completely diminished in the controls during and after metamorphosis, but fully remained in the metamorphosis-arrested larvae. Two types of dermal glands, mucous and serous glands, however, behaved differently from Leydig's cells, even though they constituted the same skin. The dermal gland cells in the controls appeared at a premetamorphic stage, gradually increased in number during the metamorphosis and fully developed after the metamorphosis. Those in metamorphosis-arrested larvae appeared much later than in the controls. Thus, aged, metamorphosis-arrested larvae had skin which consisted of larval type epidermis (Leydig's cells) and adult type dermis (mucous and serou...

The Distribution of Marine Invertebrate Larvae Near Vertical Surfaces in the Rocky Subtidal Zone

The realized dispersion patterns of marine invertebrate larvae were examined in the field for a range of larval types that differ in their potential for dispersal. Plankton samples were collected near three subtidal rock walls over a 2—yr period. Larvae to sessile invertebrates were identified and counted in each sample and analyzed for density differences among: (1) three distances from the rock surface; 1—5 cm, 1 m, and &gt;200 m, (2) 1—5 cm above crustose coralline algae—covered surfaces and 1—5 cm over invertebrate—covered surfaces and, (3) 1—5 cm and 1 m from rock surfaces on days of high flow speed vs. days of low flow. Larvae with short dispersal times, such as ascidian tadpole larvae and hydroid actinula larvae, were more abundant 1—5 cm away from rock walls than 1 or &gt;200 m away from rock surfaces. They were usually more abundant over invertebrate—covered surfaces than over crustose coralline algae—covered surfaces as well. Larvae with long planktonic dispersal times, but which were competent to settle (e.g., mussel pediveligers) were more abundant 1—5 cm away from rock walls than at 1 or &gt;200 m away from rock walls. These larvae were often more abundant over invertebrate—covered surfaces than over crustose algae—covered surfaces also. Larvae with long planktonic dispersal times, but which were not competent to settle (e.g., Anomia veligers and mussel veliconcha—stage veligers) had similar densities at all distances from rock surfaces. These larvae occurred at similar densities over both invertebrate—covered surfaces and crustose algae—covered surfaces. Larvae that exhibited differences in density with distance from vertical walls, such as ascidian larvae, mussel pediveligers, and hydroid actinula larvae, were more thoroughly mixed into the mainstream flow away from rock surfaces on days of high flow compared to days of low—flow conditions.

Regionally regulated conversion of protein expression in the skin during anuran metamorphosis

The back skin of anuran larvae undergoes drastic metamorphic changes which are quite different from the tail skin. To have an insight into the mechanism of the region-dependent changes, we made a thorough survey of metamorphic alterations in protein expression in both the body and tail skin of Xenopus laevis tadpoles utilizing high-resolution two-dimensional electrophoresis. Approximately 400 proteins were detected, 9 of which were identified as known proteins and were used as index proteins to standardize the readout of distribution of protein spots on gels. The overall pattern of polypeptide distribution (map) was classified into "larval type" and "adult type." The maps showed stage-specific changes. Up to stage 58 the back skin map remained the larval type, showed remarkable changes for the first time at stage 60 (whose pattern was a combination of both larval and adult types), and became the adult type at stage 62. The basic feature of the tail skin map remained the larval type through the climax stage of metamorphosis. Experiments using tadpoles treated with an antithyroid drug showed that most of back skin proteins detected on the map are thyroid hormone responsive in their expression. Expression of keratin II (adult-type keratin) was found to be uniquely regulated. As expected, silver staining detected it in the back but not in the tail. However, the protein-labeling experiment with [35S]methionine showed that the tail skin is actively synthesizing the adult keratin, suggesting that tail cells have a specific elimination mechanism of the protein.

Transient expression of stromelysin-3 mRNA in the amphibian small intestine during metamorphosis.

It has been suggested that a matrix metalloproteinase, stromelysin-3 (ST3), is an important enzyme for epithelial transformation; the ST3 gene is known to be regulated by thyroid hormone during Xenopus metamorphosis. In this study, we have examined the distribution of ST3 mRNA in the small intestine of Xenopus during metamorphosis by using in situ hybridization. Around stage 58, ST3 mRNA is first detectable in larval fibroblasts near the muscular layer, and then increases in amount throughout the entire region of connective tissue. By stage 61, when connective tissue cells are rapidly increasing in number, ST3 mRNA is localized in fibroblasts just beneath the epithelium. This localization of ST3 mRNA is in good temporal accordance with modification of the basement membrane and epithelial transformation from the larval to adult form. Thereafter, ST3 mRNA gradually decreases and is no longer detected after stage 63 when the adult epithelium has completely replaced the larval type. The transient expression of ST3 mRNA in the fibroblasts of connective tissue during metamorphosis indicates that it plays an important role in epithelial transformation by changing the basement membrane.

Establishment of neuronal connectivity during development of the Drosophila larval visual system.

We used confocal microscopy in conjunction with specific antibodies and enhancer trap strains to investigate the development of specific neuronal connections in a simple model system, the larval visual system of Drosophila. We find that the establishment of axonal projections from the larval photoreceptor neurons to their central nervous system targets involves a series of discrete steps. During embryogenesis, the larval optic nerve contacts several different cell types, including optic lobe pioneer (OLP) neurons and a number of glial cells. We demonstrate that OLP neurons are present and project normally in glass (gl) mutant embryos in which the larval optic nerve fails to develop, suggesting that they do not depend on interactions with the larval optic nerve for differentiation and proper axonal projection. The OLPs fail to differentiate properly is disconnected (disco) mutant embryos, where appropriate connections between the larval optic nerve and its targets in the brain are not formed. The disco gene is expressed in the OLPs and may therefore act autonomously to direct the differentiation of these cells. Taken together, our results suggest that the OLPs act as an intermediate target required for the establishment of normal optic nerve projection and connectivity.

Isolated cell type corpora allata in adults of the loreyi leafworm, Leucania loreyi duponchel (Lepidoptera: Noctuidae).

The corpora allata in adult Loreyi leafworms Leucania loreyi (Lepidoptera: Noctuidae) exhibit sexual dimorphism. The male possesses right and left corpora allata of about the same size. Each gland is composed of a cluster of approximately 40 semi-transparent, spherical, isolated cells held together by fine tracheae and nerve fibers. The largest cell diameter found in male glands was 203 pm. In contrast, the female gland cells and clusters are much smaller. The largest dimensions of one whole female gland cluster were 452 μm in length and 280 μm in width. Using bilateral and unilateral larval allatectomy, we confirmed that the adult isolated cell type glands develop ontogenetically from larval capsular type glands. Ultrastructural study re- vealed many similarities between the subcellular structures of the isolated cell type glands of L. loreyi and the more common capsular gland reported by others. These similarities include very large numbers of mitochondria, abun- dant whorled smooth endoplasmic reticulum, irregularly shaped nuclei, Golgi bodies, and free ribosomes. Compared with the corpora allata of 3- to 9-day-old adults, the glands of 1-day-old adults possessed much less smooth endoplasmic reticulum. The gland cells in females usually have more neurosecretory nerve endings, less-abundant stacked smooth endoplasmic reticulum, and less- defined interdigitations than the gland cells in males. © 1995 Wiley-Liss, Inc.

Similarities in Form and Developmental Sequence for Three Larval Shell Muscles in Nudibranch Gastropods

AbstractShell‐anchored muscles that extend into the cephalopodium of five species of planktotrophic nudibranch larvae were studied by ultrastructural examination of sequential larval developmental stages. All species, regardless of larval shell type (inflated or non‐inflated), showed a similar basic pattern of shell muscles. The larval retractor muscle (LRM) differentiates prior to hatching and its fibres insert on epithelia of the velum, apical plate, stomodeal region, or mantle fold. Many fibres also connect with subepithelial intrinsic muscles of the cephalopodium. Most but not all LRM fibres Project to left‐sided targets and are innervated from the left cerebral ganglion. Two pedal muscles, which are innervated from the pedal ganglia, differentiate during the post‐hatching larval stage and both insert primarily on pedal epithelium attached to the operculum. The left pedal muscle is anchored to the shell immediately adjacent to the attachment plaque of the LRM and consists of basal and distal tiers of muscle cells. The right pedal muscle arises on the ventral rim of the shell aperture and consists of a single tier of muscle cells. Ontogenic changes in larval retraction behaviour correlate with developmental change in the muscle effectors. Although some interspecific differences were noted, the presence of a common ground plan for larval shell muscles in these five species contrasts with previous indications of marked variability for nudibranch larval shell muscles.

Distribution of pelagic larvae of benthic marine invertebrates in the Bellingshausen Sea

During November and December 1992, plankton samples were collected using a ring net of mesh size 200 μm vertically hauled through a 600 m water column, at five stations along a transect running north from the Allison Peninsula in the Bellingshausen Sea. Three stations were located over the continental shelf; two of these were ice bound, whilst the third was at the ice edge. Two other stations were in deeper, ice-free water. Sixteen different larval and juvenile types were found representing seven phyla: Echinodermata, Nemertea, Coelenterata, Mollusca, Annelida, Arthropoda and Bryozoa, of which the first two were the most abundant. Larval numbers and types decreased with distance offshore and away from permanent sea ice. The presence of many stages of nemertean larval development within a short time scale, in an area where developmental tends to be slow, suggests that reproduction occurs over an extended period and that the larvae have a long planktonic phase. The increased size of later developmental stages of the nemertean larvae indicates they obtain nutrition within the water column during winter, when little particulate food is present.