Veliger

Abstract: On the basis of anatomy and larval behavior, the apical sensory organ (ASO) of gastropod veliger larvae has been implicated as the site of perception of cues for settlement and metamorphosis. Until now, there have been no experimental data to support this hypothesis. In this study, cells in the ASO of veliger larvae of the tropical nudibranch Phestilla sibogae were stained with the styryl vital dye DASPEI and then irradiated with a narrow excitatory light beam on a fluorescence microscope. When its ASO cells were bleached by irradiation for 20 min or longer, an otherwise healthy larva was no longer able to respond to the usual metamorphic cue, a soluble metabolite from a coral prey of the adult nudibranch. The irradiated cells absorbed the dye acridine orange, suggesting that they were dying. When larvae not stained with DASPEI were similarly irradiated, or when stained larvae were irradiated with the light beam focused on other parts of the body, there was no loss of ability to metamorphose. Together these data provide strong support for the hypothesis. Potassium and cesium ions, known to induce metamorphosis in larvae of many marine-invertebrate phyla, continue to induce metamorphosis in larvae that have lost the ability to respond to the coral inducer due to staining and irradiation. These results demonstrate that (1) the ASO-ablated larvae have not lost the ability to metamorphose and (2) the ions do not act only on the metamorphic-signal receptor cells, but at other sites downstream in the metamorphic signal transduction pathway.

Abstract: We investigated the effects of seawater equilibrated with CO2-enriched air (2000 ppm, pH 7.4) on the early development of the mussel Mytilus galloprovincialis. Mussel embryos were incubated for 144 h (6 d) in control and high-CO2 seawater to compare embryogenesis, larval growth and morphology with ordinary light, polarized light, and scanning electron microscopy. Embryogenesis was unaffected by exposure to high-CO2 seawater up to the trochophore stage, but development at the trochophore stage was delayed when the shell began to form. All veliger larvae of the high-CO2 group showed morphological abnormalities such as convex hinge, protrusion of the mantle and malformation of shells. Larval height and length were 26 ± 1.9% and 20 ± 1.1% smaller, respectively, in the high-CO2 group than in the control at 144 h. These results are consistent with our previous findings of CO2 effects on early development of the oyster Crassostrea gigas, although the severity of CO2 damage appears to be less in M. galloprovincialis, possibly due to differing spawning seasons (oyster: summer; mussel: winter). Results from this and the previous study indicate that high CO2 (2000 ppm) interferes with early development, particularly with larval shell synthesis, of bivalves; however, vulnerability to high CO2 differs between species. Taken together with recent studies demonstrating negative impacts of high CO2 on adult mussels and oysters, results imply a future decrease of bivalve populations in the oceans, unless acclimation to the predicted environmental alteration occurs.

Abstract: 1. Experimental evidence shows that the presence of a favorable substratum has a marked effect on the metamorphosis of Nassarius obsoletus larvae. In the presence of a natural substratum the percentage metamorphosis was very significantly greater than in controls without such natural substratum. Larvae postponed metamorphosis for over two weeks when a desirable substratum was not encountered.2. Certain substrata were preferred over others; a significant difference in the response of N. obsoletus larvae to different sediments was demonstrated experimentally. A correspondence exists between substrata preferred under natural conditions and those favored in experiments in the laboratory.3. The physical properties of a substratum, such as median grain size and sorting do not directly influence the metamorphosis of N. obsoletus veliger larvae. This was demonstrated both experimentally and by field observations.4. Certain biological properties are important in making the substratum attractive for the metamorphos...

Abstract: Cadlina laevis (L.) is the first British species of nudibranch shown to possess direct development. The embryo leaves the egg-capsule approximately 50 days (at 10° C) after oviposition, and resembles the adult in the general form of the body. Sufficient food reserves remain to sustain life and apparent growth for more than 1 week of benthic life. A vestigial veliger phase is passed through within the egg-capsule. The shell, velum, mantle, and metapodium-rudiment resemble transitorily those organs of opisthobranch veliger larvae, but certain veliger structures (larval kidney, velar locomotor and pedal cilia) are vestigial in Cadlina, and others are altogether absent (operculum, larval retractor muscle, subvelar ridges, metapodial mucus-gland, pedal sensory cilia, and nephrocysts).In the Opisthobranchia three distinct development types can be recognized. Type 1 includes those species which possess planktotrophic veliger larvae. Type 2 comprises the species which have lecithotrophic larvae (in the sense employed by Ockelmann, 1965). Type 3 species have direct development, in which many veliger structures may be briefly recapitulated before hatching. Species of type 3 possess eggs of the largest sizes, species of type 2 in general have smaller eggs, while species of type 1 possess the smallest eggs. There is a positive correlation between egg size and the length of the embryonic period, and an inverse correlation between both and batch size. Within any development-type, the largest species tend to produce larger eggs in greater numbers. Within any species, the largest individuals tend to produce more eggs than do the smaller individuals; egg size varies little through the geographical range.