An organismal perspective on C. intestinalis development, origins and diversification

The ascidian Ciona intestinalis, commonly known as a ‘sea squirt’, has become an important model for embryological studies, offering a simple blueprint for chordate development. As a model organism, it offers the following: a small, compact genome; a free swimming larva with only about 2600 cells; and an embryogenesis that unfolds according to a predictable program of cell division. Moreover, recent phylogenies reveal that C. intestinalis occupies a privileged branch in the tree of life: it is our nearest invertebrate relative. Here, we provide an organismal perspective of C. intestinalis, highlighting aspects of its life history and habitat—from its brief journey as a larva to its radical metamorphosis into adult form—and relate these features to its utility as a laboratory model.

Intergenerational transmission of symbiotic bacteria in oviparous and viviparous demosponges, with emphasis on intracytoplasmically-compartmented bacterial types

Recent molecular detection of vast microbial communities exclusively associated with sponges has made evident the need for a better understanding of the mechanisms by which these symbiotic microbes are handled and transferred from one sponge generation to another. This transmission electron microscopy (TEM) study investigated the occurrence of symbiotic bacteria in free-swimming larvae of two viviparous species (Haliclona caerulea and Corticium candelabrum) and spawned gametes of two oviparous species (Chondrilla nucula and Petrosia ficiformis). Complex microbial communities were found in these sponges, which in two cases included bacteria characterized by an intra-cytoplasmic membrane (ICM). When ICM-bearing and ICM-lacking bacteria co-existed, they were transferred following identical pathways. Nevertheless, the mechanism for microbial transference varied substantially between species. In C. nucula, a combination of intercellular symbiotic ICM-bearing and ICM-lacking bacteria, along with cyanobacteria and yeasts, were collected from the mesohyl by amoeboid nurse cells, then transported and transferred to the oocytes. In the case of Corticium candelabrum, intercellular bacteria did not enter the gametes, but spread into the division furrows of early embryos and proliferated in the central cavity of the free-swimming larva. Surprisingly, symbiotic bacteria were not vertically transmitted by P. ficiformis gametes or embryos, but apparently acquired from the environment by the juveniles of each new generation. This study failed to unravel the mechanism by which the intercellular endosymbiotic bacterium found in the central mesohyl of the H. caerulea larva got there. Nevertheless, the ultrastructure of this bacterial rod, which was characterized by a star-shaped cross section with nine radial protrusions, an ICM-bound riboplasm, and a putative membrane-bound acidocalcisome, suggested that it may represent a novel organization grade within the prokaryotes. It combines traits occurring in members of Poribacteria, Planctomycetes and Verrucomicrobia, emerging as one of the most complex prokaryotic architectures known to date.

The Vortex Wake of the Free-swimming Larva and Pupa of CULEX PIPIENS (Diptera)

SUMMARY The kinematics and hydrodynamics of free-swimming pupal and larval (final-instar) culicids were investigated using videography and a simple wake-visualisation technique (dyes). In both cases, swimming is based on a technique of high-amplitude, side-to-side (larva) or up-and-down (pupa) bending of the body. The pupa possesses a pair of plate-like abdominal paddles; the larval abdominal paddle consists of a fan of closely spaced bristles which, at the Reynolds numbers involved, behaves like a continuous surface. Wake visualisation showed that each half-stroke of the swimming cycle produces a discrete ring vortex that is convected away from the body. Consecutive vortices are produced first to one side then to the other of the mean swimming path, the convection axis being inclined at approximately 25° away from dead aft. Pupal and larval culicids therefore resemble fish in using the momentum injected into the water to generate thrust. Preliminary calculations for the pupa suggest that each vortex contains sufficient momentum to account for that added to the body with each half-stroke. The possibility is discussed that the side-to-side flexural technique may allow an interaction between body and tail flows in the production of vorticity.

Kronbrogia pugettensis sp. nov. (Neorhabdocoela: Fecampiidae), an endoparasitic turbellarian infesting the shrimp Heptacarpus kincaidi (Rathbun), with notes on its life-history

SUMMARYThe morphology of Kronborgia pugettensis sp. nov., an endoparasite from the haemocoele of the caridean shrimp Heptacarpus kincaidi is described from specimens collected in the San Juan Island region of the western coast of North America. Members of this species are unisexual as are other members of the genus, but only females were found. An ootype and a type of accessory reproductive gland not known from other Kronborgia spp, are described, but they are probably present in other members of the genus. The life-history is similar to that of other Kronborgia spp. After the female reaches maturity, it emerges from the host and secretes a tubiform cocoon around itself and deposits its egg capsules within it. Embryogenesis lasts about 4 months. A free-swimming larva locates the new host, attaches to its exterior, and secretes a cyst around itself. It then penetrates through the exoskeleton to reach the haemocoele. K. pugettensis is easily distinguished from other members of the genus by the shape of the cocoon, type of host, and geographic distribution. This is the second species of fecampiid described from North America, but several undescribed species are known to exist from their cocoons.

Calcium accumulation and secretion in the serpulid polychaete Spirorbis spirorbis L. at settlement

When the free-swimming larva of the polychaete tubeworm Spirorbis spirorbis settles permanently on a suitable substratum, it forms a thin, mucous, anchoring tube, which covers only the posterior half of the body. Within 3 h the worm has built a comparatively thick, calcareous tube onto the anterior end of the initial, mucous tube, which later becomes a compressed and folded remnant (Nott, 1973). The volume of calcareous material forming the tube cannot be stored within the body of the larva before settlement and must, therefore, be taken up rapidly from sea water or ingested material.

The oncomiracidia of the monocotylid monogeneans Dictyocotyle coeliaca and Calicotyle kroyeri

The eggs of D. coeliaca have been cultured successfully and the free-swimming larva has been studied for the first time. The eggs have an incubation period which for monogeneans is exceptionally long (4–5 months at 10 °C). Apart from maintenance at a low temperature no other special conditions such as high hydrostatic pressure or washing to remove host body fluid are necessary for development. It is not necessary for the eggs to come into contact with host body fluid before development can begin.The oncomiracidium of D. coeliaca has no eyes, a well-developed pair of anterior median gland cells and a pair of hamuli. The oncomiracidium of C. kroyeri is similar to that of D. coeliaca except for the presence of two pairs of conspicuous pigmented eyes, poorly developed anterior median gland cells, the presence of an extra pair of gland cells at the posterior end of the body and the absence of hamuli.A comparison of the larval features of D. coeliaca and C. kroyeri has confirmed that these parasites belong to quite distinct species.

Experiments on host-finding and host-specificity in the monogenean skin parasite Entobdella soleae

The oncomiracidium of the monogenean skin parasite Entobdella soleae finds its flatfish host, Solea solea, by chemoreception. The free-swimming larva responds to a specific substance secreted by the skin of the common sole, attaches itself to this skin and immediately sheds its ciliated epidermal cells. Larvae respond in the same way to agar jelly which has been in contact with the skin of S. solea.The oncomiracidia attach to S. solea skin in preference to that of other soleid fishes (Buglossidium luteum and Solea variegata), pleuronectid fishes (Limanda limanda, Pleuronectes platessa) and elasmobranch flatfishes (Raia spp.).Larvae respond strongly to isolated epidermis from Solea solea but show no response to the fish's cornea, indicating that the attractive substance is produced by the mucus cells in the fish's epidermis.The larvae attach with equal readiness to skin from the upper and lower surfaces of S. solea. Thus the preponderance of young parasites on the upper surfaces of soles is due not to a preference for the upper skin but to the fact that the lower skin is in contact with the substratum and cannot be reached by the larvae.These results led to speculations on the way in which host specificity evolved in the Monogenea.I am indebted to Mr J. E. Green of the Plymouth Laboratory for setting up a tank containing infected soles and for maintaining the tank and feeding the fishes for many months.I am also grateful to the Directors and Staff of the Plymouth Laboratory and the Fisheries Laboratory, Lowestoft, for hospitality and assistance. I am particularly grateful to Mr J. Riley of the Lowestoft Laboratory for providing various flatfishes.

VIII.—Structure and Biology of the Larva and Spat ofVenerupis pullastra(Montagu)

Synopsis:This is a study of the structure and biology of the lamellibranchVenerupis pullastra(Montagu) from the early larva to the spat stage. The larva is identified, and the functional significance of the changes that occur during metamorphosis from the larva to the spat is considered. Organ development in the spat up to a length of one millimetre is described. The free swimming larva is studied from the point of view of seasonal abundance, length of pelagic period, vertical distribution and diurnal migration. Spatfalls both on artificial and natural bottom are studied, and the results indicate a possible cause for the high mortality in the early stages of marine invertebrates with this type of reproduction.