Claudia E. Mills

The ctenophore genome and the evolutionary origins of neural systems

The origins of neural systems remain unresolved. In contrast to other basal metazoans, ctenophores (comb jellies) have both complex nervous and mesoderm-derived muscular systems. These holoplanktonic predators also have sophisticated ciliated locomotion, behaviour and distinct development. Here we present the draft genome of Pleurobrachia bachei, Pacific sea gooseberry, together with ten other ctenophore transcriptomes, and show that they are remarkably distinct from other animal genomes in their content of neurogenic, immune and developmental genes. Our integrative analyses place Ctenophora as the earliest lineage within Metazoa. This hypothesis is supported by comparative analysis of multiple gene families, including the apparent absence of HOX genes, canonical microRNA machinery, and reduced immune complement in ctenophores. Although two distinct nervous systems are well recognized in ctenophores, many bilaterian neuron-specific genes and genes of ‘classical’ neurotransmitter pathways either are absent or, if present, are not expressed in neurons. Our metabolomic and physiological data are consistent with the hypothesis that ctenophore neural systems, and possibly muscle specification, evolved independently from those in other animals.

Medusae, siphonophores, and ctenophores as planktivorous predators in changing global ecosystems

Medusae, siphonophores, and ctenophores are planktivorous predators operating at higher trophic levels in marine ecosystems of a wide range of productivity. It has been hypothesized that high-productivity ecosystems such as areas of upwelling tend towards food chains dominated by larger phytoplankton, large copepods, and ultimately many species of fish rather than gelatinous predators; ecosystems with lower productivity are characterized by small flagellate phytoplankton, small copepods, and ultimately numerous medusae and ctenophores. Evidence is provided that medusae, siphonophores, and ctenophores are actually important predators in both sorts of planktonic ecosystems, although uneven reporting in the literature may be cause for underestimates of the importance of these carnivores in some systems. As world fisheries begin to experience serious declines, it is relevant to recognize that the carnivorous “jellyfishes” are ubiquitous and are thus opportunistically positioned to utilize secondary production that is ordinarily consumed by fish.

Questioning the Rise of Gelatinous Zooplankton in the World's Oceans

During the past several decades, high numbers of gelatinous Zooplankton species have been reported in many estuarine and coastal ecosystems. Coupled with media-driven public perception, a paradigm has evolved in which the global ocean ecosystems are thought to he heading toward being dominated by “nuisance” jellyfish. We question this current paradigm by presenting a broad overview of gelatinous Zooplankton in a historical context to develop the hypothesis that population changes reflect the human-mediated alteration of global ocean ecosystems. To this end, we synthesize information related to the evolutionary context of contemporary gelatinous Zooplankton blooms, the human frame of reference for changes in gelatinous Zooplankton populations, and whether sufficient data are available to have established the paradigm. We conclude that the current paradigm in which it is believed that there has been a global increase in gelatinous Zooplankton is unsubstantiated, and we develop a strategy for addressing the critical questions about long-term, human-related changes in the sea as they relate to gelatinous Zooplankton blooms.

Invertebrate introductions in marine habitats: two species of hydromedusae (Cnidaria) native to the Black Sea, Maeotias inexspectata and Blackfordia virginica, invade San Francisco Bay

The hydrozoans Maeotias inexspectata Ostroumoff, 1896 and Blackfordia virginica Mayer, 1910, believed to be native to the Black Sea (i.e. Sarmatic) and resident in a variety of estuarine habitats worldwide, were found as introduced species in the Petaluma River and Napa River, California, in 1992 and 1993. These rivers are mostly-estuarine tributaries that flow into north San Francisco Bay. Both species appeared to be well-established in this brackishwater habitat. Salinities at the collection sites were about 11‰ during the summer, rising to nearly 20‰ in the early autumn and falling to near 0‰ in the winter. Large numbers of all sizes of both species of medusae were observed and collected, indicating that the hydroid stages of the life cycles of the two are also well-established in these rivers. In the Petaluma River, populations of both species were at maximum in late July, with numbers of individuals declining through August and into September; the Napa River was sampled only in October, and at that time only B. virginica was found. Examination of full guts of M. inexspectata and B. virginica medusae revealed that both species had fed nearly exclusively on small crustaceans, principally barnacle nauplii, copepods and their eggs and nauplii, and crab zoea larvae (M. inexspectata only). All the M. inexspectata medusae were males, indicating that the population has probably developed from the introduction of perhaps only a single male polyp or polyp bud. In spite of its inability to reproduce sexually, this population appears to be maintained by the prodigious ability of the polyp to bud and reproduce asexually, and is fully capable of invading additional low-salinity habitats from its present Petaluma River site. Male and female B. virginica medusae were collected in both the Petaluma River and the Napa River, indicating that B. virginica may have been introduced by either the polyp or medusa stage (or both), but that multiple individuals (of both sexes) must have arrived from another port in one or more invasions. As indicated for M. inexspectata, the B. virginica population will also probably seed new populations in San Francisco Bay and elsewhere. Based on its cnidome as well as the morphology of both medusa and polyp, M. inexspectata has been reclassified by moving it from the family Olindiidae, Limnomedusae, to the family Moerisiidae, Anthomedusae.

DENSITY IS ALTERED IN HYDROMEDUSAE AND CTENOPHORES IN RESPONSE TO CHANGES IN SALINITY

Laboratory experiments have determined the behavioral and gross physiological responses of hydromedusae and ctenophores subjected to sudden changes of salinities in the range that might be encountered in nature. Nine species of hydromedusae (Aequorea victoria, Aglantha digitale, Bougainvillia principis, Gonionemus vertens, Phialidium gregarium, Polyorchis penicillatus, Proboscidactyla flavicirrata, Sarsia tubulosa, Stomotoca atra) and two species of ctenophores (Bolinopsis infundibulum, Pleurobrachia bachei) were transferred from natural sea water of 30.5‰ to modified sea waters of 19-38‰. Most species altered their density within a few hours by osmoconforming to salinities ranging from 23-38‰, so that equilibrium buoyancy (either positive, neutral, or negative, according to species) was regained along with normal behavior. Even salinity differences of only 1-2‰ required 30-60 minutes adjustment time. Prior to regaining their equilibrium buoyancies, differences in relative density caused medusae and cteno...

Western Atlantic midwater hydrozoan and scyphozoan medusae: in situ studies using manned submersibles

Little is known about the biology and ecology of mesopelagic medusae. In part, this is because midwater trawls are used to collect fragile medusae and other gelatinous macroplankton. Additionally, nets cannot provide data on behavior and on biotic associations. Herein, in situ observations on northwestern Atlantic midwater medusae made using the Johnson-Sea-Link submersibles are reported. Included are depth and temperature ranges; notes on pigments; locomotory behavior; and notes on prey and predators.

Structure and function of the prehensile tentilla of Euplokamis (Ctenophora, Cydippida)

SummaryEuplokamis has coiled tentilla on its tentacles, which can be discharged, flicking out at high velocity, when triggered by contact with prey. The tentillum adheres to prey by means of numerous colloblasts. Discharge, which takes 40–60 ms, is accomplished by contraction of striated muscles, found only in this genus among the Ctenophora. Restoration of the coiled state is attributable to passive, elastic components of the mesogloea. Rows of “boxes” (fluid-filled compartments) along the sides of the tentillum appear to stiffen the structure so that it does not collapse, kink or buckle during discharge. Smooth muscle fibres present in the tentillum may help pull the tentillum tight after prey have been captured.In addition to the rapid discharge response, the tentillum can perform slower, spontaneous, rhythmic movements which, it is suggested, resemble the wriggling of a plank-tonic worm, enabling the tentillum to function as a lure. These movements appear to be executed by contraction of two sets of myofilament-packed cells which differ in several important respects from conventional smooth muscle. They belong to a novel and distinct cytological subset (“inner-ring cells”), other members of which are packed with microtubules and seem to be involved in secondary structuring of the collagenous component of the mesogloea.Study of tentilla in different stages of development shows that the striated muscle fibres, originally nucleated, become enucleate as they differentiate and that the colloblasts form in association with accessory cells, as proposed by K. C. Schneider and G. Benwitz. The refractive granules which adhere to the outside of all mature colloblasts derive from these accessory cells. The colloblast nucleus undergoes changes during development suggestive of progressive loss of its role in transcription and protein synthesis, but it remains intact, contrary to statements in the literature.The tentillum of Euplokamis can be regarded as a true food-capturing organ and it is probably the most highly developed organ in the phylum.

EVIDENCE THAT ION REGULATION IN HYDROMEDUSAE AND CTENOPHORES DOES NOT FACILITATE VERTICAL MIGRATION

Medusae and ctenophores, like many types of gelatinous zooplankton, actively exclude sulphate ion from their mesogloeal body fluid and thus gain lift(buoyancy). It is hypothesized that vertically migrating species might show day/night variations in the rate of sulphate elimination to regulate buoyancy dynamically and that such changes are a major factor in vertical migration. To test this hypothesis, a series of laboratory experiments were conducted using medusae and ctenophores. Concentrations of radioactive sulphate were measured in equilibrium (uptake) experiments and concentrations of sodium, magnesium, potassium, and calcium ions were measured using atomic absorption spectrophotometry. No evidence of day/night (light/dark) changes in ion concentrations were found for the hydromedusae Aequorea victoria, Aglantha digitale, Gonionemus vertens, Mitrocoma cellularia, Phiaildium gregarium, Polyorchis penicillatus, Sarsia tubulosa, and Stomotoca atra or for the ctenophore Pleurobrachia bachei. It is conclud...

In Situ Foraging and Feeding Behaviour of Narcomedusae (Cnidaria: Hydrozoa)

Narcomedusae are a small and mostly oceanic group of hydromedusae whose tentacle morphology and comportment sets them off behaviourally and perhaps ecologically from most other medusae. Their tentacles are relatively few in number (2–40), stiff, and noncontractile, with points of insertion located well above the bell margin. Eleven species representing eight narcomedusan genera ( Aegina, Aeginura , an undescribed aeginid, Cunina, Pegantha, Solmaris, Solmissus , and Solmundella ) were observed and collected in situ in the NW Atlantic, Arctic and Antarctic, using scuba and manned submersibles. In life, the tentacles of narcomedusae are nearly always held upwards over the bell or projected laterally. The major prey were other gelatinous zooplankton, especially salps and doliolids. In the laboratory, these relatively large prey were caught on the tentacles which bend inward and coil at the tips to bring food to the mouth. By extending the tentacles perpendicular to the swimming path, these medusae achieve a relatively large encounter area, thus increasing the probability of contact with prey, for the amount of protein invested in tentacles.

PODOCORYNE SELENA, A NEW SPECIES OF HYDROID FROM THE GULF OF MEXICO, AND A COMPARISON WITH HYDRACTINIA ECHINATA

1. A new species of hydroid, Podocoryne selena, is described from the north Florida Gulf of Mexico. This hydroid has mistakenly been called P. carnea in the past; polyps are very similar in the two species. It cannot be determined whether P. carnea actually occurs in subtropical waters until mature medusae are examined in other regions.2. Observations on the behavior of the polyps and medusae of P. selena are given.3. The species Podocoryne selena and Hydractinia echinata are both found living on hermit crab shells in the Gulf of Mexico. The two species are compared and morphological differences are illustrated with scanning electron micrographs.