Antarctic Asteroid Odontaster validus: Constancy of Reproductive Periodicities.

The fact that samples of Odontaster validus from the Balleny Islands (67 degrees S) and Robertson Bay (71 degrees S) closely resembled reproductively samples taken from McMurdo Sound (77 degrees S) indicates reproductive synchrony in this species over much or all of their circumcontinental antarctic distribution. This synchrony suggests that the reproductive periodicities of O. validus are both adapted to and synchronized by the summer period of phytoproduction and that neither light nor temperature changes have any direct synchronizing role.

Bacterivory: a novel feeding mode for asteroid larvae.

Planktotrophic larvae that occur beneath the annual sea ice in the Antarctic assimilate organic solutes and preferentially ingest bacteria, whereas they actively exclude phytoplankton. In regions where phytoplankton biomass is temporally limited by light or nutrient concentrations, the growth and development of planktotrophic larvae may not be directly coupled to phytoplankton production.

Population structure and speciation in tropical seas: global phylogeography of the sea urchin Diadema.

The causes of speciation in the sea are rarely obvious, because geographical barriers are not conspicuous and dispersal abilities or marine organisms, particularly those of species with planktonic larvae, are hard to determine. The phylogenetic relations of species in cosmopolitan genera can provide information on the likely mode of their formation. We reconstructed the phylogeny of the pantropical and subtropical sea urchin genus Diadema, using sequences of mitochondrial DNA from 482 individuals collected around the world, to determine the efficacy of barriers to gene flow and to ascertain the history of possible dispersal and vicariance events that led to speciation. We also compared 22 isozyme loci between all described species except D. palmeri. The mitochondrial DNA data show that the two deepest lineages are found in the Indian and West Pacific Oceans. (Indo-Pacific) Diadema setosum diverged first from all other extant Diadema, probably during the initiation of wide fluctuations in global sea levels in the Miocene. The D. setosum clade then split 3-5 million years ago into two clades, one found around the Arabian Peninsula and the other in the Indo-West Pacific. On the lineage leading to the other species of Diadema, the deepest branch is composed of D. palmeri, apparently separated when the climate of New Zealand became colder and other tropical echinoids at these islands went extinct. The next lineage to separate is composed of a currently unrecognized species of Diadema that is found at Japan and the Marshall Islands. Diadema mexicanum in the eastern Pacific separated next, whereas D. paucispinum, D. savignyi, and D. antillarum from the western and central Atlantic, and (as a separate clade) D. antillarum from the eastern Atlantic form a shallow polytomy. Apparently, Indo-Pacific populations of Diadema maintained genetic contact with Atlantic ones around the southern tip of Africa for some time after the Isthmus of Panama was complete. Diadema paucispinum contains two lineages: D. paucispinum sensu stricto is not limited to Hawaii as previously thought, but extends to Easter Island, Pitcairn, and Okinawa; A second mitochondrial clade of D. paucispinum extends from East Africa and Arabia to the Philippines and New Guinea. A more recent separation between West Indian Ocean and West Pacific populations was detected in D. setosum. Presumably, these genetic discontinuities are the result of water flow restrictions in the straits between northern Australia and Southeast Asia during Pleistocene episodes of low sea level. Diadema savignyi is characterized by high rates of gene flow from Kiribati in the central Pacific all the way to the East African Coast. In the Atlantic, there is a biogeographic barrier between the Caribbean and Brazil, possibly caused by fresh water outflow from the Amazon and the Orinoco Rivers. Diadema antillarum populations of the central Atlantic islands of Ascension and St. Helena are genetically isolated and phylogenetically derived from Brazil. Except for its genetic separation by the mid-Atlantic barrier, Diadema seems to have maintained connections through potential barriers to dispersal (including the Isthmus of Panama) more recently than did Eucidaris or Echinometra, two other genera of sea urchins in which phylogeography has been studied. Nevertheless, the mtDNA phylogeography of Diadema includes all stages expected from models of allopatric differentiation. There are anciently separated clades that now overlap in their geographic distribution, clades isolated in the periphery of the genus range that have remained in the periphery, clades that may have been isolated in the periphery but have since spread towards the center, closely related clades on either side of an existing barrier, and closely related monophyletic entities on either side of an historical barrier that have crossed the former barrier line, but have not attained genetic equilibrium. Except for D. paucispinum and D. savignyi, in which known hybridization may have lodged mtDNA from one species into the genome of the other, closely related clades are always allopatric, and only distantly related ones overlap geographically. Thus, the phylogenetic history and distribution of extant species of Diadema is by and large consistent with allopatric speciation.

Hybrids of two closely related tropical sea urchins (genus Echinometra): evidence against postzygotic isolating mechanisms.

A series of cross-fertilization experiments were conducted with two unnamed, sympatric species of sea urchins in the Echinometra mathaei species complex, Echinometra sp. A (Ea) and Echinometra sp. C (Ec). Heterogametic fertilization success was high when eggs of Ec and sperm of Ea were involved, and low with eggs of Ea and sperm of Ec. Hybrids produced from crosses in either direction developed normally to sexually mature adults; Ea x Ea were largest in test size, followed by Ec (ova) x Ea (sperm), Ea (ova) x Ec (sperm), and Ec x Ec, respectively. Color patterns of the hybrids were closer to the maternal coloration, whereas other characters such as relative test dimensions and spine lengths, morphology of tubefoot and gonad spicules, and gamete sizes were intermediate. Fertilization rates in F1 backcrosses were high, minimizing the possibility that hybrid infertility is a postzygotic mechanism of reproductive isolation. On the other hand, intensive surveys failed to find individuals with hybrid characteristics in the field, suggesting that natural hybridization between the two species is rare. Prezygotic isolating mechanisms, such as microhabitat separation and gamete incompatibility, at least between Ea eggs and Ec sperm, most likely maintain the genetic integrity of these two closely related species.

Evidence for a Programmed Circannual Life Cycle Modulated by Increasing Daylengths in Neanthes limnicola (Polychaeta: Nereidae) From Central California.

Timing of parturition, fecundity, and life span were determined in laboratory cultures of the semelparous, self-fertilizing, viviparous polychaete Neanthes limnicola. Worms were exposed to fixed daylengths (short--8h light: 16h dark; neutral--12h:12h; long--16h:8h), switched between different fixed daylengths, and switched from fixed daylengths to increasing or decreasing daylengths. Timing of parturition was synchronized when under neutral daylength, but became asynchronous under both short and long daylength, as well as when any of the fixed daylength was followed by decreasing daylengths. Worms under neutral daylength had the highest fecundities and shortest life spans, while those under long days had the lowest fecundities and longer life spans. When fixed daylength (short, neutral, long) was followed by increasing daylengths, timing of parturition was synchronized, fecundity was high, and life span shortened. These and earlier published experiments on the influence of seasonally changing photoperiods indicate that the life cycle of the estuarine N. limnicola is programmed to be completed in somewhat less than a year, and that seasonally changing photoperiods modulate it to determine the optimal time of parturition.

Consistent synchronization and circaseptennian (about-7-yearly) modulation of circannual gonadal index rhythm of three marine invertebrates.

A circannual rhythm in mean gonadal weight, expressed as percentage of overall body weight, is quantified by cosinor and linear-nonlinear least-squares rhythmometry in three marine invertebrates. Data series from the purple sea urchin (Strongylocentrotus purpuratus), a mollusk, the black chiton (Katharina tunicata), and the ochre sea star (Pisaster ochraceus), exhibiting circannual periods of 368, 361, and 365 days, have relatively tight 95% limits of 356-387, 356-367, and 359-371 days, respectively, when scrutinized by nonlinear least-squares rhythmometry. These limits overlap a precise year (365.25 days). This findings supports the occurrence of a 1 year-synchronized circannual rhythm in these marine invertebrates. For the ochre sea star, the about-yearly timing of greatest gonadal weight in relation to body mass, analyzed for a circannual rhythm by the fitting of a 1-year cosine to the data, remains virtually identical within 30 years. The 95% confidence intervals of the acrophase for mean gonadal weight (as percentage of body weight), range from March 3 to April 12 in the 1950s and from March 8 to April 12 in the 1980s. A MESOR (midline-estimating statistic of rhythm) difference may relate to geographic factors (Pacific Grove, California, from January 1954 to December 1958 vs. Terrace Point near Santa Cruz, California, from December 1978 to June 1985) that in turn may reflect a different food supply and/or temporal factors, e.g., it may result from changes in the ecology of the sea. A possible circaseptennian rhythm is only suspected in the data from the ochre star (P = 0.14); it is rigorously described for the purple sea urchin (P less than 0.001) and also detected in the black chiton (P = 0.029) in time series covering 9.5 years, from September 1953 to April 1963 for the purple sea urchin and from January 1957 to September 1966 for the black chiton. Least-squares rhythmometry (with trial periods between 1 and 15 years in 6-month increments) resolves the two (circannual and circaseptennian) components concomitantly below the 3% level of statistical significance. Nonlinear least-squares rhythmometry validates a period of 7.01 years for the purple sea urchin, although within broad 95% limits (ranging from 5.58 to 10.41 years). The life span of these marine invertebrates is not determined. One deals with a population rhythm sampled with serial independence.(ABSTRACT TRUNCATED AT 400 WORDS)