Many animals generate new body segments sequentially from a posterior growth zone, and this is generally thought to be the case for the annelids. Most annelids, including polychaetes, have an indirect life cycle and generate their earliest segments during larval life. We have characterized the nature of the growth zone in two polychaetes, Hydroides elegans and Capitella sp. I, during both larval and juvenile stages of segment formation by examining cell division patterns with 5-bromo-2'-deoxyuridine incorporation. Cell division patterns show commonalities between the two species, even though they have distinct body plans and life history characteristics. In both polychaetes, larval segments arise from a field of dividing cells located in lateral regions of the body, rather than from a localized posterior growth zone. Circumferential expansion of the forming segmental tissue is particularly pronounced in Capitella sp. I. Post-metamorphic segments, in contrast, originate from a classical posterior growth zone, with the exception of four posterior thoracic segments of H. elegans, which appear to arise from an area in the middle of the body, indicating plasticity of segment-generating mechanisms present in different annelid life histories. The distinct nature of larval versus juvenile growth zones in H. elegans and Capitella sp. I raises the question of the mechanistic relationship between these two growth zones. The results of this study increase our understanding of the cellular origins of segments in annelids, and serve as a basis for interpretation of molecular expression patterns associated with segment formation in polychaetes.

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Substantial accumulation of dimethylsulfoniopropionate (DMSP) is documented in tissues of all six of the common species of giant clams (Tridacnidae). Results include measures of DMSP concentrations in siphonal mantle, byssal mantle, adductor muscle, and gill tissues obtained by gas chromatography of alkalized extracts plus evidence of DMSP from mass spectrometry. Formation of dimethylsulfide (DMS) by tissues after death is documented. The tridacnid clams maintain symbiotic associations with populations of dinoflagellates, which live within the enlarged siphonal mantle. We postulated that because of their association with dinoflagellates, the clams would chronically accumulate DMSP to high concentrations. Our results show that DMSP occurs at over 30 mmol·kg -1 in many tissues of tridacnid clams (sometimes 50-60 mmol·kg -1 ), meaning that these clams accumulate DMSP to the highest documented tissue concentrations in the animal kingdom. DMSP at such concentrations could affect multiple properties and functions. Our particular interest for this research was to assess whether postmortem breakdown of DMSP is responsible for the rapid development of potent off-odors and off-tastes that have blocked the commercial success of giant clam aquaculture. High concentrations of DMS produced in the day after death probably account for the peculiar perishability of giant clam tissues.Résumé : Nous présentons des données qui montrent une accumulation importante de diméthylsulfoniopropionate (DMSP) dans les tissus des six espèces communes de moules géantes (Tridacnidae). Ces données comprennent des dosages des concentrations de DMSP par chromatographie en phase gazeuse d'extraits alcalinisés du manteau du siphon, du manteau du byssus, du muscle adducteur et des tissus branchiaux; d'autres données sur le DMSP ont été obtenues par spectrométrie de masse. Il y a aussi formation de sulfure de diméthyle (DMS) dans les tissus après la mort. Les moules tridacnidées maintiennent des relations symbiotiques avec des populations de dinoflagellés qui vivent dans le manteau élargi du siphon. Nous avons émis l'hypothèse que cette association avec les dinoflagellés amène les moules à accumuler de façon chronique de fortes concentrations de DMSP. Nos données confirment l'existence de concentrations de plus de 30 mmol·kg -1 (même 50-60 mmol·kg -1 ) dans plusieurs tissus des moules tridacnidées, ce qui représente les plus fortes accumulations tissulaires connues de DMSP dans le règne animal. À de telles concentrations, le DMSP doit affecter de nombreuses propriétés et fonctions. Nous nous sommes intéressés, en particulier, dans ce travail à savoir si la dégradation après la mort du DMSP est responsable de l'apparition rapide des fortes odeurs et des saveurs désagréables qui nuisent au succès commercial de l'élevage des moules géantes. Les fortes concentrations de DMS qui se produisent durant la première journée qui suit la mort des moules géantes expliquent probablement la détérioration inusitée de leurs tissus.[Traduit par la Rédaction] ...

Changes in total mercury concentration (T‐Hg), diet, and growth were monitored in three age‐classes and four cohorts of mimic shiners Notropis volucellus throughout the ice‐free season of 1995 and in early May 1996 and 1997 in Devils Lake, Wisconsin. All age‐classes of fish fed exclusively on Daphnia in early spring and late fall but had a mixed diet during the summer consisting of zooplankton, other invertebrates, and filamentous algae. Concentration of methyl Hg in Daphnia varied during the summer. Concentration of T‐Hg in age‐1 fish also varied during the summer, but all age‐classes increased in T‐Hg concentration during winter. Body burden increased steadily throughout the year for all age‐classes and cohorts. Each successively older age‐class of fish had increasingly higher concentrations of T‐Hg and body burdens. The T‐Hg concentration and body burden were significantly different between cohorts in early spring for age‐1 and age‐2 fish. Differences between cohorts corresponded to differences in Hg concentrations in the water and Daphnia during the cohort's year of birth. Our results suggest that small planktivorous fish are a useful sentinel for monitoring short‐term changes in the availability of Hg in lakes.