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Despite their potential importance as predators, the early life-history stages of ctenophores are relatively understudied. Measurements of digestion times for larvae are especially limited compared with adults, even though these measurements are required to calculate feeding rates using gut content analysis. This study reports digestion times for larval Mnemiopsis leidyi for a wide range of prey, Larval M. leidyi consumed the copepodite and naupliar stages of copepods, polychaete larvae, bivalve larvae, rotifers, rotifer eggs, ciliates and flagellates. Digestion times varied significantly among prey taxa and ranged from ~ 1 min for aloricate ciliates to 2 h for copepodites. Although ciliates and flagellates are frequently observed in ctenophore guts, feeding on protistan microplankton cannot be quantified adequately using gut content analysis because these prey items are often digested too rapidly to be identified in ctenophore guts.

Mnemiopsis leidyi larvae were preserved quantitatively for up to 1 year in 5―20% acid Lugol's solution. The highest preservation efficiency and least shrinkage and deformation occurred in the 5% treatment.

Observations of ctenophore species were made in the Gulf of Trieste between 2003 and 2006. We examined native ctenophore species with special attention to representatives of the orders Lobata and Beroida, and we recorded among them two non-native ctenophores: Mnemiopsis leidyi A. Agassiz 1865 and Beroe ovata sensu Mayer, 1912. The validity of the Mediterranean species Beroe ovata is discussed. We determined that among the native species, it is not Beroe ovata but rather Beroe cucumis sensu Mayer, 1912 that occurs in the Mediterranean Sea.

Mnemiopsis leidyi. larvae depend on microplankton ( 0.5 mm in length and can successfully capture and consume mesozooplankton prey. Feeding and growth rates of newly hatched M. leidyi larvae were measured in controlled laboratory experiments. When fed natural microplankton assemblages, newly hatched larvae consumed significant quantities of both autotrophic and heterotrophic prey, including diatoms, phototrophic, heterotrophic and mixotrophic dinoflagellates, euglenoid flagellates, aloricate and tintinnid ciliates, and rotifers. Average per capita clearance rates were 1.99-7.59 mL individual -1 h -1 (X = 4.01 mL individual -1 h -1 ; SD = 1.95) and total per capita ingestion was 0.01-4.70 μg C individual -1 day -1 x 10 2 (X = 0.83 μg C individual -1 day -1 × 10 2 ; SD = 1.89). Larval growth rates were -0.13 to 0.56 mm individual -1 day -1 (equivalent to -1.72 to 4.33 μgC individual -1 day -1 ) over a range of larval sizes from 0.5 ( 2 weeks, and growth rate decreased when larvae reached 4-5 mm in length, corresponding to the beginning of their morphological transition from tentaculate to lobate feeding mode. The gross growth efficiency of larvae fed natural microplankton assemblages was ∼3%.

Larval stages of the ctenophore Mnemiopsis leidyi rely on metazoan prey, such as Acartia tonsa nauplii and copepodites, to support high growth rates. However, M. leidyi larvae 2.0 mm, nauplii REs plateaued at a mean of 91.03 ± 1.70%. All larvae 2.5 mm reached a maximum copepodite RE of 63.75 ± 3.01%.

Larvae and post-larvae of the lobate ctenophore Mnemiopsis leidyi consumed significant quantities of microphytoplankton and microzooplankton prey in controlled experiments where they were fed in situ assemblages of microplankton prey. Prey taxa ingested included phototrophic and heterotrophic dinoflagellates, choreotrich ciliates, cyclotrich ciliates and euglenoid flagellates. Per capita clearance and ingestion rates varied with consumer size, prey taxon and prey abundance. Average per capita clearance rates for the 11 size classes of larvae and post-larvae examined ranged from 9 to 230 mL individual � 1 h � 1 . Average per capita clearance rates for tentaculate-stage, transition-stage and lobate-stage individuals were 40, 31 and 95 mL individual � 1 h � 1 , respectively. Total ingestion rate varied similarly with consumer size, prey taxon and prey abundance, ranging from 1 to 6259% body carbon day � 1 . All size classes of M. leidyi larvae and post-larvae derived more nutrition from dinoflagellates than from ciliates. Both positive and negative selection of specific prey taxa were observed, and varied with larval size and prey taxonomic composition. The results contradict the prevailing view that all life history stages of M. leidyi are primarily carnivorous. The measured clearance rates are sufficiently high that the larvae, when abundant, have the potential to exert a significant predation impact on their prey populations in the water column. Understanding the processes that influence survival during early developmental stages may provide one key to understanding the initiation and maintenance of the adult M. leidyi blooms that are commonly observed in coastal waters.

SCUBA observations confirm dense aggregations of the ctenophore Mnemiopsis leidyi in surface and bottom layers of waters off southern Argentina. These observations are consistent with results from prior studies indicating directed vertical migration of M. leidyi.