Jilda Alicia Caccavo

Caccavo JA, Ashford JR, Ryan S, Papetti C, Schröder M, Zane L (2019) Spatial structuring and life history connectivity of Antarctic silverfish along the southern continental shelf of the Weddell Sea. Marine Ecology Progress Series. 624:195-212.

A multidisciplinary approach was employed to examine a physical-biological population hypothesis for a critical forage species, the Antarctic silverfish (Pleuragramma antarctica). A previous study had shown strong gene flow along the westward Antarctic Slope Current, in addition to spatially recurring length modes that provided evidence for episodic connectivity. In this paper, otolith nucleus chemistry from a subset of fish collected in the southern Weddell Sea as part of a hydrographic survey of the Filchner Trough system was used to test between connectivity scenarios. Nucleus chemistry, which reflects environmental exposure during early life, showed significant spatial structuring despite homogeneity in microsatellite allele frequencies. Mg⋅Ca-1 and Sr⋅Ca-1 differentiated length modes, and Mg⋅Ca-1 showed significant contrasts between Atka Bay, Halley Bay, and Filchner Trough. Physical-biological mechanisms may help reconcile structuring shown by otolith chemistry, length, and abundance data with prior evidence of gene flow. Such mechanisms include self-recruitment shaped by circulation associated with the Filchner Trough, fluctuations in mixing between immigrant and locally-recruited fish, and feeding opportunities between inflowing Modified Warm Deep Water and outflowing Ice Shelf Water. The results illustrate how comparisons between multi-disciplinary techniques based on integrated sampling designs that incorporate hydrography can enhance understanding of population structure and connectivity around the Southern Ocean.

Caccavo JA, Papetti C, Wetjen M, Knust R, Ashford J, Zane L (2018) Along-shelf connectivity and circumpolar gene flow in Antarctic silverfish (Pleuragramma antarctica). Scientific Reports. 8:17856.

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The Antarctic silverfish (Pleuragramma antarctica) is a critically important forage species with a circumpolar distribution and is unique among other notothenioid species for its wholly pelagic life cycle. Previous studies have provided mixed evidence of population structure over regional and circumpolar scales. The aim of the present study was to test the recent population hypothesis for Antarctic silverfish, which emphasizes the interplay between life history and hydrography in shaping connectivity. A total of 1067 individuals were collected over 25 years from different locations on a circumpolar scale. Samples were genotyped at fifteen microsatellites to assess population differentiation and genetic structuring using clustering methods, F-statistics, and hierarchical analysis of variance. A lack of differentiation was found between locations connected by the Antarctic Slope Front Current (ASF), indicative of high levels of gene flow. However, gene flow was significantly reduced at the South Orkney Islands and the western Antarctic Peninsula where the ASF is absent. This pattern of gene flow emphasized the relevance of large-scale circulation as a mechanism for circumpolar connectivity. Chaotic genetic patchiness characterized population structure over time, with varying patterns of differentiation observed between years, accompanied by heterogeneous standard length distributions. The present study supports a more nuanced version of the genetic panmixia hypothesis that reflects physical-biological interactions over the life history.

Brooks CM, Caccavo, JA, Ashford J, Dunbar R, Goetz K, La Mesa M and Zane L. (2018) Early life history connectivity of Antarctic silverfish (Pleuragramma antarctica) in the Ross Sea. Fisheries Oceanography, 00, 1-14.

A recent population hypothesis for Antarctic silverfish (Pleuragramma antarctica), a critical forage species, argued that circumpolar distributions documented over the continental shelf are maintained by interactions between life history processes and the circulation associated with glacial trough systems. In the Ross Sea, aggregations of cryopelagic eggs and larvae are found under fast ice in Terra Nova Bay, and dispersing larvae are predicted to encounter the shelf outflow along the western side of the Drygalski Trough. The outflow advects the larvae towards the shelf-break, where mixing with the trough inflow facilitates return toward the inner shelf. To examine the hypothesis, we compared samples of P. antarctica collected near Coulman Island in the outflow, along Crary Bank in the inflow, and a third set taken over the rest of the Ross Sea. Any misidentification was ruled out using an innovative genetic validation. Silverfish larvae comprised more than 99% of the overall catch from all combined tows, and the highest population densities were found in the Drygalski Trough. The results provided no evidence to reject the population hypothesis. Instead, abundance indices, back-calculated hatching dates, length distributions and growth were congruent with a unified early life history for larval silverfish in the western Ross Sea, constrained by cryopelagic early stages in Terra Nova Bay. By contrast, a single tow in the Bay of Whales revealed much smaller larvae, suggesting either a geographically separate population with a coherent early life history in the eastern Ross Sea; or westward connectivity with spawning fish sourced from troughs located upstream in the Amundsen and Bellingshausen Seas. These results illustrate how hypotheses that integrate population structure with life history processes can provide precise spatial predictions across geographically separated systems, to facilitate highly targeted sampling designs in subsequent testing.

Papetti C, Schiavon L, Milan M, Lucassen M, Caccavo JA, Paterno M, Boscari E, Marino IAM, Congiu L, Zane L (2018) Genetic variability of the striped venus Chamelea gallina in the northern Adriatic Sea. Fisheries Research. 201, 68-78.

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Chamelea gallina is a valuable commercial species in the Mediterranean Sea. The strong fishing pressure on C. gallina in the northern and central Adriatic Sea has paralleled a clear-cut decrease in clam population density and the occurrence of several irregular mortality events. Despite the commercial interest in this species, nothing is known about its genetic sub-structuring at the geographic and/or temporal scale, nor its levels of genetic variability. Analyzing microsatellite genotypes for samples collected in the Adriatic Sea, we detected large geographic genetic homogeneity with gene flow guided by broad scale circulation in the north-south direction. Our results also indicate weak genetic differentiation among adults and juveniles at local and temporal scales . These small genetic differences might be determined by variability of local circulation and reproductive success as suggested by our estimates of effective number of breeders. In fact, global effective population size estimates are medium-high, but a low number of breeders are responsible for the yearly recruitment. Notwithstanding, it was not possible to detect signatures of bottleneck. Future efforts in fishery management should aim to maintain genetic diversity –– essential to the long-term sustainability of the resource –– and limit effective population size fluctuations while considering the need to improve water quality to avoid mass mortality events.

Caccavo JA, Brooks C, Zane L, Ashford JR (2015) Identification of Pleuragramma antarctica larvae in the Ross Sea via mitochondrial DNA. CCAMLR Working Group on Fish Stock Assessment, WG-FSA 15.

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Research into the early life stages of Pleuragramma antarctica is essential to understanding how oceanographic variation will impact spatial distributions over time. The recent findings of nursery grounds in Terra Nova Bay have led to added inquiry into larval distribution and life history traits in the Ross Sea. A report submitted to the CCAMLR-EMM working group last year provided abundance, length and growth data for larvae found in the western and eastern Ross Sea during the austral summer of 2013, which were identified as P. antarctica based on morphological characteristics. We extracted genomic DNA from a sample of these larvae and, using fish universal primers, amplified part of the 16S rDNA and the D-Loop region. Despite evidence of DNA degradation, sequencing was nevertheless successful in a fraction of samples. Sequences were aligned with known GenBank sequences for P. antarctica and several of related notothenioids, which confirmed the species identity of larvae in the western Ross Sea as P. antarctica. Consistent with the previous report, D-Loop sequences also demonstrated that recently hatched larvae sampled from the eastern Ross Sea were from the same species, suggesting the possibility of another nursery ground for P. antarctica in the vicinity of the Bay of Whales. This is a novel use of mitochondrial DNA to test morphological identification when examining spatial distributions of P. antarctica that depart from expectation.