Ontogenetic Changes in Body Shape and the Scaling of Metabolic Rate in the American Eel (Anguilla rostrata).

AbstractThe body mass (M) scaling of resting metabolic rate (RMR) may vary significantly throughout ontogeny for multiple reasons that are not perfectly understood. To compare two major geometric theories of metabolic scaling, surface area (SA) theory and resource transport network (RTN) theory, we tested whether ontogenetic shifts in metabolic scaling relate to changes in body shape in the American eel (Anguilla rostrata). To do so, we compared the log-linear scaling exponents of RMR to M (bR) and M to body length (bL) in juvenile and subadult eels (glass and yellow eel life stages, respectively). Glass eels exhibited a bL>3 and bR significantly <2/3, as predicted by SA theory. Yellow eels also had a bL>3, but their bR was not significantly different from 2/3 or 3/4. We hypothesize that two developmental changes contribute to bR being higher for yellow eels: (1) a greater reliance on branchial respiration than body-surface-dependent cutaneous respiration and (2) a lower rate of thickening during subadult growth. An ontogenetic decrease in the ratio of cutaneous to gill respiration may have increased the relative importance of the physical constraints of a single-pump, closed circulatory system on the body-size-dependent rate of resource supply to metabolizing tissues (as predicted by RTN theory) in subadult eels. Future research is needed to quantify these developmental changes and their potential mechanistic effects on metabolic scaling, especially in the elver, a critical life stage between the glass and yellow eel stages, that was not analyzed in this study.

Reestablishing a host-affiliate relationship: migratory fish reintroduction increases native mussel recruitment.

Co-extirpation among host-affiliate species is thought to be a leading cause of biodiversity loss worldwide. Freshwater mussels (Unionida) are at risk globally and face many threats to survival, including limited access to viable host fish required to complete their life history. We examine the relationship between the common eastern elliptio mussel (Elliptio complanata) and its migratory host fish the American eel (Anguilla rostrata), whose distribution in the Chesapeake Bay watershed is limited, in part, by dams. We examined population demographics of E. complanata across locations in the Chesapeake Bay watershed, primarily in the Susquehanna River in the absence of American eels, and conducted experimental restocking of eels to assess potential impacts on mussel recruitment. Compared to surveys completed ~20 yr prior, E. complanata could be experiencing declines at several historically abundant sites. These sites also had extremely limited evidence of recruitment. Restoration of host fish improved recruitment, but results were not equivalent between stocking sites, indicating that host reintroduction alone may not be fully effective in reestablishing mussel populations. One site where eels were introduced (Pine Creek, Tioga County, Pennsylvania, USA) experienced an increase from 0 juveniles found during quantitative surveys prior to eel stocking to 151 (21% of individuals collected during quantitative surveys) E. complanata juveniles found four years after stocking. A second site (Buffalo Creek, Union County, Pennsylvania) experienced a more moderate increase from 2 to 7 juveniles found during 2010 and 2014 quantitative surveys, respectively. Continued examination of other potential interacting factors affecting recruitment, including water quality or habitat conditions, is necessary to target favorable sites for successful restoration.

RNA sequencing analysis of transcriptional change in the freshwater mussel Elliptio complanata after environmentally relevant sodium chloride exposure.

To identify potential biomarkers of salt stress in a freshwater sentinel species, we examined transcriptional responses of the common mussel Elliptio complanata to controlled sodium chloride (NaCl) exposures. Ribonucleic acid sequencing (RNA-Seq) of mantle tissue identified 481 transcripts differentially expressed in adult mussels exposed to 2 ppt NaCl (1.2 ppt chloride) for 7 d, of which 290 had nonoverlapping intervals. Differentially expressed gene categories included ion and transmembrane transport, oxidoreductase activity, maintenance of protein folding, and amino acid metabolism. The rate-limiting enzyme for synthesis of taurine, an amino acid frequently linked to osmotic stress in aquatic species, was upregulated, as was the transmembrane ion pump sodium/potassium adenosine 5'-triphosphatase. These patterns confirm a primary transcriptional response to the experimental dose, albeit likely overlapping with nonspecific secondary stress responses. Substantial involvement of the heat shock protein 70 chaperone family and the water-transporting aquaporin family was not detected, however, in contrast to some studies in other bivalves. A subset of the most significantly regulated genes was confirmed by quantitative polymerase chain reaction in an independent sample. Cluster analysis showed separation of mussels exposed to 2 ppt NaCl from control mussels in multivariate space, but mussels exposed to 1 ppt NaCl were largely indistinguishable from controls. Transcriptome-scale analysis of salt exposure under laboratory conditions efficiently identified candidate biomarkers for further functional analysis and field validation. Environ Toxicol Chem 2017;36:2352-2366. © Published 2017 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Transcriptomic analysis of the mussel Elliptio complanata identifies candidate stress-response genes and an abundance of novel or noncoding transcripts.

Mussels are useful indicator species of environmental stress and degradation, and the global decline in freshwater mussel diversity and abundance is of conservation concern. Elliptio complanata is a common freshwater mussel of eastern North America that can serve both as an indicator and as an experimental model for understanding mussel physiology and genetics. To support genetic components of these research goals, we assembled transcriptome contigs from Illumina paired-end reads. Despite efforts to collapse similar contigs, the final assembly was in excess of 136,000 contigs with an N50 of 982 bp. Even so, comparisons to the CEGMA database of conserved eukaryotic genes indicated that ∼ 20% of genes remain unrepresented. However, numerous candidate stress-response genes were present, and we identified lineage-specific patterns of diversification among molluscs for cytochrome P450 detoxification genes and two saccharide-modifying enzymes: 1,3 beta-galactosyltransferase and fucosyltransferase. Less than a quarter of contigs had protein-level similarity based on modest BLAST and Hmmer3 statistical thresholds. These results add comparative genomic resources for molluscs and suggest a wealth of novel proteins and noncoding transcripts.

The effects of salinity exposure on multiple life stages of a common freshwater mussel, Elliptio complanata.

There is growing concern over the effects of increased salinization on freshwater organisms, which are largely unknown for unionid mussels. Adult and larval Elliptio complanata were exposed to low-level salt concentrations to determine the effects on mussel survival, physiology, and reproduction. Adults were exposed to salt concentrations of 0 parts per thousand (ppt), 2 ppt, 4 ppt, and 6 ppt NaCl and monitored over 7 d for mortality. Treatment groups exposed to 6 ppt and 4 ppt experienced 50% mortality at day 3 and day 4, respectively, with complete mortality by day 7. No mortality was observed in the other treatments. Adults were also exposed to sublethal salinity levels of 1 ppt and 2 ppt NaCl for 4 wk to determine physiological consequences of prolonged salinity exposure. Mussels exposed to 1 ppt and 2 ppt experienced reduced metabolic rates within the first 24 h of exposure that recovered to control levels in the 1-ppt treatment within 7 d. Metabolic recovery did not occur in the 2-ppt treatment by the end of 28 d. Glochidia exposed to 3-ppt NaCl during attachment to their host fish suffered a reduction in attachment success and metamorphosis, resulting in a 10-fold reduction in the number of juveniles produced per host fish. The present study demonstrates that low levels of salt can have a dramatic effect on the reproduction, physiology, and survival of freshwater mussels.

Shoaling preferences of two common killifish (Fundulus heteroclitus and F. diaphanus) in the laboratory and in the field: a new analysis of heterospecific shoaling.

Heterospecific grouping behavior of mummichogs (Fundulus heteroclitus) and banded killifish (Fundulus diaphanus) was analyzed in the laboratory and in a freshwater tidal marsh in Cremona, Maryland. Several parameters of wild, intact shoals were measured, including species composition, body length, parasite load, gender, and any physical abnormalities. Fish collected were used for laboratory analysis of shoaling preferences. When size was equal, banded killifish and mummichogs preferred conspecific shoals to heterospecific shoals, consisting of mummichogs, banded killifish, and sheepshead minnows (Cyprinodon variegates). Shoal collection from the field resulted in mixed species shoals with individuals predominantly unaffected by parasites or other physical abnormalities. Size appeared to be a sorting mechanism. A temporal shift in lengths was evident. Initial shoals caught contained significantly smaller fish compared to the final shoals caught. Results are compared with previous studies on heterospecific shoaling in killifish and new characteristics of heterospecific shoals inhabiting freshwater tidal marshes are discussed.

Octopuses (Octopus bimaculoides) and cuttlefishes (Sepia pharaonis, S. officinalis) can conditionally discriminate.

In complex navigation using landmarks, an animal must discriminate between potential cues and show context (condition) sensitivity. Such conditional discrimination is considered a form of complex learning and has been associated primarily with vertebrates. We tested the hypothesis that octopuses and cuttlefish are capable of conditional discrimination. Subjects were trained in two maze configurations (the conditions) in which they were required to select one of two particular escape routes within each maze (the discrimination). Conditional discrimination could be demonstrated by selecting the correct escape route in each maze. Six of ten mud-flat octopuses (Octopus bimaculoides), 6 of 13 pharaoh cuttlefish (Sepia pharaonis), and one of four common cuttlefish (S. officinalis) demonstrated conditional discrimination by successfully solving both mazes. These experiments demonstrate that cephalopods are capable of conditional discrimination and extend the limits of invertebrate complex learning.