Sublethal salinity stress contributes to habitat limitation in an endangered estuarine fish

Abstract

As global change alters multiple environmental conditions, predicting species' responses can be challenging without understanding how each environmental factor influences organismal performance. Approaches quantifying mechanistic relationships can greatly complement correlative field data, strengthening our abilities to forecast global change impacts. Substantial salinity increases are projected in the San Francisco Estuary, California, due to anthropogenic water diversion and climatic changes, where the critically endangered delta smelt (Hypomesus transpacificus) largely occurs in a low-salinity zone (LSZ), despite their ability to tolerate a much broader salinity range. In this study, we combined molecular and organismal measures to quantify the physiological mechanisms and sublethal responses involved in coping with salinity changes. Delta smelt utilize a suite of conserved molecular mechanisms to rapidly adjust their osmoregulatory physiology in response to salinity changes in estuarine environments. However, these responses can be energetically expensive, and delta smelt body condition was reduced at high salinities. Thus, acclimating to salinities outside the LSZ could impose energetic costs that constrain delta smelt's ability to exploit these habitats. By integrating data across biological levels, we provide key insight into the mechanistic relationships contributing to phenotypic plasticity and distribution limitations and advance the understanding of the molecular osmoregulatory responses in nonmodel estuarine fishes.

Keywords: Hypomesus transpacificus; anadromous fish; climate change; delta smelt; environmental stress; osmoregulation; transcriptome.

Figure 1

(A) Map of salinity seascape and adult delta smelt abundance in San Francisco Estuary; salinity interpolation based on mean salinity at each sampling station, and station symbols weighted to mean delta smelt abundance utilizing Fall Midwater Trawl Survey (FMWT) data 2000–2015. (B) Distribution of adult delta smelt in relation to environmental salinity during FMWT surveys 1967–2015, overlaid with their demonstrated physiological tolerance range from Komoroske et al. (2014).

Figure 2

Effects of exposure time and salinity in Experiment 2 (acute exposure, longer‐term duration) on delta smelt (A) blood plasma osmolality (mmol kg⁻¹) and (B) condition index = weight (g)/fork length (mm)³; mean ± SEM for each salinity per exposure time point. Fish were taken for pre‐experiment samples randomly from each tank under control conditions (2.3 ppt) prior to 6 h gradual increases to target salinities (0‐h time point denotes when tanks reached target salinity); tanks remained at target salinities (2.3 control, 18.5, or 34.0 ppt) for the duration of the experiment. Asterisks indicate where responses within a treatment are significantly different from the pre‐experiment value, while lettering designates differences between treatments within each time point.

Figure 3

Heat map of microarray genes in Experiment 1 (acute exposure, short‐term duration) with q ≤ 0.01 for main effect of salinity (salinity) and the interaction of salinity x exposure time (I) (yellow shading, Venn diagram). Duplicate probes and replicates were averaged across treatments for visualization, while numbers in the Venn diagram refer to total numbers of significant probes for each factor or interaction; n = 4–6 individual fish for each salinity and exposure time group. Genes clustered on averaged probe similarity (dendrogram displayed on left, based on Euclidean distance matrix and complete agglomeration); a three‐color scale (value bar depicted above) was applied to visualize fold changes of mean normalized values between treatments (See Supplementary information for expanded heat map and table for all genes q ≤ 0.05).

Figure 4

Two‐dimensional nonmetric multidimensional scaling plot of microarray genes in Experiment 1 (acute exposure, short‐term duration) with q ≤ 0.01 for main effect of salinity (salinity) and the interaction of salinity x exposure time (I) (yellow shading, Venn diagram). Shapes correspond to exposure time (triangle = pre‐experiment, circle = 0 h, i.e. when target salinities were reached after 6 h gradual increases or decreases, square = 18 h, diamond = 42 h) and color corresponds to salinity treatment (black = 2.3 ppt pre‐experiment, yellow = 0.4 ppt, gray = 2.3 ppt handling control, blue = 6.0 ppt, purple = 12.0 ppt, red = 18.0 ppt). Arrows overplotted to visualize mean trajectory of transcriptomic changes over the exposure time course of the experiment.

Figure 5

Composite bar charts for biological processes represented by genes in Experiment 1 (acute exposure, short‐term duration) affected by probes of primary interest (combined list of genes displaying differential expression at q ≤ 0.1 for salinity main effect and salinity x exposure time interaction). Panels represent: (A) overall biological processes, (B1) subcategories of metabolic processes in (A), (B2) subcategories of primary metabolic processes in (B1), (C1) subcategories of cellular processes in (A).

Figure 6

Effects of salinity and exposure time on the expression of genes in Experiment 1 (acute exposure, short‐term duration) representative of key biological processes (mean ± SEM for each salinity per exposure time point). Y‐axes units are normalized log2 expression, reversed for ease of interpretation of transcriptional changes (i.e., lower number is higher transcription). Post hoc analyses of biological interest were conducted where appropriate as determined by main models (see Materials and methods). Points and error bars represent mean ±SEM; colored symbols and letters correspond to treatment colors in the legend. Uppercase letters denote main effect contrasts for salinity treatments, while lowercase letters denote pairwise contrasts between salinities within each time point. Asterisks denote statistical difference from time point 0; ✪ denotes main effect contrasts, while * denotes pairwise contrasts.