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1. Juvenile social status predicts primary sex allocation in a sex changing fish

   https://doi.org/10.1111/ede.12195  

   Solomon‐Lane, Tessa K. ; Shvidkaya, Polina ; Thomas, Alma ; Williams, Megan M. ; Rhyne, Andrew ; Rogers, Lock ; Grober, Matthew S. ( July 2016 , Evolution & Development)

   Both individual sex and population sex ratio can affect lifetime reproductive success. As a result, multiple mechanisms have evolved to regulate sexual phenotype, including adult sex change in fishes. While adult sex change is typically socially regulated, few studies focus on the non‐chromosomal mechanisms regulating primary sex allocation. We investigated primary sex determination in the bluebanded goby (Lythrypnus dalli), a bidirectionally sex‐changing fish. Of the studies investigating primary sex determination in species with adult sex change, this is the first to incorporate the roles of social status and size, key factors for determining adult sex allocation. ForL. dalli, adult sex is regulated by social status: dominants are male; subordinates are female. In social groups of laboratory‐reared juveniles, we demonstrate that status also predicts primary sex. Dominant juveniles developed male‐typical genitalia, and their gonads contained significantly less ovarian tissue than subordinates, which developed female‐typical genitalia. To better understand natural development, we quantified the distribution of juveniles and adults on the reef and analyzed genital papilla and gonad morphology in a sample of wild‐caught juveniles. Juveniles were observed in various social environments, and most grouped with other juveniles and/or adults. The majority of field‐caught juveniles had female‐typical genitalia and bisexual, female‐biased gonads. These data are consistent with a single mechanism that regulates sexual phenotype throughout life. Social status could first cause and then maintain through adulthood a female‐biased population, allowing individuals to regulate sex based on local conditions, which is important for optimizing lifetime reproductive success.

    

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2. Microplastic in riverine fish is connected to species traits

   https://doi.org/10.1038/s41598-018-29980-9  

   McNeish, R. E. ; Kim, L. H. ; Barrett, H. A. ; Mason, S. A. ; Kelly, J. J. ; Hoellein, T. J. ( August 2018 , Scientific Reports)

   Microplastic is a contaminant of concern worldwide. Rivers are implicated as major pathways of microplastic transport to marine and lake ecosystems, and microplastic ingestion by freshwater biota is a risk associated with microplastic contamination, but there is little research on microplastic ecology within freshwater ecosystems. Microplastic uptake by fish is likely affected by environmental microplastic abundance and aspects of fish ecology, but these relationships have rarely been addressed. We measured the abundance and composition of microplastic in fish and surface waters from 3 major tributaries of Lake Michigan, USA. Microplastic was detected in fish and surface waters from all 3 sites, but there was no correlation between microplastic concentrations in fish and surface waters. Rather, there was a significant effect of functional feeding group on microplastic concentration in fish.Neogobius melanostomus(round goby, a zoobenthivore) had the highest concentration of gut microplastic (19 particles fish⁻¹) compared to 10 other fish taxa measured, and had a positive linear relationship between body size and number of microplastic particles. Surface water microplastic concentrations were lowest in the most northern, forested watershed, and highest in the most southern, agriculturally dominated watershed. Results suggest microplastic pollution is common in river food webs and is connected to species feeding characteristics. Future research should focus on understanding the movement of microplastic from point-source and diffuse sources and into aquatic ecosystems, which will support pollution management efforts on inland waters.

    

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3. Pelagic–littoral resource polymorphism in Hovsgol grayling Thymallus nigrescens from Lake Hovsgol, Mongolia

   https://doi.org/10.1111/eff.12464  

   Olson, Kirk W. ; Krabbenhoft, Trevor J. ; Hrabik, Thomas R. ; Mendsaikhan, Bud ; Jensen, Olaf P. ( January 2019 , Ecology of Freshwater Fish)

   Resource polymorphism is a widespread phenomenon in post‐glacial fishes where multiple morphotypes of a species occur sympatrically and exhibit distinct resource use. Availability of open niches and high levels of within and among species competition are thought to drive differences in morphology and may provide insights into early stages of speciation. Hovsgol grayling (Thymallus nigrescens) are endemic to Lake Hovsgol, a lake colonised by fish following the Pleistocene, and are threatened with habitat loss due to climate change and illegal harvest. Previous analysis of Hovsgol grayling diet inferred through C and N stable isotopes suggested the presence of littoral and pelagic foraging groups. We hypothesised that morphology of the two foraging groups would reflect predictions of functional morphology, indicating the presence of resource polymorphism. To test this hypothesis, we evaluated evidence from C and N stable isotopes, stomach contents, fish ages, capture location and morphology. Two foraging groups of Hovsgol grayling were identified through stomach content and C and N stable isotope analysis. Individuals with greater zooplankton consumption were more frequently captured in the pelagic zone, exhibited higher gill raker counts, larger orbit (eye) sizes, longer paired fins, narrower head width, larger maxilla and smaller size at age than the group with greater reliance on benthic invertebrate prey. These differences were generally consistent with those described in other fish species exhibiting littoral–pelagic resource polymorphism. Our study provides the first example of littoral–pelagic resource polymorphism in the salmonid subfamily Thymallinae and highlights the potential influence of competition on fish evolution.

    

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4. Functional impact and trophic morphology of small, sand‐sifting fishes on coral reefs

   https://doi.org/10.1111/1365-2435.14087  

   Brodnicke, Ole Bjørn ; Hansen, Camilla Elisabeth ; Huie, Jonathan Michael ; Brandl, Simon Johannes ; Worsaae, Katrine ( May 2022 , Functional Ecology)

   Oligotrophic tropical coral reefs are built on efficient internal energy and nutrient cycling, facilitated by tight trophic interactions. In the competition for available prey, some small fishes have evolved to feed on apparently barren sand patches that connect hard‐substratum patches in many reef habitats.

   One strategy for obtaining prey from a particulate matrix is to sift out small prey items from the sediment (often called ‘winnowing’). Yet, the trophic link between small winnowing consumers and their prey are poorly resolved, let alone the morphological specialisations that enable this foraging behaviour.

   We used aquarium‐based feeding experiments to quantify the impact of winnowing by two sand‐dwelling goby species (Valenciennea sexguttataandValenciennea strigata) on meiobenthos abundance and diversity and examined their actual ingestion of meiobenthos using gut content analysis. To identify potential morphological structures involved in winnowing, we investigated the gobies' feeding apparatus with electron microscopy (SEM) and micro‐computed tomography (micro‐CT).

   After 4 days of sifting through the sand matrix, the two species significantly reduced meiobenthic prey abundance by 30.7% ± 9.2SE(V. sexguttata) and 46.1% ± 5.1SE(V. strigata), but had little impact on the meiobenthic diversity. The most abundant prey groups (copepods and annelids) experienced the greatest reduction in number, suggesting selection by size, shape and density of prey items. Furthermore, gut content analysis confirmed that winnowing gobies can efficiently separate meiobenthic prey from heavier inorganic particles (sand), likely facilitated by a specialised epibranchial lobe, pharyngeal jaws and highly abundant papillose taste buds in the oropharyngeal cavity.

   Our results provide important background on the trophic link between the meiobenthos and winnowing gobies on coral reefs. The revealed specialisations of the goby feeding apparatus facilitate sand‐sifting foraging behaviour and access to an otherwise inaccessible trophic niche of microscopic prey. By having evolved a specialised strategy to obtain nutritious and highly abundant prey from seemingly barren sand, we suggest that winnowing gobies act as an important conduit for sand‐derived energy to higher trophic levels.

   Read the freePlain Language Summaryfor this article on the Journal blog.

    

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5. Reef Location and Client Diversity Influence the Skin Microbiome of the Caribbean Cleaner Goby Elacatinus evelynae

   https://doi.org/10.1007/s00248-022-01984-z  

   Pereira, A ; Soares, M.C. ; Santos, T ; Poças, A ; Pérez-Losada, M ; Apprill, A ; Sikkel, P.C ; Xavier, R. ( March 2022 , Microbial ecology)

   Fish-associated microorganisms are known to be affected by the environment and other external factors, such as microbial transfer between interacting partners. One of the most iconic mutualistic interactions on coral reefs is the cleaning interactions between cleaner fishes and their clients, during which direct physical contact occurs. Here, we characterized the skin bacteria of the Caribbean cleaner sharknose goby, Elacatinus evelynae, in four coral reefs of the US Virgin Islands using sequencing of the V4 region of the 16S rRNA gene. We specifically tested the relationship between gobies’ level of interaction with clients and skin microbiota diversity and composition. Our results showed differences in microbial alpha- and beta-diversity in the skin of gobies from different reef habitats and high inter-individual variation in microbiota diversity and structure. Overall, the results showed that fish-to-fish direct contact and specifically, access to a diverse clientele, influences the bacterial diversity and structure of cleaner gobies’ skin. Because of their frequent contact with clients, and therefore, high potential for microbial exchange, cleaner fish may serve as models in future studies aiming to understand the role of social microbial transfer in reef fish communities. 

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