Parallel and non-parallel divergence within polymorphic populations of brook stickleback, Culaea inconstans (Actinopterygii: Gasterosteidae)

Studying parallel evolution allows us to draw conclusions about the repeatability of adaptive evolution. Whereas populations likely experience similar selective pressures in similar environments, it is not clear if this will always result in parallel divergence of ecologically relevant traits. Our study investigates the extent of parallelism associated with the evolution of pelvic spine reduction in brook stickleback populations. We find that populations with parallel divergence in pelvic spine morphology do not exhibit parallel divergence in head and body morphology but do exhibit parallel divergence in diet. In addition, we compare these patterns associated with pelvic reduction in brook stickleback to well-studied patterns of divergence between spined and unspined threespine stickleback. Whereas spine reduction is associated with littoral habitats and a benthic diet in threespine stickleback, spine reduction in brook stickleback is associated with a planktonic diet. Hence, we find that pelvic spine divergence is associated with largely non-parallel ecological consequences across species.

Predation in littoral habitats is a complex process: Comment on Whitfield (2020)

Shallow littoral waters in estuaries provide important nurseries for a diversity of fish species. The recent review of Whitfield (2020; Mar Ecol Prog Ser 649:219-234) reinforced the long-held paradigm that the value of shallow nurseries is due, in part, to these habitats containing few predators and having low predation pressure. We argue that the provision of refuge does not necessarily mean that a habitat must have few predators and low predation pressure, and present evidence that shallow nurseries contain abundant predator assemblages, may have high predation pressure, and be the habitats where much of the high mortality suffered by juvenile cohorts occurs. Shallow littoral waters are clearly important nurseries, and we highlight the importance of predation in nursery ground functioning.

Terminalia catappa (Singapore almond).

T. catappa is a perennial tree species that has been extensively introduced into littoral habitats, coastal forests, gardens and parks to be used as an ornamental, shade tree, and sand-dune stabilizer (Orwa et al., 2009; ISSG, 2017). This species has become of the most common trees in littoral habitats and beaches across tropical and subtropical regions of America, India, southeastern Asia, and the Pacific Ocean, due in part to human-mediated introductions, the adaptation of its fruits to be dispersed over long-distances by sea currents and its tolerance to salt-spray, coastal-winds and drought conditions (Thomson and Evans, 2006; Brown and Cooprider, 2013). T. catappa is a prolific seed producer and fruits may remain viable for a long time, even after floating in salt water for considerable time periods. This species naturalizes readily in littoral habitats and has been listed as invasive in the United States (Florida and Hawaii), Brazil, the Bahamas, Cuba, Dominican Republic, Trinidad and Tobago, Puerto Rico and Virgin Islands where it is displacing native vegetation and altering coastal dynamics (Smith, 2010; Oviedo Prieto et al., 2012; Mir, 2012; Rojas-Sandoval and Acevedo-Rodríguez, 2015; FLEPPC, 2017; I3N-Brazil, 2017; ISSG, 2017).

Littoral habitats as major nursery areas for fish species in estuaries: a reinforcement of the reduced predation paradigm

For many decades, the role of estuaries as important nursery areas for fishes was accepted as fact by scientists and environmental managers. At the turn of the 21st century, a question mark was raised in relation to the reduced predation component of the nursery function, with some scientists contending that both large and small piscivorous fish species had access to the estuarine habitats that juvenile fishes in estuaries occupied. If true on a global scale, the nursery designation for these habitats would be compromised and the long-held paradigm that estuaries are important nursery areas for fishes would need to be revised. In this review, I examine the nature of fish nursery areas in estuarine littoral habitats from a mainly predation perspective and, based on a variety of ichthyofaunal and avifaunal studies, come to the conclusion that apart from a few selected estuarine systems, there is limited predation on juvenile fishes in these particular areas. This, coupled with the abundant suitable food resources for juvenile fish from different trophic categories, shelter from high-energy marine wave action and biological connectivity between a variety of submerged and emergent macrophyte communities, renders shallow estuarine littoral areas ideal nursery areas for the juveniles of mostly euryhaline marine fish species, the dominant component of estuarine ichthyofaunas globally. In addition, there are strong indications from the fossil record that these littoral estuarine nursery areas have been functioning since the Devonian, more than 350 million years ago.

Differentiation in Aquatic Metabolism between Littoral Habitats with Floating-Leaved and Submerged Macrophyte Growth Forms in a Shallow Eutrophic Lake

The metabolic balance between gross primary production (GPP) and ecosystem respiration (R) is known to display large spatial and temporal variations within shallow lakes. Thus, although estimation of aquatic metabolism using free-water measurements of dissolved oxygen concentration has become increasingly common, the explanation of the variance in the metabolic regime remains an extremely difficult task. In this study, rates of GPP, respiration (R) and the metabolic balance (net ecosystem production, NEP) were estimated in four littoral habitats with different macrophyte growth forms (floating-leaved vs submerged) over a 28-month period in lake of Kastoria (Greece), a shallow eutrophic lake. Our results showed that net heterotrophy prevailed over the studied period, suggesting that allochthonous organics fuel respiration processes in the littoral. Temporal variation in the metabolic rates was driven mainly by the seasonal variation in irradiance and water temperature, with the peak of metabolic activity occurring in summer and early autumn. Most importantly, significant spatial variation among the four habitats was observed and associated with the different macrophyte growth forms that occurred in the sites. The results highlight the importance of habitat specific characteristics for the assessment of metabolic balance and underline the potentially high contribution of littoral habitats to the whole lake metabolism.

Native bluegill influence the foraging and aggressive behavior of invasive mosquitofish

Two fish species that are common invaders of aquatic ecosystems world-wide are Gambusia affinis and G. holbrooki, commonly known as mosquitofish. In North America, introduced G. affinis are thought to have contributed to the population decline of several native fish species. Sunfish (family Centrarchidae) naturally occur across much of North American, thus mosquitofish and sunfish are likely to come into contact and interact more frequently as mosquitofish spread. However, the nature of this interaction is not well known. We used a lab experiment to explore whether and how the aggressive and foraging behaviors of G. affinis might be influenced by a representative and ubiquitous native centrarchid (Lepomis macrochirus; bluegill sunfish), a species with juveniles that inhabit littoral habitats also preferred by mosquitofish. The experiment partnered an individual male or female mosquitofish (focal fish) with a juvenile bluegill, or a same- or opposite-sex conspecific, filmed these one-to-one interactions, and quantified foraging and aggressive actions for the focal mosquitofish. We found that juvenile bluegill affect foraging in male mosquitofish, resulting in lower percent of handling attempts and handling time in which the male consumed a food item. The presence of juvenile bluegill also led to a reduction in the number of aggressive acts by mosquitofish compared to aggression levels when focal mosquitofish were with conspecifics. In nature, when mosquitofish encounter juvenile bluegill in littoral habitats, our results suggest that the foraging and aggressive behaviors of mosquitofish will be modified, especially for males. This mechanism may influence the rate or geographic extent of the spread of mosquitofish into North American waterbodies.