In a river's estuary, there are special conditions for fish species. Pelagic and benthic (or almost, because this paper wisely separates the benthic types) groups can be termed "guilds" because of their habitat, and contain most species of fish, while migratory species such as lampreys and Atlantic salmon also pass through the estuary. These guilds are investigated today in an interesting paper from the Proceedings of the Royal Society B: Biological Sciences.

They are basically groups of niches which each species exploits, within the available habitat, but in different ways. Anne E. Magurran and Peter A. Henderson argue that abundance doesn't account for the community's structure on its own.

Biomass explains the allocation of each of the habitat's resources between the members. Common and rare species inhabit each guild, with their abundance shaped by natural selection. Darwin would have been so proud!

One of these guilds would be the sprat and herring group, in open water; another would be the soft benthic group on the bottom. This famously flavourful flounder (sorry to mention actually eating him) with its anatomical mods and great camouflage, joins the Dover sole in that guild:

The "soft benthic" flounder, Platichthys flexus Flounder image; Credit: © Shutterstock

With dynamic change important to a system, this study compared 30 years of samples in Bridgwater Bay in the UK. The dates were 1980 till 2010. The community structure was conserved over this period as species and their abundance varied. Core groups for the guilds were present over the whole study.

So a few species were dominant in exploiting most resources, while transients came in if conditions created environmental change. The authors quote the North Atlantic Oscillation, river flow and colder winters as prime factors. They work at St. Andrews University in Fife and PISCES Conservation Ltd in England.

Predation is always a selection pressure on animal body size. Habitats provide another constraint while pathogens (like predators), mating systems and trophic (feeding) levels also act as selective agents.

The conger, Conger conger (easy one huh?) is an simple animal to place. He has a niche literally in the rocks, but his benthic niche is classed as "hard benthic", with soft sediment nearby. Another member of his guild would be the rockling, Ciliata mustela - Conger image; Credit: © Shutterstock

Other researchers have used guilds among specialist herbivores, with single species such as oak or even their leaves alone. Here, multiple functional groups comprise the estuarine communities, complicating the system. Available capacity in the system is allocated among the species leaving biomass variable according to individuals.

Species Abundance Distribution (abbreviated as SAD) is a measure of how abundance is inequal among community members. So biomass in each animal is linked to their use of resources and how big they are as individuals.

That means the biggies produce more biomass per capita, but if a fish relies on safety in numbers, in open water, then the biomass is more efficiently spread among many individuals. So the titchy fish are selected as the more successful! We loved reading your paper, Anne. I hope the report lives up to your achievement!