Ian Potter

This chapter examines how zoogeography and estuarine typology can influence fish assemblages. There is a focus on the global classification of estuary‐associated fish species that places them into functional groups and guilds according to the ways in which they utilise these systems, especially in terms of their life cycle, feeding and reproductive strategies. Such an approach facilitates ecological comparisons of estuarine fish assemblages on a regional, continental and global scale, and is particularly useful in documenting estuarine degradation or rehabilitation using functional fish guilds as indicators of responses to anthropogenic impacts.

The characteristics of the biota and fisheries of estuaries are considered in the context of a contemporary definition that recognizes the physico-chemical features of the different estuary types found worldwide, i.e. macrotidal v. microtidal and permanently open v. seasonally open v. normally closed. The ways in which fishes use estuaries as a habitat and for feeding and reproduction are categorized and exemplified by fishery-important species from their constituent guilds. Marine species, and particularly their juveniles, dominate the fish faunas of permanently open estuaries. The prevalence and abundance of species either confined to estuaries or represented by estuarine populations that are discrete from their marine counterparts are far greater in systems that are periodically closed to the sea by sandbars at their mouths. Anthropogenic activities are continuing to have marked effects on estuarine environments and, in extreme circumstances, have had highly deleterious impacts on the fish faunas and fisheries of these systems.

Tidal range is a master factor governing the differences in physico-chemical and biological characteristics between microtidal (<2 m) and macrotidal (>2 m) estuaries, which, for convenience, thus include mesotidal estuaries (2–4 m). Microtidal estuaries differ from macrotidal estuaries in geomorphology, tidal water movements, salinity regimes, residence times, turbidity, sedimentology and intertidal area. Consequently, their phytoplankton, microphytobenthos and macrophytes communities differ in biomass and production, areal extent, distribution patterns and composition. Mesozooplankton comprise predominantly autochthonous species in microtidal estuaries and allochthonous species in macrotidal estuaries. Meiobenthos in microtidal estuaries have greater densities in subtidal than intertidal areas and species persist along the estuary. Macrobenthos is dominated by small deposit-feeding species in microtidal estuaries, whereas macrotidal estuaries contain some larger species and suspension feeders. Species richness and abundance of estuarine-resident fish species and the contributions of diving piscivorous birds and wading invertebrate-feeding birds are greater in microtidal estuaries. As paradigms regarding estuarine ecology have been based mainly on northern hemisphere macrotidal systems, this review has redressed this imbalance by detailing the extent of differences between microtidal and macrotidal estuaries. In particular, it uses data and case studies for southern hemisphere microtidal systems to demonstrate that the physico-chemical characteristics and ecology of the main flora and fauna of microtidal estuaries are frequently not consistent with those paradigms.

The lampreys (Petromyzontiformes), one of the two surviving groups of agnathan (jawless) vertebrates, currently consist of 41 recognized species. This group has an antitropical distribution, with the 37 species of Northern Hemisphere lampreys assigned to the Petromyzontidae, whereas the four species of Southern Hemisphere lampreys are separated into either the Geotriidae (one species) or Mordaciidae (three species). All lamprey species have a blind and microphagous, burrowing larva (ammocoete), which spends a number of years in the soft sediment of creeks and rivers, after which it undergoes a radical metamorphosis. Eighteen lamprey species then embark on an adult parasitic phase (nine at sea and nine in fresh water) during which they increase markedly in size, whereas the other 23 species do not feed as adults and remain in fresh water. On the basis of morphology, 17 of the 23 non-parasitic species each evolved from a particular parasitic species whose descendants are still represented in the contemporary fauna. The remaining six non-parasitic species, the so-called “southern relict” species, have no obvious potential ancestral parasitic species, implying they have diverged markedly from their parasitic ancestor or that the parasitic ancestor is now extinct. Many of the main taxonomic characteristics reside in features that are associated with parasitic feeding, for example, the type and arrangement of the teeth on the suctorial disc and tongue-like piston. The phylogenetic relationships, derived by maximum parsimony analyses of morphological and anatomical data for the 18 parasitic species, were similar in most respects to those obtained by subjecting molecular data (cytochrome b mitochondrial DNA sequence data) for those species to Bayesian analyses. However, in contrast to the results of morphological analyses, the genera Eudontomyzon and Lampetra were not monophyletic when using molecular analyses. When non-parasitic species were included in the molecular analyses, some of the six relict non-parasitic species formed clades with parasitic species which, from their morphology, had been allocated by taxonomists to different genera. More genes, and particularly nuclear genes, should be used to help resolve the basis for these differences between the morphological and molecular phylogenies.