Global-scale distribution of marine fish: An analysis of taxonomic richness, phylogenetic diversity, and functional traits in latitudinal and depth gradients

Abstract

The latitudinal diversity gradient (LDG) has long interested biologists because it provides clues about where species have evolved and survived on evolutionary time scales. Depth is another factor affecting the distribution of marine species. Temperature is thought a critical factor to influence the latitudinal and depth gradients. Thus, the species-energy hypothesis, temperature-size rule (TSR), gill-oxygen limitation theory (GOLT), and temperature constraint hypothesis (TCH) have been proposed to explain the gradients of species richness, body size, and trophic level of marine species, respectively. This thesis used AquaMap’s modelled distributed range data of worldwide marine fish, which minimised the bias of sampling effort, to describe the gradients of richness on five taxonomic levels, higher taxonomic richness, phylogenetic relationship, body size and trophic level in the 100 m depth band, and in the 5-degree latitude band of four depth zones. Also, this thesis tested whether these results support the aforementioned temperature-related hypotheses. The results showed that LDGs of all taxonomic levels were not generally unimodal in all depth zones, and their peaks of richness moved poleward with higher taxonomic levels. However, the tropics across the equator still had 40% more species than high latitudes. Also, taxonomic richness from class to species was highest in warmer shallow depths, then decreased with depth. These results support the species-energy hypothesis. Species and higher taxonomic richness were positively correlated, and they were higher in the subtropics with a dip at the equator. Species assemblages had closer phylogenetic relationships in the warmer (low latitudes, shallow depths) than the colder environments (high latitudes, deep sea). In addition, below 200 m depth, both poles had distinct fish assemblages other than the rest of the world. Also, although the Arctic and the Antarctic are polar environments, their fish assemblages and endemism were different because of at least 23 different physical and biological conditions. Mean body size and trophic level of marine fish were smaller and lower in the warmer latitudes and larger and higher in the high latitudes except for the Southern Ocean. These results support the hypotheses of TSR, GOLT, and TCH. Fish species’ mean maximum body size declined with depth because of decreased dissolved oxygen. Fish species with trophic levels ≤ 2.80 were dominant in the warmer environment but absent in the colder environment. These results support the TCH hypotheses. Thus, because temperature seems the primary factor affecting the distribution and biological traits of marine fish, fish species and assemblages will change rapidly in response to climate change.