Decapod crustacean diversity along Australia's western continental margin

Abstract

A challenge for biodiversity conservation on continental margins is the lack of information on species distributions. Australia has an expansive continental margin that is largely unexplored. To protect and manage biodiversity in Australia’s deep marine environments, biological and abiotic surrogates have been used to classify biodiversity. The aim of this thesis is to describe patterns of decapod diversity on an extensive continental margin and investigate the ability of physical and biological surrogates to represent underlying diversity patterns. The description of biological patterns at large spatial scales requires reliable taxonomic identifications, and consequently a substantial part of the thesis is taxonomic in nature.

Surveys of Australia’s western continental margin (~100 to 1000 m depths) were undertaken in 2005 and 2007 by CSIRO Marine and Atmospheric Research in conjunction with museum taxonomists. Species identified from the north-west margin (survey SS05/2007) are reported here and the results of both surveys are summarised. In total, 890 provisional species of decapod crustaceans were discovered during the two surveys, of which 30% are new to science. Many of the species collected and identified (327 or 37%) are known to occur elsewhere, from the tropical Indian Ocean to the West Pacific, and 142 species were recorded in Australia for the first time.

A small component of the new fauna discovered during the surveys is described here. These include two new species of the hippolytid shrimp genus Lebbeus and six new species of squat lobster of the family Chirostylidae. Although 45 species of Lebbeus exist worldwide, only one had previously been described from Australia. Squat lobsters including those of the family Chirostylidae are rapidly advancing our understanding of deep-sea environments across broad spatial scales and therefore the taxonomy and distribution of these animals is a research priority. In addition to the six new species described, seven new records of Indo-West Pacific species are reported for Australia. This study increases the number of chirostylid species in Australia from 40 to 53. Keys to Australian species of the genera Gastroptychus, Uroptychodes and Uroptychus are provided.

The distributional records of decapods along the margin were used to determine the relative importance of environmental and spatial predictor variables on both species richness (alpha diversity) and species turnover. The best predictors of species turnover were temperature, oxygen and salinity, factors that reflect the oceanographic features that dominate distinct depth bathomes along the slope. On both the shelf and the upper slope, I differentiated an assemblage north of 22°S from another south of 23°S in the vicinity of North-west Cape. This location correlates with changes in oxygen concentration along the margin and marks the head of the Leeuwin Current system. The number of species within samples was highly variable, but a small significant increase in diversity towards the tropics was evident. On the shelf edge (~100 m) temperature was correlated with latitude, oxygen and salinity, and thus the independent effects of each variable could not be separated. On the shallow upper slope (~400 m) temperature was disassociated from latitude, and latitude proved to be the best predictor of sample species richness. The predictive power of latitude over other variables indicates that proximity to the highly diverse Indo-West Pacific is important.

Management of both terrestrial and marine environments often uses vertebrates as a surrogate for the diversity of the overall fauna, as their distributions are better documented than those of most invertebrates. In the case of Australia’s deep-water marine planning, the distributions of fishes were used to classify bioregions. To ensure this classification represents underlying biodiversity, the spatial patterns of fishes with three invertebrate taxa were compared along a latitudinal gradient. Changes in community structure along the margin were broadly congruent for fishes and all invertebrate taxa. In contrast, broad-scale species richness patterns differed between major taxa, leading to the conclusion that one taxonomic group cannot be taken to represent others in terms of species richness or taxonomic distinctness.

The results of this thesis lend support to Australia’s marine planning framework in which bioregions are defined according to the distributions of fishes and major oceanographic features. As in other large-scale studies which seek to examine the drivers of diversity in the deep sea, attributing causal relationships is difficult as many covariates are correlated. However, it was possible to distinguish some variables which have greater explanatory power than others. Future research will invariably consider the distributions of fauna over much larger scales as biological and environmental datasets are assembled at global scales, and this may help to explain the physical and historical constraints of deep-sea distributions.