Global Habitat Suitability and Ecological Niche Separation in the Phylum Placozoa

Abstract

The enigmatic placozoans, which hold a key position in the metazoan Tree of Life, have attracted substantial attention in many areas of biological and biomedical research. While placozoans have become an emerging model system, their ecology and particularly biogeography remain widely unknown. In this study, we use modelling approaches to explore habitat preferences, and distribution pattern of the placozoans phylum. We provide hypotheses for discrete ecological niche separation between genetic placozoan lineages, which may also help to understand biogeography patterns in other small marine invertebrates. We, here, used maximum entropy modelling to predict placozoan distribution using 20 environmental grids of 9.2 km2 resolution. In addition, we used recently developed metrics of niche overlap to compare habitat suitability models of three genetic clades. The predicted distributions range from 55°N to 44°S and are restricted to regions of intermediate to warm sea surface temperatures. High concentrations of salinity and low nutrient concentrations appear as secondary factors. Tests of niche equivalency reveal the largest differences between placozoan clades I and III. Interestingly, the genetically well-separated clades I and V appear to be ecologically very similar. Our habitat suitability models predict a wider latitudinal distribution for placozoans, than currently described, especially in the northern hemisphere. With respect to biogeography modelling, placozoans show patterns somewhere between higher metazoan taxa and marine microorganisms, with the first group usually showing complex biogeographies and the second usually showing "no biogeography."

Fig 1. The phylogram of placozoan haplotypes (species) based on 16S sequences and Bayesian inference (Modified after Eitel et al [7]).

The three clades highlighted in red have been investigated this study.

Fig 2. Global distribution of placozoans according to Eitel et al. [7] and unpublished data (see the text).

Three red, green, and blue colors represent three investigated clades, clade I, III and V, respectively. Note that the number of localities on the map does not add to 79, because of points overlying in many localities.

Fig 3. Global maps showing predicted habitat suitability for placozoans based on a 10^th percentile training presence threshold (see text).

Three red, green, and blue colors represent the three placozoan clades, clade I, III and V, respectively. Records with black colors belong to other clades. Yellow represents regions where at least two clades overlap.

Fig 4. Occurrence probabilities for three placozoan clades in relation to four important abiotic factors, mean temperature, temperature range, nitrate, and salinity.

Raw data of the occurrence probability can be found as supporting information (S1 Data).