Feeding strategies and competition between terrestrial Haemadipsa leeches (Euhirudinea: Arhynchobdellida) in Danum Valley rainforest (Borneo, Sabah)

Larger leeches attack from higher ground – size-dependent preferences for ambush sites in the Bornean terrestrial leech Haemadipsa picta

The rain forests of South-East Asia represent a biodiversity hotspot of terrestrial leeches, but we have only fragmentary and often anecdotal information on this component of tropical communities. To address the foraging tactics of terrestrial leeches, we studied the vertical distribution of Haemadipsa picta on foliage in a Bornean tropical rain forest. We investigated the links between leech body length and the above-ground height of their ambush positions under natural conditions and in a choice experiment performed under semi-natural conditions. We studied 167 leeches, which varied considerably with respect to body length (4–29 mm). On average, the leeches ambushed at lower heights under natural conditions than under experimental conditions (47.7 cm vs. 67.5 cm), though the heights of ambush positions overlapped considerably. Leeches that chose higher ambush positions under natural conditions consistently chose higher ambush positions in the experiment (Pearson r = 0.29). Under both natural and experimental conditions, leech body length was positively correlated with the height of ambush positions (Pearson r = 0.48); on average, a 1-mm increase in body length corresponded to a 2–3 cm higher ambush position. Our findings suggest that H. picta individuals actively choose hunting locations according to their above-ground height, shifting their ambush sites over ontogeny to higher foliage. We hypothesize that growing leeches might target different prey following ontogenetic shifts in the foraging optima, ultimately decreasing intraspecific competition.

Using metabarcoding to compare the suitability of two blood-feeding leech species for sampling mammalian diversity in North Borneo

The application of high-throughput sequencing (HTS) for metabarcoding of mixed samples offers new opportunities in conservation biology. Recently, the successful detection of prey DNA from the guts of leeches has raised the possibility that these, and other blood-feeding invertebrates, might serve as useful samplers of mammals. Yet little is known about whether sympatric leech species differ in their feeding preferences, and whether this has a bearing on their relative suitability for monitoring local mammalian diversity. To address these questions, we collected spatially matched samples of two congeneric leech species Haemadipsa picta and Haemadipsa sumatrana from lowland rainforest in Borneo. For each species, we pooled ~500 leeches into batches of 10 individuals, performed PCR to target a section of the mammalian 16S rRNA locus and undertook sequencing of amplicon libraries using an Illumina MiSeq. In total, we identified sequences from 14 mammalian genera, spanning nine families and five orders. We found greater numbers of detections, and higher diversity of OTUs, in H. picta compared with H. sumatrana, with rodents only present in the former leech species. However, comparison of samples from across the landscape revealed no significant difference in mammal community composition between the leech species. We therefore suggest that H. picta is the more suitable iDNA sampler in this degraded Bornean forest. We conclude that the choice of invertebrate sampler can influence the detectability of different mammal groups and that this should be accounted for when designing iDNA studies.