Size and eyes

Yesterday we collected 17 live opilioacarids from under stones near the Chamela station greenhouses.  Their light purple colour makes them stand out relatively well against the reddish soil encrusted on the bottoms of the rocks. But they are pretty small and getting one off a rock can involve quite a chase with a paintbrush as they dash in and out of crevices. In case you want to know how big they are, here is a photo of one in a vial with my left index fingerprint for scale. The closeup shows how the camera’s flash is reflected by a tapetum in the eye of the mite – a feature unique to opilioacarids among both parasitiform and acariform mites. In spiders, presence of a tapetum seems be an adaptation for night vision (but not all nocturnal spiders with good vision have a tapetum according to this  Australian Museum post). Does the tapetum in opilioacarids mean that they come out from under rocks at night? Or that they use these fancy eyes to better navigate their dark sublithic world?

opilioacarid with fingers for scale and tapetum compilation16 Feb 2014

It walks!

Guess where you can get plenty of live opilioacarids? In the Viveros area of the Chamela field station, just next to the greenhouses, under rocks. It is with pride and some surprise that I present the very first Youtube video of a Neocarus* walking across a piece of herbarium paper.

*thanks to Bruce Smith for correcting my previous typo – Neoacarus is a water mite genus, Neocarus is the opilioacarid.

Daddy-longlegs-like mites

Anystis offered a challenging etymological quest (see previous post) but sometimes a spade is just called a spade. I came to the UNAM Chamela field station in Jalisco, Mexico, to hunt for a mite whose lifestyle is mysterious but whose name is straightforward. Mites of the suborder Opilioacarida were once thought to be the most ancient lineage of Acari because they bear remnants of abdominal segmentation and have a strange combination of morphological features of the two main groups mites, the Parasitiformes and the Acariformes, suggesting that opilioacarids may have arisen prior to the split between these two lineages.  However, molecular data don’t support this story, as they place Opilioacarida firmly in the Parasitiformes, and the Acariformes way the heck far off in another part of the arachnid phylogeny (i.e., mites appear to be diphyletic).

Nevertheless, opilioacarids remain very odd parasitiforms, with their big eyes, soft bodies, and gut contents that include particles (most other parasitiforms are blind, crunchy, fluid-feeders).  Their semi-segmented opisthosomas and long gangly legs make them look like a cross between a daddy-longlegs (Opiliones) and mites, hence their name. We don’t know how they catch their food or how they have sex.  My mission in coming to Chamela is to collect live opilioacarids and watch them eat and (fingers crossed) transfer sperm.  But after a week here, I hadn’t even managed to extract a single specimen from the soil, litter, wood and bark that I’ve been putting through my Tullgren funnels.  Until today – I extracted three lovely Neocarus from an unprepossessing habitat consisting of dry soil, scattered small rocks, and crispy leaf litter.  Unfortunately, these three are dead, having fallen into the ethanol at the bottom of the funnels, and hence will not be useful subjects for behavioural observations.  But at least I now know they’re here.

first opilioacarid and habitat nr Jose Maria Morelos 13 Feb 2014

First opilioacarid and habitat nr Jose Maria Morelos