Marking Cave Harvestmen: A Study of Great Basin's Model Cave Harvestman

First published in The Midden*, Great Basin National Park's newsletter, Vol. 22, No. 2, Winter 2022. By Shiloh McCollum, biologist, Great Basin Institute.*

Arachnids — especially shy, cave-dwelling ones — keep their secrets. To learn them, scientists turn to careful fieldwork.

Harvestmen

Harvestmen are solitary omnivores that avoid light, and can sometimes outweigh the spiders around them in total biomass. Cave species tend to vary less with the seasons than surface ones, but little is known about them: some breed continuously and some do not; some guard their eggs and young and some do not; some wander widely and some stay put.

The Model Cave harvestman

Sclerobunus ungulatus, the Model Cave harvestman, is found only in Great Basin caves, and even less is known about it — whether it eats droppings or springtails, how its populations work, how far it moves.

• Its type specimen — the one the species was described from — was collected in Model Cave in 1952.
• It differs from its close relative S. cavicolens in several ways, including lack of pigment, and from S. madhousensis, found only in caves near Provo, Utah, by its longer second leg and limited range.
• It is known from the Baker Creek watershed and two high-elevation caves.

The study

To learn more, researchers began a mark-and-recapture study in Model Cave, following a method used for harvestmen in Brazil:

1. Four transects, each 3 metres long, were laid out in the cave.
2. Every month, the same two researchers search — to limit variation — each taking a 15-minute pass over a transect, collecting every harvestman and noting other animals nearby that might be prey.
3. Each harvestman is marked with fluorescent acrylic paint in a unique pattern: a different colour for each capture date (green in September, pink in October) and a different leg for each transect.
4. Each is photographed against a microscale, to record its size and age class and its identifying marks, then returned to the transect it came from.

After a year, the data will be analysed to estimate population size, size classes, life span and reproduction, along with anything else the observations reveal — a start on unravelling the species' secrets.

Sources

• National Park Service, Great Basin National Park, "Mark and Recapture Study for the Model Cave Harvestman, Sclerobunus ungulatus," The Midden, Vol. 22, No. 2, Winter 2022. https://www.nps.gov/articles/000/mark-and-recapture-study-for-the-model-cave-harvestman-sclerobunus-ungulatus.htm
• The article cites Mestre and Pinto-da-Rocha (2004) on a Brazilian harvestman population; Derkarabetian and Hedin (2014) on the genus Sclerobunus; Krejca and Taylor (2003) on eight Great Basin caves; and Taylor, Krejca and Slay (2008) on the park's cave life, published by the Illinois Natural History Survey (given in the article as "Illinois National Historical Survey").
• The article's photographs, by Jean Krejca and the author, are not reproduced.
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.