Report of a mass aggregation of isopods in an Ozark cave of Oklahoma with considerations of population sizes of stygobionts

Danté Fenolio, G Graening
On 28 January 2002 S. Hensley (United States Fish and Wildlife Service) and DBF entered an Adair County, Oklahoma, cave (code AD-8) to perform a bioinventory. The cave seasonally accommodates a maternity colony of Myotis grisescens Howell 1909 and has substantial guano deposits within the main passage. The cave also has a shallow stream running through it which feeds several large pools before exiting the system. The stream meanders directly between guano piles and is in the flight path of bats
more » ... exiting/entering the cave. The first large pool within the mouth of the cave (roughly 10 m in from the dripline) is just beyond the twilight zone. At the time of the survey, this was a shallow, roughly circular pool, slightly larger than 3 m in diameter. The pool depth was 10 cm or less at its deepest point. The bottom of the pool was covered with fine sediment, a significant portion of which was decaying guano. To the very shoreline of the pool, the bottom was covered by thousands of stygobiotic isopods, Caecidotea macropropoda Chase and Blair 1937 which formed a fairly uniform mat, in some places stacked at least 3 individuals deep (Figure 1). In 1981, Lewis 1 had described this aggregation in cave AD-8 as, "a large population of isopods in a guano bog." To estimate the number of isopods in the pool, we calculated the surface area of the pool and multiplied by a number of isopods per square cm (derived from visual examination of images taken during the event). We used a radius of 150 cm for the circular pool, a density of 1.5 isopods per cm 2 , and roughly 50% of the pool having isopods stacked 2 or 3 individuals deep; we estimate between 100,000 to 150,000 isopods were in the pool. The purpose of the aggregation was not inherently obvious in that individuals with eggs were not observed. Further, guano deposits covered the bottom of the aquatic system throughout the waterway and the length of the cave, including the bottoms of large pools farther into the system. While many individual C. macropropoda could be found throughout the course of the waterway, the aggregation was only observed in this last large pool, before the cave's stream exited the system. No fewer than 10 larvae of the grotto salamander, Eurycea spelaea (Stejneger, 1892), were observed in the pool where the isopods were aggregated. The larval salamanders ranged from 4 to 8 cm in total length. All salamander