Review Article |
Corresponding author: Thomas R. Sawicki ( thomas.sawicki@famu.edu ) Academic editor: Oana Teodora Moldovan
© 2016 Julian J. Lewis, Thomas R. Sawicki.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Lewis JJ, Sawicki TR (2016) Mexistenasellus floridensis sp. n., the first stenasellid isopod discovered from the Floridan aquifer (Crustacea, Isopoda, Asellota). Subterranean Biology 17: 121-132. https://doi.org/10.3897/subtbiol.17.7703
|
Mexistenasellus floridensis sp. n. is described from Hole in Wall Cave, Jackson County, Florida. The discovery of this unique isopod nearly doubles the range of the Family Stenasellidae in North America, which was previously known from Oaxaca, Mexico to southern Texas, USA. This is the first stenasellid reported swimming in the water column of a cave, a curious finding for an isopod that belongs to a group that is generally more adapted to inhabiting interstitial spaces. Mexistenasellus floridensis occurs syntopically with the subterranean asellid Caecidotea putea. Both species have been observed swimming in the water column, although neither has apparent anatomical adaptations for swimming as compared to the natatory pereopods of Remasellus parvus, an asellid that occurs in caves in Alachua, Madison and Wakulla counties, Florida.
Isopoda , Stenasellidae , Mexistenasellus floridensis , Remasellus , Caecidotea , Florida, cave
The northern edge of the range of stenasellid isopods in North America was previously the Balcones Fault Zone in southcentral Texas, with the eight known species distributed from approximately 30° north latitude in Texas southward along eastern Mexico to Oaxaca at about 17° (
The history of the discovery of stenasellids in the Nearctic is relatively brief. The first species described from the New World was Mexistenasellus coahuila Cole & Minckley (1972) discovered along with other crustaceans in the remarkable Cuatro Ciénegas basin of northern Mexico (
USA: FLORIDA: Jackson County, Hole in Wall Cave, approximately 7 km east of Marianna (N30.78334 W85.15671), male holotype, 3 male and 2 female paratypes, collected 19 October 2013, Thomas R. Sawicki and Michael Stine; same locality, 3 male and 1 female paratypes, 2–3 January 2009, Thomas R. Sawicki. The specimens are deposited in the collection of the US National Museum of Natural History, Smithsonian Institution, Washington, D.C.
Eyeless, unpigmented, longest male approximately 9.0 mm, female 9.4 mm. Body slender, linear, about 4.3× as long as wide. Head about 1.4× as wide as long, rostrum and lateral incisions absent. Coxae not visible in dorsal view. Body becoming more dorsally spinose on pereonites 6–7 and pleon. Pleotelson about 1.65× as long as wide, caudomedial lobe moderately produced, broadly rounded.
Antenna 1 of 20 articles, distal 6 articles each with one esthete, then alternate articles with one esthete (8 esthetes total). Antenna 2 broken or detached in most specimens, flagellum 57–58 merous, esthetes absent. Mandibles with 4-cuspate incisors and lacinia; spine row with 5 spines on left, 4 spines on right, both with simple stout spines adjacent to incisors that resemble the cusps of the incisors and spines distad from incisors with complex plumosity; left molar with row of 15 plumose setae, right molar with row of 12 plumose setae. Mandibular palp 3-merous, with plumose setae on distal articles. Maxilla 1, inner lobe with 4 apical stout plumose spines; outer lobe with 12 dentate apical spines. Maxilliped without retinaculae.
Pereopods with sexual dimorphism not apparent. Pereopod 1, dactyl with elongate spine resembling accessory claw, 0.8× length of claw; propodus about 2.1× as long as wide, palmar margin with 4–6 robust plumose spines; carpus with 3–4 spines continuing from propodus. Pereopods 2–7 increasing in length, pereopod 7 longest; dactyls with prominent elongate claw-like spine parallel to claw.
Male pleopod 1, protopod with retinaculae absent; exopod oval with 5 elongate disto-lateral setae. Female pleopod 2 triangular, about 2.1× as long as wide, with 3 setae inside mesial margin, 1 distal seta and 1 lateral seta at mid-point. Male pleopod 2, protopod elongate, about 1.7× as long as wide; exopod, proximal article sub-equal in length to distal article, setae absent, distal article broadly rounded apically, with 5 lateral setae; endopod, setation absent, distal article about 2× length of proximal article, bent at approximately 60 degree angle to proximal article, tip bi-lobed, separated by endopodial groove terminating in sub-conical stylet (cannula). Pleopod 3, exopod with transverse suture, distal area slightly longer than proximal, with submarginal spines in row along mesial margin, spines and setae along distal and lateral margins; endopod about 0.6× length of exopod, bifurcated distally. Pleopod 4, exopod with oblique suture, about 2.5× as long as wide, area distal to suture about 0.67× length of proximal area, with about 32 marginal setae; endopod about 0.6× length of exopod, bifurcated distally. Pleopod 5, exopod with oblique suture, setae absent, about 2.5× as long as wide, area distal to suture about 0.3× length of proximal area; endopod sub-equal in length to exopod, bifurcated distally.
Uropods about 2.4mm in length, equal to pleotelson; rami slender, linear, endopod 5× length of protopod, 1.25× length of exopod.
Named for the state of Florida, in recognition of the first stenasellid discovered in the United States east of the Mississippi. Suggested vernacular name is the Florida cave isopod.
Nearctic Mexistenasellus species can be tentatively identified by their ranges (Fig.
When
The hypothesis of Magniez entails colonization of groundwater by the stenasellid ancestors prior to the separation of Africa and South America during the early Cretaceous (140–150 million years before present). This idea requires the presence of stenasellids in South America that remain to be discovered, and subsequent dispersal to North America during the Cenozoic. Magniez acknowledged the flaw in the hypothesis presented by the fact that the North and South American continents were separated during the majority of the Cenozoic. This problem is avoided by pushing back the invasion of freshwater habitats by stenasellids even further, to a time prior to the rifting of Pangea during the Jurassic (about 175 million years b.p.).
The molecular genetic analysis of stenasellids by
The Hole in Wall Cave is a water-filled cave system that is a popular dive site located in Merritt’s Mill Pond, east of Mariana, Florida. The cave was mapped by
This observation of the isopods swimming is curious since the pereopods of Mexistenasellus floridensis do not exhibit the dense rows of setae present on the legs of Remasellus parvus that appear to be natatory adaptations. Regardless of how ill-prepared the isopods were for swimming that was precisely what they appeared to be doing each time specimens were collected. The asellid Caecidotea putea
On 17 July 2015 and 31 July 2015, dives were conducted in Hole in the Wall Cave to conduct careful behavioral observations of the isopods as well as to collect physicochemical data from the cave and surface pond. During these dives isopods were observed crawling on the floor, walls, and ceiling of the cave, as well as swimming in the water column. The swimming behavior observed in these animals may be in response to physical disturbance. This hypothesis is based on the fact that isopod swimming behavior was observed more frequently by the second diver than the lead diver as the team swam through the cave. Both Mexistenasellus floridensis and Caecidotea putea exhibited a fluid, graceful swimming motion. This fact at least suggests that although fin kicks and bubbles exhaled from SCUBA equipment may have induced their movement, it is not an unfamiliar, artificial behavior. It is easier to visually spot the isopods as they swim in the water column, where their unpigmented bodies are set against the dark cave background, than against the white limestone walls and ceilings or the silt-covered bottom. Due to this fact divers were focused on collecting animals that were swimming in the water column, and therefore likely missed many isopods that may have been crawling on surfaces within the cave. In total, these facts may explain why animals, seemingly poorly adapted for swimming, were collected solely from the mid-water column.
This swimming behavior may be an adaptive flight response to escape predation from the crayfish Cambarus cryptodytes, the Georgia cave salamander Eurycea wallacei or the trogloxene yellow bullhead catfish Ameiurus natalis. Other members of the community, a subset of the Florida subterranean fauna discussed by
During the 31 July 2015 dive, physicochemical data was taken using a Hydrolab HL4 sonde. Readings of depth, temperature, pH, conductivity, and dissolved oxygen (DO) were recorded every 20 seconds continuously during the dive. Dive bottom time (time spent swimming through cave passages and not entering and exiting the cave) was 51 minutes, and 155 separate readings were taken with depth varying between 19.57 and 31.53 meters. Readings were also taken in the open water of Merritt’s Mill Pond. For each parameter measured, the shallowest regions of the cave most closely approximated the pond water; however, temperature, pH, and DO progressively decreased with depth and specific conductivity increased with depth (Table
Physicochemical data correlated with depth. The 2 m reading was outside of the cave in the open water of Merritt’s Mill Pond. The deepest sections of the cave correlated with the lowest temperature, pH, and DO, and highest specific conductivity.
Depth m | Temperature °C | pH | Dissolved Oxygen | Conductivity µS/cm | |
---|---|---|---|---|---|
mg/l | % Saturation | ||||
2,21 | 20,58 | 7,37 | 5,82 | 64,7 | 315 |
19.57–29.5 | 20.08–20.22 | 7.36–7.39 | 5.22–5.28 | 57.6–58.3 | 318–327 |
30.0–31.53 | 18.73–19.97 | 7.31–7.32 | 3.01–4.88 | 32.2–53.6 | 333–359 |
Concerning reproduction, a 7.4 mm female M. floridensis was ovigerous with a brood pouch containing eggs approximately 0.5 mm in diameter. Another post-ovigerous female released 32 juveniles about 1.4 mm in length.
The senior author gratefully acknowledges conversations and suggestions regarding stenasellids with his friend and colleague, the late Dr. Guy Magniez. The authors would like to thank Dr. Michael Stine for sharing his time and cave diving expertise in assisting with the collection of specimens for this project. We would also like to thank the staff at Cave Adventurers for their professionalism and sharing their local knowledge. Funding for this project was provided by the American Public University System Faculty Research Grant.