Gerald R. Johnston

• Ph.D.
• Professor (Full) at Santa Fe College

Macrochelys suwanniensis (Suwannee Alligator Snapping Turtle) coexists with 2 other native large omnivorous turtle species (Chelydra serpentina [Snapping Turtle] and Trachemys scripta scripta [Yellow-bellied Slider]) in a 9-km section of the Santa Fe River in northern Florida. A major shift in dominant submersed aquatic vegetation prompted us to qu...

The published literature does not provide a consensus regarding maximum body sizes of North America's largest wild freshwater turtles (genus Macrochelys). The largest known wild M. suwanniensis (Suwannee Alligator Snapping Turtle) currently exists as a curated specimen measuring 713 mm straight-midline carapace length, 801 mm maximum carapace lengt...

Herein we briefly provide a background context for and summary overview of the articles comprising this special issue of the Southeastern Naturalist on Macrochelys spp. (alligator snapping turtles). We also make a case for standardization in collecting and reporting morphometric data on these species, clarify the current taxonomic status within the...

Alligator snapping turtles (genus Macrochelys) are the largest freshwater turtles in North America. Despite strong interest in their conservation, we still lack a comprehensive understanding of their natural history, including behaviors associated with terrestrial activity. We reviewed the literature and solicited unpublished observations of terres...

While monogenean worms are mainly parasites of the gills and skin of fish, and to a lesser extent parasites of the oral cavity, urinary bladder, and/or conjunctival sacs of amphibians and freshwater turtles, Oculotrema hippopotami Stunkard, 1924 is the single monogenean polystome reported from a mammal, the common hippopotamus (Hippopotamus amphibi...

Damage to a turtle’s shell can provide evidence of past events such as vehicle collisions, disease, predator encounters, or even a behavioural interaction between members of the same species. Documenting shell damage as part of long-term mark and recapture studies enables researchers to determine population trends, intraspecific interactions and...

Polystomatids are platyhelminth parasites that infect mainly amphibians and freshwater turtles. For more than seven decades, chelonian polystomes were classified into three genera according to the number of hamuli, i.e. absent for Neopolystoma , one pair for Polystomoidella and two pairs for Polystomoides . Following re-examination of morphological...

A multitude of different statistical models are commonly used to monitor trends in wildlife populations. Most are used to estimate abundance or survival (or both), and these estimates are then examined over time to infer trends in a population. The choice of which model to use is influenced by the key research question of interest and the types of...

Freshwater megafauna populations, which are declining worldwide, are well known but often overlooked and understudied compared with marine and terrestrial megafauna. One species of freshwater megafauna is the Suwannee alligator snapping turtle (Macrochelys suwanniensis), which is endemic to the Suwannee River drainage in Georgia and Florida. Severa...

The Suwannee alligator snapping turtle (Macrochelys suwanniensis) was first described in 2014. The species is thought to occur in blackwater and spring-fed streams of sufficient size throughout the Suwannee River drainage, but we have limited detailed information regarding its range. To clarify the distribution of M. suwanniensis, we compiled 111 m...

We conducted a survey of freshwater turtle populations in the spring-fed Ichetucknee River in northern Florida to assess temporal variation in assemblage structure. Following the same protocols as an initial 2007 survey, we hand-captured turtles while snorkeling in 2014 and found changes in species composition, abundance of 3 species, and demograph...

Animals aggregate to reduce predation risk, facilitate mating, and access resources with patchy distributions. During a long-term study of turtle populations in the Santa Fe River (SFR) ecosystem in northern Florida, we observed a large aggregation of turtles at Gilchrist Blue Springs Park (GBSP) in August-October 2013 and again in March-May 2014....

An aggregation of Suwannee Cooters (Pseudemys concinna suwanniensis) prompted an investigation of their effects on vegetation in Blue Spring, a 2nd-magnitude spring in Gilchrist County, Florida. We counted turtles and documented % cover and wet biomass of vegetation in September and October 2013. The maximum mean density of turtles (1566 ± 319 turt...

The Santa Fe River (SFR) in northern Florida and its springs provide a unique ecosystem for a wealth of flora and fauna, and support a unique freshwater turtle assemblage. We conducted a 6-year mark-recapture study of ten turtle species to assess how habitat heterogeneity among sites affects riverine turtle populations. Because the SFR ecosystem ha...

Macrochelys suwanniensis is a newly described species endemic to the Suwannee River drainage in the southeastern United States. We conducted a study of M. suwanniensis in the Santa Fe River (SFR), the major Florida tributary of the Suwannee River, between 2004 and 2011. We captured 109 individuals (24% immature, 44% adult female, 32% adult male). A...

We report on the occurrence of South American suckermouth armored catfishes (Loricariidae) in the Suwannee River basin, southeastern USA. Over the past few years (2009-2012), loricariid catfishes have been observed at various sites in the Santa Fe River drainage, a major tributary of the Suwannee in the state of Florida. Similar to other introduced...

Snapping turtles (Chelydra serpentina) occur in nearly every type of freshwater habitat in North America east of the Rocky Mountains, but little is known about the ecology of populations in the southern part of their range, as well as those in lotic habitats. We conducted a mark-recapture study of the Florida snapping turtle (C. s. osceola) in the...

Snapping turtles (Chelydra serpentina) occur in nearly every type of freshwater habitat in North America east of the Rocky Mountains, but little is known about the ecology of populations in the southern part of their range, as well as those in lotic habitats. We conducted a mark-recapture study of the Florida snapping turtle (C. s. osceola) in the...

A decreased tannin load during 2006–2007 in a northern Florida blackwater river allowed us to make observations on the ecology of the Suwannee cooter (Pseudemys concinna suwanniensis), a species otherwise usually studied in clear, spring-fed rivers. We conducted a capture–mark–recapture study of this protected species and recorded the locations of...

We conducted a mark-recapture study of the turtle assemblage inhabiting a 1.1 km reach of the Santa Fe River within River Rise Preserve State Park in northern Florida during drought conditions in 2006 and 2007. In 154 man-hours of snorkeling, we hand-captured eight species, including two Florida listed Species of Special Concern – the Suwannee Coot...