Description
Isthmura maxima is a large salamander in the I. bellii species complex. The snout-vent length for adult males ranges from 90 to 125 mm (based on seven specimens), and snout-vent length for adult females ranges from 88.8 to 128.1 mm (based on five specimens). The head is proportionately broad and long, with a rounded snout and somewhat protruding eyes. There are no parotid glands. Sexually mature males have a prominent mental gland that increases in size with age. The trunk possesses 13 costal grooves. The limbs do not overlap when adpressed to the body, with males and females showing a 1.5 - 2.5 and 1.5 - 3.0 costal groove difference in distance between limbs respectively. All four limbs possess short digits with no basal webbing and well-developed subterminal pads. Finger length in decreasing order is as follows: III, II, IV, I. Toe length in decreasing order is as follows: III, IV, II, V, I. The tail is both large and robust, with a constriction at its base that indicates the placement of the autotomy plane. It is relatively long compared to total body length. Males possess longer tails than females. The tail ends in a blunt tip (Parra-Olea et al. 2005).

DIAGNOSIS:

Isthmura maxima can be distinguished from other members of its genus based on coloration, overall body size, and the proportions of its head and limbs. It possesses a broader head and longer limbs than I. bellii and I. boneti, and it can also be distinguished from I. boneti based on its paired spots (rather than chevrons), lack of shoulder markings, and lack of combined pelvic spot complex. The lack of the shoulder markings and combined pelvic spot complex also distinguishes I. maxima from I. naucampatepetl, along with a broader head and larger overall size. Unlike I. gigantea, I. maxima has no combined spot at the start of its dorsal marks (Parra-Olea et al. 2005).

COLORATION:

In life, though coloration can be somewhat variable, I. maxima typically has a mostly black body coloration with vivid orange marks arranged in parallel rows down the head, body, and proximal region of the tail. The exact shade of these marks varies by individual. Most collected specimens have had broken occipital spots, but none have possessed scapular spots or a nuchal spot. The dorsum has 10 - 15 pairs of spots, with some unpaired spots interspersed throughout this pattern on some individuals. There is a pair of spots directly over the sacrum that is then followed by a pair of spots on each caudosacral segment. There are no spots at the autonomy plane. The tail has irregular marks up to the tenth caudal segment. The venter is the same color as the dorsum, with occasional lightening around the gular region. Males may also exhibit a lightened area over their mental gland (Parra-Olea et al. 2005).

VARIATION:

The exact pattern of the orange markings can vary between individuals, both in number of marks and precise location. The intensity of the orange shade is also often different. Males tend to have both a notable mental gland and a longer tail than females (Parra-Olea et al. 2005).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Mexico

View distribution map in BerkeleyMapper.

Isthmura maxima is found in Mexico in western and southern Oaxaca, with the initial descriptions of the species reporting a very limited range centered mostly around Putla de Guerrero (Parra-Olea et al. 2005). Further expeditions have revealed a somewhat larger geographic range for the species, with individuals found in more eastern areas of the state such as Villa de Tututepec de Melchor Ocampo (Rocha et al. 2019).

Isthmura maxima is found at elevations between 750 and 2000 m above sea level (Rocha et al. 2019). The species is found in wet tropical forests and somewhat montane areas, and several specimens have been collected along roads or within agricultural areas. It can usually be found in burrows (Parra-Olea et al. 2005).

Life History, Abundance, Activity, and Special Behaviors
Information on the abundance of this species is limited, as relatively few specimens have been collected over a series of separate expeditions, and many of the known individuals have been observed in disparate areas (Rocha et al. 2019). However, I. maxima appears to spend a significant amount of time in burrows that can be over 30 cm deep (Parra-Olea et al. 2005).

Larva
As a member of family Plethodontidae, I. maxima is presumed to reproduce via direct development.

Trends and Threats
The IUCN classifies I. maxima as “Endangered” due to habitat loss and alteration from changing farming methods in its limited range (from forest-based products to till-heavy crops). The exact population trend is unknown due to the dearth of information available on this species. Thus, the assessment has been partially based on the decline of suitable habitat conditions rather than a direct species survey (IUCN 2016).

Possible reasons for amphibian decline

General habitat alteration and loss
Habitat modification from deforestation, or logging related activities
Intensified agriculture or grazing

Comments

PHYLOGENETIC RELATIONSHIPS:

Bayesian analysis of ultraconserved elements nuclear DNA show that I. maxima and I. boneti are likely sister species in a clade that diverged from the rest of the I. bellii complex (Bryson et al. 2018).

The species epithet is presumably a reference to the species’ large body size.

Synonyms:
Pseudoeurycea maxima (Parra-Olea, García-París, Papenfuss, and Wake 2005)
Pseudoeurycea (Isthmura) maxima (Dubois and Raffaëlli 2012)

References

Bryson, R.W., Zarza, E., Grummer, J.A., Parra-Olea, G., Flores-Villela, O, Klicka, J., and McCormack, J.E. (2018). “Phylogenomic insights into the diversification of salamanders in the Isthmura bellii group across the Mexican highlands.” Molecular Phylogenetics and Evolution, 125, 78-84. [link]

Dubois, A., Raffaelli, J. (2012). ''A new ergotaxonomy of the order Urodela Duméril, 1805 (Amphibia, Batrachia).'' Alytes, 28(3-4), 77-161. [link]

IUCN SSC Amphibian Specialist Group. (2016). "Isthmura maxima." The IUCN Red List of Threatened Species 2016: e.T61907A53989368. https://dx.doi.org/10.2305/IUCN.UK.2016-3.RLTS.T61907A53989368.en. Downloaded on 08 October 2020.

Parra-Olea, G., Garcia-Paris, M., Papenfuss, T. J. and Wake, D. B. (2005). ''Systematics of the Pseudoeurycea bellii (Caudata: Plethodontidae) species complex.'' Herpetologica, 61, 145-158.

Rocha, G., Mata-Silva, V., Mata-González, S., Ramíerez-Bautista, A., Berriozabal-Islas, C., Wilson, L.D. (2019). “Geographic distribution of the endemic salamander Isthmura maxima (Amphibia: Plethodontidae) from southern Mexico.” Herpetology Notes 12, 1231-1233. [link]

Species Account Citation: AmphibiaWeb 2022 Isthmura maxima: Southern Giant Salamander <https://amphibiaweb.org/species/6435> University of California, Berkeley, CA, USA. Accessed Apr 18, 2024.

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 18 Apr 2024.

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