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Dr. Dean Blinn

Regent's Professor of Biology

Research Location Site: Montezuma Well, Central Arizona

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Specialized open water feeding by the leech, Erpodbella montezuma (Hirudinea)

Dean W. Blinn, Ronald W. Davis and Behrooz Dehdashti Northern Arizona University


An endemic leech, Erpodbella montezuma, lives in the isolated, fishless waters of Montezuma Well in central Arizona. Interestingly, the leech's diet consists almost entirely of one pelagic amphipod, Hyalella montezuma. This restricted diet is confirmed by the gut content of the leech, though other potential prey are available in the vegetation and sediments of the pelagic and littoral zones of the Well. Some organisms in Montezuma Well follow certain behavioral patterns that repeat themselves daily, such as vertical leech migrations through the water column. Predictive patterns have contributed to the very close predator-prey relationship between the leech and amphipod.


The range of prey species taken by freshwater predatory leeches is normally quite high and changes seasonally with prey availability. While leeches sometimes do ingest whole body prey, they more commonly suck out body fluids. When whole bodies are ingested the exoskeletons are normally quickly evacuated.

Erpodbella montezuma, a newly described species of leech endemic to Montezuma Well in central Arizona, is quite different. (Look for how, when, and what the leech feeds on and compare it to leeches in normal freshwater ecosystems) The leech routinely undergoes a twilight vertical migration up into the surface waters of Montezuma Well, each night feeding on the large numbers of the amphipod Hylella montezuma, which migrates horizontally to the side of the Well each night. E. Montezuma return to the bottom of the 5-meter water column near sunrise and remain in or on the sediment until the following twilight. In this study the diel feeding pattern of the leech is researched by performing a microscopic analysis of the gut content. The dual migration pattern and close predator-prey relationship of endemic leeches and amphipods in Montezuma Well make it an excellent place to study reproductive isolation and speciation in general.

Site Description

Montezuma Well is a circular collapsed travertine spring mound believed to have formed between 4,000 and 10,000 years ago. The Well encloses an area of 0.76 hectares and has a mean depth of 6.7 meters (An area roughly the size of a football field with a mean depth of almost three stories). Warm water (24 degrees Celsius) continuously enters at the bottom of the well through artesian spring vents and exits through a side wall cavern; the water level remains constant throughout the year (Cole and Barry 1973). Water temperature never varies more than 4 degrees throughout the year with an annual mean temperature of 21.1 degrees Celsius (Boucher et al. 1984). Importantly, there are no fish in Montezuma Well (Cole and Watkins 1977, Boucher et al. 1984) apparently due to a high dissolved carbon dioxide concentration of over 550 mg l (Cole 1983). Montezuma Well's water level, temperature, and carbon dioxide levels are nearly the same day after day, year after year, producing an interesting aquatic environment.


E. Montezuma was collected from Montezuma Well on 11 different days to examine the seasonal diet of the leech. Leeches (0.2-0.6 g wet wt.) were collected in nets at different time intervals 2 hours prior and up to 6 hours after sunset on 8 dates and near sunrise on the remaining 3 dates. To sample various specimens and locations, different sampling techniques were used. Petite dredges were used to collect specimens in sediment, vertical nets collected from the water column to quantify plankton prey, and net tows were used in the littoral zone to quantify potential prey in the vegetation. (To make sure the scientists had all the information they needed, different types of collections at different locations were made at varying times in Montezuma Well.)

In collections taken for microscopic analysis of leech gut contents, animals were individually packaged and preserved to stop digestion.(If the leeches were kept alive, they would continue to digest they food and the scientist wouldn't be able to accurately find out what they were eating.) All visible prey in the dissected gut was counted for each animal.

To determine the approximate minimum time required for E. Montezuma to evacuate exoskeletons of Hyalella montezuma, 50 live prey animals (Hyalella montezuma) were placed in 1-liter containers filled with Well water and one leech starved for 24-30 hours. Feeding trials with a total of 36 leeches were performed in the dark at 20 degrees C (because leeches feed at night and at this water temp. in Montezuma Well) with each animal allowed to feed for 2 hours, after which they were placed in clean separate containers of Well water and observed for 1 hour intervals for 20 hours. (The leeches were placed in the clean water so the evacuation of exoskeletons could be measured more easily and accurately than in if they remained in Well water with live amphipods.) Number of prey eaten, exoskeletons evacuated, and times of occurrence were recorded during each observation. At the end of the 20-hour period, all leeches were dissected and gut contents examined.


The major prey in the open water column of Montezuma Well was the amphipod, Hyalella montezuma, although other potential prey were abundant in the sediment and littoral vegetation throughout the year. The amphipod, H. montezuma was observed in over 90% of the dissected guts of E. Montezuma collected from Montezuma Well with other prey, including Odonata (dragonflies), Gastropada (snails), and Oligochaeta (earthworms), present in much lower numbers. The number of H. montezuma present in the guts of E. montezuma were low in leeches collected 1-2 hours before sunset (<1.5) but increased to over ten in leeches collected at midnight. The greatest increase in H. montezuma in the gut contents of E. montezuma occurred within 2-4 hours after sunset, while numbers decreased from midnight to sunrise. The frequency of H. montezuma in the gut contents of E. montezuma support the diel feeding pattern(The pattern of repeated itself every sunrise and sunset). There was no evidence of feeding without ingestion, i.e., sucking out the body fluids.(The leech consistently ingested the amphipod whole!) Based on laboratory observations, orange colored exoskeletons of H. montezuma prey were evacuated from E. montezuma through the mouth and anus as early as 6 hours following ingestion with guts completely cleared 16 hours after feeding.(So if a leech stops feeding at 4 A.M., by 8 P.M. later that day, it's gut is completely empty, thus driving it upward in it's daily migration in search for food.)


In summary, the constant and highly predictable environmental conditions in Montezuma Well (Cole and Barry 1973, Boucher et al. 1984) may have sustained an abundant and highly synchronous twilight horizontal migration of H. montezuma for at least 10,000 years (Hevly 1974) or even longer (Batchelder 1974). The accompanying twilight vertical migration of E. Montezuma to the surface water and the high degree of selection of prey suggest a close evolutionary history in this predator-prey relationship.(When it gets dark the leech moves up in the water column search for food and the amphipod moves over towards the shore to escape the predaceous leech) In normal freshwater systems leeches usually feed on many different species throughout the lake or pond, including humans. In contrast, the leeches in Montezuma Well feed on the abundant and predictable amphipod species almost exclusively, in the pelagic zone, even though other common food items are available. Since there are no fish in Montezuma Well the top predator is the leech, E. Montezuma, making this aquatic ecosystem unique. The high carbon dioxide content has historically restricted the number of species that can live in the Well. New species have evolved that are able to tolerate these conditions. The Well environment has produced a close predator-prey relationship that isn't found in normal freshwater ecosystems. Montezuma Well, surrounded by desert, may be considered a biological island as many animals living in the Well's unique environment are unable to move or live in other freshwater ecosystems.

Key Words

Amphipod: Any small, flat-bodied crustacean of the order Amphipoda, having one set of limps adapted for swimming and another for hopping. They look like tiny shrimp.

Diel: Pertaining to a 24-hour period, a regular daily cycle. The leeches and amphipods follow one diel cycle.

Endemic: Indigenous to, and restricted to, a particular area. The leech and amphipod studied are endemic to Montezuma Well, meaning they're found only in this location and nowhere else on earth.

Littoral Zone: The zone in a body of fresh water where light penetration to the bottom is sufficient for the growth of plants. This is where most of the aquatic plants grow in Montezuma Well and where the amphipods migrate to in the evening, from the pelagic zone.

Mean: The average; statistical term for the average of a series of values, calculated by adding together all of the values and dividing by the number of different values.

Pelagic: Living in or relating to open water, especially surface waters to the middle depths. This would be away from the shore of Montezuma Well where the leeches feed on the amphipod at night.

Quantitative: Numerical, or capable of being expressed in numbers; derived from measurements or other numerical values, e.g., 10 leeches.

Travertine: A mineral deposit of calcium carbonate that forms through precipitation in a hot spring, like Montezuma Well, or from the calcium-rich water on cave dripstones.


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- and Proctor, R. M. 1985. Oligochaetes as a food resource of lake dwelling leeches. - J. Freshwat. Ecol. 3: 181-187.

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