Diversity of the Order Amphipoda (Suborder Gammaridea) in Wisconsin

by Chris Arrowood
BIOL/WATER 361, Fall 2012

Key taxa: Arthropoda, Crustacea, Malacostraca, Peracarida, Amphipoda, Senticaudata

The goal of this paper is to answer some of the major questions regarding the diversity of order Amphipoda in Wisconsin. The original focus of this paper was to achieve that goal by answering the question, “Which families, genera and species of order Amphipoda are in Wisconsin?” However, science still has its mysteries, and so only a very rough estimate could be arrived at for inclusion in this paper. Due to this lack of quality information, the focus of the paper is instead more based on answering the following questions: “What are the historical variables that have led to the current diversity of amphipods in Wisconsin?” and “What are the current factors that add to amphipod diversity in Wisconsin, and how will those factors relate to the future development of amphipod diversity in Wisconsin?” These questions were able to be answered to a much more satisfying effect.

In response to the question, “How many families, genera and species of order Amphipoda are in Wisconsin?” there are approximately fifty-five species of amphipods in eleven genera and six families that are all housed in one suborder which is named Gammaridea. This relatively high number was very difficult to arrive at, which was due to the fact that there is currently no database that keeps track of amphipods in Wisconsin and many assumptions went into this creating this number. The process was reductive, starting with the total number of amphipod species, genera and families in the United States provided by Thorp et al. (2009). Then using Thorp & Rogers (2010), Holsinger (1976) and the website Encyclopedia of Life (2012), the species, genera, and families that were confirmed not to live in Wisconsin were slowly and painstakingly removed. If a species was discovered, but had no described range, it was assumed to be present in Wisconsin waters. This assumption would lead towards an overestimate of species, which will hopefully serve to keep the estimate more relevant over time due to the consistent discovery of new species of amphipods (Vainola et al, 2008).

Responding to the question, “Which families, genera and species of order Amphipoda are in Wisconsin?” the six families, and their genera names with and the number of species in each genus are as follows. The richest family by species is family Gammaridae. It contains two genera: Echinogammarus with three species, and Gammarus, with nineteen possible species. Family Crangonyctidae is the largest family with respect to genera with four genera: Bactrurus, with seven species; Crangonyx, with three species; Stygobromus, with two species; and Stygonyx, with one species. Family Anisogammaridae has only one genus, Ramellogammarus containing eight species, that may occur in Wisconsin. Family Pontoporeiidae includes two genera: Diporeia, with four species and Monoporeia, with one species. Family Hyalellidae has one genus, Hyalella, which contains four species. The smallest family is family Gammaracanthidae, which contains as single genus, Gammaracanthus, which has only one species. All of these families and genera were confirmed as living in Wisconsin or having an undescribed range and therefore could live in Wisconsin.

Now that we can put a rough number and name on the families and genera, and a number on the species of amphipods in Wisconsin, we can move on to answering the question: “What are the historical variables that have led to the current diversity of amphipods in Wisconsin?” To that end there are many events that can be considered important, as life has a long history on Earth. That being said, for Wisconsin amphipods a single event stands out above all others. This event has greatly hindered amphipod survival, but also helped the amphipods seen in Wisconsin today, that event is the arrival and retreat of the glaciers.

In Wisconsin´s recent geologic history most of Wisconsin was covered with a layer of ice, which is not conducive to amphipod survival. Some of the amphipods that were present in Wisconsin at that time may have adapted to this change by moving underground (Kornobis et al., 2010), which may have served to increase the diversity. This is not hard to believe as nearly two-thirds of North American amphipods are subterranean (Vainola et al, 2008). However, most amphipods failed to adapt in this way and there was a reduction in diversity that can be seen in the amphipod community today (France, 1992).

Glaciers can be blamed for the extinctions of many amphipod species and the creation of a few others. The retreat of the glaciers can be said to be the exact opposite. As the glacier retreated, it left carved-out and filled glacial lakes, providing plenty of habitats for the cold adapted amphipod species that surged northward from southern areas. Ever since that time, Wisconsin has had a lot to offer all freshwater organisms to promote their growth and diversification (WASAL, 2003). So while the presence of the glacier can be viewed as a bad thing, its retreat and the effects it had, have been crucial to the development of amphipod diversity today.

Now that the historical variables of Wisconsin´s geologically recent history have been considered, the focus can again shift to the final questions, “What are the current factors that influence amphipod diversity in Wisconsin, and how will those factors relate to the future of development of amphipod diversity in Wisconsin?” To this we can respond that the important factors for amphipod diversity include high food quality and abundance, habitat variability and amenable seasonal temperatures. These factors are all important because they effect the reproduction of amphipods, which ensures they survive and continue to diversify.

Seasonal temperatures are a critical component of amphipod habitat because amphipods are cold-water, stenothermic species that reproduce best in warmer water (Kruschwitz, 1978). If the temperature is too low they reproduce less often, and if it is too high adult survival rates are reduced. An ideal temperature regime for amphipods would be one that has a low annual temperature average, to promote adult survival, with a period of elevated temperatures to promote reproductive success. Fortunately, Wisconsin has over 15,000 lakes and has mean annual lake temperature that ranging from 4°C to 10°C which facilitates adult survival and temperatures that rise to 30°C in the summer to facilitate the amphipod breeding season (France, 1992, and Rose et al, 2008). This means that Wisconsin´s seasonal temperatures are almost perfect for the persistence and eventual diversification of amphipods in its waters.

A second significant factor that serves to continue to enhance Wisconsin´s amphipod diversity is its habitat variety. Wisconsin has plenty of habitat to offer with 15,000 lakes, 32,000 miles of river, 5.3 million acres of wetlands and 1.2 quadrillion gallons of groundwater (WASAL, 2003). No matter what particular habitat a species of amphipod might prefer, they stand a good chance of finding it in Wisconsin. All of this potential habitat also means that Wisconsin is an area that is particularly vulnerable to invasive species. Invasive species are considered a negative thing in the eyes of most natural resource professionals, but in terms of species diversity they can be a good thing. This is especially the case with amphipods which, it has been proven, can accommodate some invasive species right alongside native species through habitat and resource partitioning (Palmer & Ricciardi, 2004). It is vital to this process of allowing invasive species to persist, that a variety and volume of habitats exist for amphipods. In Wisconsin it does, and that allows for both a large current diversity and a future where increase amphipod diversity can be seen.

The final major component that has resulted in Wisconsin´s diversity of amphipods is its high food quantity and quality. Most amphipods are scavengers that tend to get most of their food from plant material or are just plain herbivores (Anteau & Afton, 2008). Studies have shown that sessile amphipods can survive with minimal ill effects in environments with poor quality food in high quantities, but more mobile amphipods require high quality food in a reasonably high abundance to survive (Cruz-Rivera & Hay, 2000). In order for the aquatic plants that make up amphipod diets to grow well, the plants need to have a good balance of nutrients and sunlight. Wisconsin has very tough water regulation laws that serve to protect its wetlands from having this balance altered by farming and development practices. This protection ensures that the plants grow and die in a natural way. In turn this stable, high quality food source ensures that more species of amphipods will be able to survive and reproduce effectively (Cruz-Rivera & Hay, 2000). It is clear to see that Wisconsin is maintaining and possibly enhancing its amphipod diversity through this protection of the habitats that produce the high quantity of high quality food on which all types of amphipods can survive.

With the consideration of current factors in Wisconsin´s amphipods it can be seen that the diversity of amphipods in Wisconsin is likely to increase because Wisconsin has three incredibly important beneficial factors. First, Wisconsin has a seasonal temperature regime that ensures both adult amphipod survival and reproductive success. Second, Wisconsin has a high variety and number of habitats that allow for amphipods that prefer certain environments to survive or to share environments with invasive species that may enter the ecosystem. Last, Wisconsin has tough laws that do their best to protect the habitat that its amphipods call home. All of these elements work together to preserve amphipod diversity in Wisconsin, and serve to lay the building blocks for future diversification.

References Cited

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