Author Spotlight: Sexual dimorphism in the horned isopod

Sexual dimorphism and physiological correlates of horn length in a South African isopod crustacean

D.S. Glazier, S. Clusella-Trullas and J.S. Terblanche


Figure 1
Dr. Susana Clusella-Trullas on the left and Dr. John Terblanche in the middle along the rocky shore where specimens of the horned isopod (Deto echinata) were collected. Photo by Douglas Glazier

During October 2012 to March 2013 I spent a wonderful sabbatical leave at Stellenbosch University in South Africa.  There I was hosted by two superb physiological ecologists, Drs. John Terblanche and Susana Clusella-Trullas, who also happen to be husband and wife.  My major objective was to compare the rates of metabolism and water loss of several species of amphipod and isopod crustaceans with differing degrees of adaptation to land life. During one of my field trips to Pringle Bay (Western Cape Province) along the southern coast of South Africa, John and Susana acquainted me with the unusual horned isopod (Deto echinata) that lives abundantly amongst piles of wave-washed rocks along the shore (see Fig. 1).   I was immediately fascinated by the extremely long dorsal spines of the males of this species (Fig. 2), which I had never seen before in any other isopod species that I had collected in many places around the world.

Figure 2
A male horned isopod. Photo by Douglas Glazier

Most isopod species that dwell along rocky shores are notoriously difficult to capture – their association with sea shores and their quick escape responses to intruders has led to them to be called “sea roaches”.  However, I soon learned a useful technique for capturing dozens of these animals in a matter of seconds!  All I had to do was to reach my hand into one of the wet rock crevices (Fig. 3) and legions of these creepy crawlers would quickly scurry up my arm and beyond, reminding me of the scary scarab beetle attack scene in the 1999 movie “The Mummy”.   All I had to do next was to shake or brush my arm over a collection bucket and my collection was complete!

Figure 3
Example of a rocky crevice where I collected numerous individuals of the horned isopod. Photo by Douglas Glazier

Back at John Terblanche’s laboratory I soon realized that the dorsal spines of D. echinata are not only longer in males than females, but they increase disproportionately in length as a male grows in size.  In fact, the scaling slope of male horn length in relation to body length is one of the steepest ever observed for a morphological trait.  My guess was that this extreme “positive allometry” of a sexually dimorphic trait was likely due to sexual selection.  My working hypothesis was that male horn length is a sign of “health”, “strength” or genetic fitness that could influence female mate choice.  An important aspect of fitness is the successful ability to acquire, conserve and store resources.  With the help of John and Susana, I tested this hypothesis by measuring the body condition (body mass per length), activity levels, and rates of energy use and water loss of male D. echinata with different horn lengths (Figs. 4, 5).

Figure 4a

Figure 4b
Examples of male Deto echinata with relatively short and long dorsal spines. Photo by Douglas Glazier
Figure 5
Dr. Terblanche’s laboratory at Stellenbosch University where the rates of metabolism and water loss were measured in individual males of the horned isopod. Photo by Douglas Glazier

As expected, males with relatively long horns had significantly better body condition and faster rates of resting metabolism than males with shorter horns.  However, activity level and rate of water loss were not related to horn length.  It thus seems possible that male horn length is a reliable signal of “good genes” to potential female mates or rival males, thus favoring its evolutionary increase via sexual selection.   Direct observations of female mate choice and male-male competition for mates are needed to further test this hypothesis.

Douglas Glazier

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