Gonadosomatic Index: a how-to

Why am I measuring gonads? I measure the whole weight of a male crab, then I measure his gonad weight to calculate gonadosomatic index (GSI). GSI can be used as a proxy for reproductive fitness: the greater the index, the more that male has to offer any female with whom he mates.

Step 1. Determine shell condition:

Shell condition is a rough estimate of timing post-molt for a crab which I am using for comparisons:
• Shell 2 is really fresh and clean, bright, with no barnacles or other epibionts
• Shell 3 is a little darker, some scratches visible on the carapace, with few epibionts like small barnacles or leech egg cases
• Shell 4 is dark, lots of abrasions on the carapace and legs, larger barnacles (given that they’ve had a longer time to grow), but not at the “very-old-shell” phase of beginning to deteriorate and become malleable
   

shell conditions: click for a larger view

Step 2. Wet weight:

Step 3. Estimating missing leg weight:

Some crabs may not have had all their legs (crab fights, trawl injuries, etc.) so I needed to estimate their missing weight in order to have the complete whole crab wet weight. This guy was missing three legs on his right and half a leg on his left (written as MR2, MR4, MR5, and DL2). Using his remaining legs, I can measure approximately what he was missing and add it to his wet weight.

MR2, MR4, MR5, DL2

Step 4. Morphometrics:

Measure the carapace width and chela, or claw, height. These measurements will be used later to determine claw size.

Step 5. Gonad removal:

It’s time to open the crab and weigh those gonads!

Step 6. Do the math:

GSI = gonad weight / whole crab wet weight * 100%

And there you have it: an easy 6-step process to measuring gonadosomatic index!