M. benedicti is the smallest of the amphipods present on the sandy beaches in Santa Barbara, ranging up to around 10mm to 15mm. Their presence is a significant kelp removal on the beaches, as their numbers can be high. These amphipods have a distinct blue, red and black checkered pattern on their dorsal side, making them easy to identify with the naked eye. This pattern, however, can change possibly due to regional differences and diet. A dark marking can also be seen along most to all of the lateral segmentation (coxa), compared to M. corniculata, having only two of these markings over the six and seven coxa. These beach hoppers are known to scavenge and cannibalize.