Cycle 3 — 2025

CSHH Horseshoe Crab Monitoring:

Night 1:

The first night of the third cycle, two nights before the full moon. We counted 119 horseshoe crabs and deployed seven tags. We noticed that there were far fewer females than on previous monitoring nights. A black skimmer was skimming the water for food.

Above: a tagged horseshoe crab making its way back into the water.

How can you tell the difference between a male and female horseshoe crab? Horseshoe crabs are dimorphic, meaning there is a visible difference between males and females; the males are much smaller than females. Additionally, males have different pedipalps, the first pair of walking legs—they look like small boxing gloves, whereas females have scissor-shaped pedipalps. The male horseshoe crab “boxing glove” allows them to clasp on to the back of a female horseshoe crab during spawning.  

a) Male horseshoe crab “boxing glove” pedipalps. b) Female horseshoe crab scissor-shaped pedipalps.

Source: Identification and Morphology of the Two Horseshoe Crab Species, Tachypleus tridentatus (Leah, 1819) and Carcinoscorpius rotundicauda (Latreille, 1802) (Merostomata: Limulidae) from Honda Bay, Palawan, Philippines. Juliana C. Baylon and Lota B. Alcantara-Creencia.

Night 2:

The night of the Strawberry Moon (full moon). We counted 263 horseshoe crabs, deployed six tags, and had three tag recoveries!  Another black skimmer was skimming the water for food.

Night 3:

Two nights after the full moon. We counted 117 horseshoe crabs and had one tag recovery. We counted fewer females this night as well, compared with previous monitoring nights. We saw another black skimmer and one black-crowned night heron. This lunar cycle was predicted to be the peak of spawning for our location and others along the north shore, however, based on our observations, it seems that the first full moon cycle, back in May, was potentially our peak cycle.