I've written a weekly natural history column for more than 40 years, and one of the favorite topics I address is what to get a naturalist for a present. Using myself as a proxy, I've always maintained that in the gift-giving department, naturalists are easy to buy for, since there's a world of natural history items out there. Case in point: my daughter, herself a splendid naturalist and high-school biology teacher, came up with the perfect birthday present for me: a guided trip to a local salt pond with the Weekapaug Inn's veteran natural history liaison Mark Bullinger to watch the horseshoe crabs come into the shallows to enact a rite of spring that has been going on for the last 400 million years. Tonight, with my grandsons Ezra and Lucas in tow, we joined an enthusiastic, headlamp-equipped group to wade into the water on a warm night close to the full moon and marvel at the mating crabs. The bigger one is the female; the smaller one along for the ride is the male. Soon, she'll be laying eggs in the sand that he'll fertilize, and when the Strawberry Moon rises next month, those eggs will hatch, and the age-old cycle will begin anew.