//metrognome logo// THE Sex God of Brooklyn was on the phone, inviting me to an orgy. Before you get all hot and bothered, the Sex God is a 66-year-old professor named Martin Schreibman, and the orgy was going to be filled with crabs. Horseshoe crabs, that is. Yes, it's the annual horseshoe crab mating season - an early-summer ritual that goes back more than 3.5 million years - and Schreibman was as excited as a 13-year-old who's just sneaked into an X-rated movie. "This is as good as it gets," said Schreibman, who runs the Brooklyn College Aquatic Research and Environmental Assessment Center, which breeds the pre-historic beasts for research. "It's very romantic. A high tide. A full moon. We should bring wine." When it comes to the sexual mores of the aquatic set, Schreibman is like a gossip columnist, chronicling all the canoodling of the creatures in the sea. If a horseshoe crab anywhere in the metropolitan region so much as sheds a gamete, Schreibman knows about it. Last year, Schreibman spiked the temperature in the AREAC fish tanks to trick his tilapia into thinking it was mating season. They responded with a breeding frenzy that made Plato's Retreat look like a Howard Johnson's. And the horseshoe crab orgy sounded even more raucous. We got to Brooklyn's Plumb Beach just as high tide came in - and the horseshoe crabs were going at it as if it were a porno version of "From Here to Eternity." Actually, that's inaccurate. In that movie, Burt Lancaster and Deborah Kerr mated a deux. But with horseshoe crabs (who are actually in the spider family), group sex is a matter of survival. The male-female ratio is similar to the MIT engineering program. If they want to have any chance of fertilizing an egg, males need to latch onto any female who's moving - even if she's already in flagrante with three other partners. Luckily, the female is larger than the male, so she can handle all the guys hanging on her. At one point, three females were buried under a dozen males, all latched on like geeks at a frat party. "They're not particularly selective, as you can see," Schreibman said. "Basically, the male grabs a girl and she buries herself in a hole." (Sounds like my mating ritual.) In two hours of action, I saw only one lonely female. Schreibman suggested a personal ad in Crabmopolitan: "Single, brown horseshoe crab female seeking partner for walks on the beach and minutes of pleasure half-buried in mud. Appearance irrelevant, but must be cool with open relationships and multiple partners." --30-- --30-- gersh.kuntzman@verizon.net