It is very likely that you have found Pontodrilus litoralis (family Megascolecidae), a circumtropical "morphospecies" common in not so exposed beaches all around the globe. Historically, several nominal taxa were described for various forms and populations, but they were all merged under a single species name by Easton in 1984. An additional species, Pontodrilus primoris was described from Tasmanian seashores by Blakemore (2000); otherwise there seem to be no other "marine" earthworms".
I was always intrigued by this, so a student of mine and I now have a paper in the pipeline that will show that the genetic variation is considerable even among Pontodrilus worms from the same locality. [In fact, our study includes specimens from the Fort Pierce area, where we found two distinctly separate genotypes, likely to represent different (sibling) species.]
All the best,
From: "Scott Jones" msjoneser from gmail.com
Date: Thu, 06 Dec 2007 21:57:43 +0100
To: annelida from magpie.bio.indiana.edu
Subject: [Annelida] Marine Megadrile?
>> I have collected some large ( 68 mm and longer, 2.5 mm wide ) megadrile
> oligochaetes on an intertidal flat in Fort Pierce, FL. They were located by
> flipping over rocks.
>> Body is naked and somatic chaetae are simple and paired; there is no
> visible clitellum. I have not dissected these animals yet.
>> Anyone have suggestions about what group this may belong to, or at least
> where to start?
> Scott Jones