A shell disease that has plagued the southern New England lobster industry by making lobsters unsightly and in some cases unmarketable appears to be creeping northward to the lobster-rich grounds off Maine. The number of lobsters with shell disease remains tiny in Maine – only three out of every 1,000 lobsters sampled last year had it. But scientists and lobstermen are concerned because the prevalence grew fivefold from 2010 to 2012. About one in every three or four lobsters caught off southern Massachusetts and Rhode Island has the disease, which is not harmful to humans. In Maine, observers a few years ago were finding one diseased lobster for every 2,000 sampled. The rate rose to four out of 2,000 in 2011, and to six out of 2,000 in 2012.