The Macroinvertebrate Biotic Integrity Index (MBII) was developed from data collected at 574 wadeable stream reaches in the Mid-Atlantic Highlands region (MAHR) by the U.S. Environmental Protection Agency's (USEPA) Environmental Monitoring and Assessment Program (EMAP). Over 100 candidate metrics were evaluated for range, precision, responsiveness to various disturbances, relationship to catchment area, and redundancy. Seven metrics were selected, representing taxa richness (Ephemeroptera richness, Plecoptera richness, Trichoptera richness), assemblage composition (percent non-insect individuals, percent 5 dominant taxa), pollution tolerance [Macroinvertebrate Tolerance Index (MTI)], and one functional feeding group (collector-filterer richness). We scored metrics and summed them, then ranked the resulting index through use of independently evaluated reference stream reaches. Although sites were classified into lowland and upland ecoregional groups, we did not need to develop separate scoring criteria for each ecoregional group. We were able to use the same metrics for pool and riffle composite samples, but we had to score them differently. Using the EMAP probability design, we inferred the results, with known confidence bounds, to the 167,797 kilometers of wadeable streams in the Mid-Atlantic Highlands. We classified 17% of the target stream length in the MAHR as good, 57% as fair, and 26% as poor. Pool-dominated reaches were relatively rare in the MAHR, and the usefulness of the MBII was more difficult to assess in these reaches. The process used for developing the MBII is widely applicable and resulted in an index effective in evaluating region-wide conditions and distinguishing good and impaired reaches among both upland and lowland streams dominated by riffle habitat.

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Reports of new habitats for a major group of organisms are rare. Fishes display diverse adaptations for temporary (amphibious) existence on land, but to our knowledge, none have ever been reported regularly living inside emergent logs. Here, we show that the mangrove killifish, Kryptolebias marmoratus, a species previously known to emerse (leave the water) regularly, is now known to emerse and aggregate in large numbers inside decaying mangrove logs that have been "galleried" by terrestrial insects. This behavior has now been documented in both Belize, Central America, and Florida, U.S.A., populations and represents the first known case of fishes entering terrestrial woody material. The dense packing of fish in the narrow log galleries may imply a novel social context in which intraspecific aggressive behaviors are reduced, possibly mediated by the physiological limitations imposed within this restrictive habitat.

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Recombination is unknown in natural popu-

The killifish Rivulusmarmoratusis the only known selfing hermaphroditicvertebrate, and males of the species are usually exceedingly rare or non-existent in nature. Collections on several Belize cays in 1988 and 1989 yielded 13.5-24% males. Factors responsible for this unusually high proportion of males are currently not understood. Likewise, the biological significance of males in populations which otherwise consist of selfing hermaphrodites (with internal fertilization) is problematic and awaits further study.

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