The Desmosomatidae (Isopoda, Asellota) Of The Gay Head-Bermuda Transect
This study treats asellote isopods of the family Desmosomatidae from benthic samples taken>on, or in the vicinity of, a transect between Woods Hole, Massachusetts, and the islands of Bermuda. Samples came from a depth range of 20-5,100 meters and represent the continental shelf, slope, and rise, the abyssal plain beneath the Sargasso Sea, and the Bermuda Rise.
Thirty-nine species have been found in this region, and all but ten of them are new. Nearly the full range of morphologies known to exist in the family is represented. Study of this material revealed the necessity for a complete revision of the family. Fifteen genera belonging to two subfamilies are recognized. This classification reflects a basic evolutionary pattern within the family. The Desmosomatinae n. subf. includes the most primitive genera and those genera in which the first pereopod has become reduced and attenuated. The Eugerdellatinae n. subf. contains genera whose first pereopod has become robust and raptorial or chelate.
The large number of individuals in single species from some of the samples has allowed close examination of development, sexual dimorphism, and individual variation. Eugerda tetarta n. sp. is described in detail from these points of view. In the female there are three manca and three juvenile stages. These are followed by two or three cycles of reproductive maturity in which each brooding stage is preceded by a preparatory one. Molting into the preparatory stage is accompanied by growth, but none is exhibited as the animal achieves brooding condition.
Males are first distinguishable at the third manca stage. This stage is followed by two juvenile and one preparatory stage in which the male looks like the female except for the usual differences in the second antenna and pleopods I and II. Profound metamorphosis accompanies the ecdysis into the copulatory stage, and as a result the mature male bears little resemblance to the female. The adaptive meaning of these changes is discussed.
Analysis of several specimens from one station reveals little individual variation. Differences in the shape of body and limb segments are usually too subtle to be detected in visual inspection. Variation in setal count is slight and tends to be allometric.
Our knowledge of geographic distribution must be regarded as fragmentary because adequate samples are lacking from most oceans. Present information indicates that nearly all the genera are cosmopolitan or will prove to be so. Species, on the other hand, have limited distributions. Only seven have been found in more than one ocean or sea. The limited distribution of species is undoubtedly related to the absence of dispersive stages in development.
The Desmosomatidae is primarily a cold-water group. Typically its members occur in shallow water only at high latitudes. In those rare cases where species have been found in shallow warm water, the annual range of temperature change is slight. On the transect, the distribution of desmosomatids in water shoaler than 200 meters is limited to the outer portion of the shelf, thus avoiding the main effects of the pronounced temperature fluctuations along this part of the coast. All species show some degree of zonation with depth. Only two display depth ranges extending beyond 2,500 meters. A few species exhibit polar emergence.
All species found on the transect are fully described and illustrated. The descriptions of previously known species are based on transect material, not on the original types.