ON THE DISTRIBUTION OF A MARINE CLADOCERAN, PENILIA AVIROSTRIS DANA (CRUSTACEA, BRANCHIOPODA), WITH A NOTE ON ITS REPORTED BIOLUMINESCENCE

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Abstract
1. Penilia has recently become a regular member of the summer plankton off the southern coast of New England, much farther north than would formerly have been expected. 2. Its reproduction in New England waters is mainly by ovoviviparous parthenogenesis. but males also occur, together with females bearing fertilized, resting eggs. There is reason to believe that the species overwinters locally in the form of resting eggs, rather than being brought in each year by wind or currents. 3. The poleward distribution of Penilia seems to be limited only by the need of a certain minimum water temperature in summer. Adults can survive at temperatures down to below 9° C., and are able to reproduce at and perhaps below 16° C. But the species has not become permanently established except in areas where summer water temperatures exceed 21° C. The resting eggs probably can survive winter temperatures of below — 2° C. 4. Penilia is found in every ocean of the world, often as one of the dominant plankton organisms, yet it is absent from many regions where it might be expected to thrive. Possible reasons for this discontinuous distribution are discussed. Sudden appearances of Penilia in new areas perhaps are the result of recently laid, resting eggs becoming caught up in wind-driven spray. 5. The restriction of Penilia to coastal waters may be because the resting eggs sink to the bottom. Salinity is not the limiting factor, since Penilia has been found in salinities ranging from fresh water to 49%o. 6. Penilia shows no obvious responses to light or to pressure which might account for its restriction to surface waters. 7. The fact that Penilia is the only filter-feeding branchiopodan to have become truly marine, may possibly be because in sea water it is difficult for members of this group to maintain the necessary turgor within their phyllopodia. 8. A report that Penilia is bioluminescent could not be confirmed for specimens examined in August at Woods Hole, Massachusetts. The possibility remains that in some localities it may be luminescent in winter.