Excerpts from Jim Conrad's
from the June 14, 2015 Newsletter issued from Río Lagartos, on the north-central coast of Yucatán, MÉXICO
I've mentioned that sometimes masses of minnow-sized, grayish-brown fish appear at the water's surface in pools, such as where the estuary's brackish water flows back and forth beneath culverts in the levee south of town separating the estuary and the mangroves. We've seen that these schools are mostly Yucatan Gambusias with a few Yucatan Mollies. Rainwater Killifish, LUCANIA PARVA, also are part of the mix, shown below:
These look a good bit like Yucatan Gambusias, which you can compare at http://www.backyardnature.net/mexnat/gambusia.htm.
But notice that the Rainwater Killifish's mouth is narrower, more "puckery," and that each of the killifish's scales is two-toned, highlighting each scale. The species is very flexible in its water requirements, occurring in marine, brackish and freshwater. FishBase.Org and similar sites describe the species as living coastally from Massachusetts south to Mexico's northern Gulf of Mexico, which doesn't include here. However, the 2010 Peralta-Meixueiro and Vega-Cendejas work I often refer to reports them as present in Ría Lagartos Estuary, in both seawater and brackish estuary water, as well in hypersaline waters at the estuary's eastern end.
Rainwater Killifish feed on tiny crustaceans such as copepods and cyclopoids, mosquito larvae, small worms, and mollusks. Our fish were about 1½ inches long (4cm), and that's within the range of adult lengths.
And who knows why the fish is named "Rainwater"?