Native to northern South America north up to Florida, the Mangrove Rivulus favours shallow, swampy, brackish water, but is extraordinarily hardy and able to tolerate a huge range of environmental conditions, including anywhere from freshwater conditions to salinities of up to 68 parts per thousand (for context, seawater is typically only around 35 parts per thousand) and water temperatures of as high as 38 degrees Celsius (over 100 degrees Fahrenheit.) Water conditions in coastal mangrove environments change frequently as a result of weather and tidal activity, and while the immense hardiness of members of this species allows them to endure most adverse conditions, if conditions become too unsuitable for them they are able to survive by temporarily leaving the water entirely, leaping onto land and taking shelter in tree cavities or abandoned burrows, respiring through their skin and remaining in their shelter for up to 9 weeks at a time until conditions improve. While little is known about this species' life cycle in the wild, it is known that they lay their eggs (which can survive being exposed to air for prolonged periods of time, but which will only hatch when submerged) in shallow areas that are difficult for predators to access, and that there are no females of this species; every Mangrove Rivulus is born as a hermaphrodite and eventually develops the ability to fertilize their own eggs and essentially reproduce asexually, but upon reaching an advanced age some individuals may lose their ability to produce eggs and transition into males that will actively court hermaphroditic individuals in hopes of being allowed to fertilize their eggs, presumably to maintain genetic diversity. Members of this species feed mainly on small invertebrates and are noted to be territorial and highly aggressive towards conspecifics, although they will neither fight nor eat during periods spent on land.
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Various creatures have their flexibilities and coping methods for change, as we know. Some are more unusual than others: In humid heat in tropical mangroves, tiny rivulus fish actually jump out of the water in order to cool off, says a new study from the University of Guelph.
The researchers said that the fish, Kryptolebias marmoratus, lowered their body temperatures by chilling themselves in the air. Also, later, when they were more accustomed to the heat after having experienced higher temperatures for a week, they coped better with warm water, according to a release.
Before that happens, though, jumping is important to the fish in leaving behind rising temperatures for a bit, said Pat Wright, lead author on the study and integrative biology professor at University of Guelph, in the release. "If the fish are prevented from jumping out of the water, they would die."
The study was recently published in the journal Physiological and Biochemical Zoology.
Family Rivulidae, the Rivulines, also known as New World Killifish
The rivulines, or New World killifish, are fish that are closely related to the "other" killifish families (such as the topminnows and the pupfish), though they are much more rarely seen in the trade (or even in the wild in North America) than their relatives. They are colorful, interesting fish with a handful of traits that make them stand out, though acquiring them is the hard part.
Species
The New World killifish, family Rivulidae, is a family under the order Cyprinodontiformes (Toothcarps) along with topminnows, pupfish, and the poeciliid livebearers, among others. This family contains about 39 Genera and 341 species, of which 5 are native to North America:
Very few members of Rivulidae are sold commercially with any real frequency, but this does not include any of the North American native species aside from the Mangrove Rivulus, which is fairly uncommon. The American Killifish Association may be helpful for acquiring specimens as well.
Conservation Status
The Mangrove Rivulus populations are considered stable and of Least Concern by the IUCN. The other species have either not been evaluated or are considered Data Deficient.
Distribution
More common in Mexico than the US, rivulines seem to be rather well dispersed throughout that county. Only the Mangrove Rivulus is found in the United States and is mainly limited to the Southern part of the state, such as in the Florida Keys.
Description
Like other killifish and livebearers, these are typically small fish. The Mexican Rivulus gets to be 1.5 inches in length, while the Mangrove Rivulus and Dogtooth Rivulus grow to be closer to 3 inches. The maximum length of the other two species does not appear to be known at this time, though if the other members of their genus are to be any indication, they will likely grow to 2 to 3.5 inches.
Like many killifish, they are not especially active fish and may prefer to lurk in shaded areas of the tank (floating plants are a plus!). These fish are definite jumpers and a secure lid is a must. If using a hang on the back filter, care should be taken to minimize the openings in the back of the lid. The Mangrove Rivulus has been noted for being territorial and somewhat intolerant of conspecifics in the same tank, though larger tanks would likely increase social behavior as these fish are often packed into very small areas without squabbling in the wild.
The Mangrove Rivulus is peculiar for a fish due to it's ability to spend up to 66 days out of the water at a time. Studies have been performed and seem to suggest that these fish spend about two-thirds of their time out of the water, given an option. A muddy area can be provided for these fish if the aquarist desires to replicate these conditions.
Feeding
These fish should be fine consuming most any live or frozen foods given to them, including brine and mysis shrimp, bloodworms, mosquito larvae, or blackworms. They may not ever except dry foods (mostly noted for the Mangrove Rivulus, though attempting to feed dry foods to see if other fishes will accept is worth trying). When fed foods with insufficient iodine concentrations, the Mangrove Rivulus seemed to be prone to developing goiters (a problem also seen with sharks in captivity). Adding iodine to the water at a rate of 1-2 drops per 10 gallons per week may be beneficial here.
Water Quality
Most of the rivulines seem to appreciate water that is close to neutral pH (7-7.5) and moderately hard (10-15 dH) though slightly more acidic is unlikely to cause any harm. The Mangrove Rivulus, on the other hand, would probably do best in significantly alkaline (pH 7.5-8.5) and hard (12-20 dH) water, given its ability to tolerate sea water. All of these fish are tropical species and warm water (74-80 F) is appreciated, with colder waters (60 F) noted as being lethal.
Brackish Suitability
Like other members of their order, some rivulines can make suitable brackish aquarium inhabitants. Cynodonichthys myersi has been noted as being brackish tolerant, however it's exact tolerances have not been noted and low end brackish conditions (up to 1.003) are likely best.
The Mangrove Rivulus is a different story, being found in brackish conditions predominantly, and even sometimes in marine. It has been noted as tolerating salinity anywhere in the range of 0 to 68 parts per thousand (1.000-1.052... twice the salinity of sea water!). Though they can stand freshwater for a time, they should be kept with at least some salt in their water for long term health. Low end conditions (around 1.005 or higher) should be acceptable.
Tank Size
Several sources, from very well respected authors and aquarists, have noted that 1 gallon tanks for the smaller species to be adequate for long term keeping. From personal experience with smaller tanks, I would not suggest something smaller than 10 gallons for even the 1.5 inch Mexican Rivulus. For the larger species, 15-20 gallons would be better, though a 10 gallon specimen tank (for a single fish) may likely suffice.
Breeding
The rivulines can be bred the same as the egg scattering killifish, using a mop (or similar threaded object) to simulate plants. Eggs seem to be laid at regular intervals, as opposed to a sizeable spawn all at once. The mop should then be moved to another aquarium for rearing, as the adults will eat the fry.
The Mangrove Rivulus is slightly different, being a simultaneous hermaphroditic species. Males seem to make up about 25% of the population, with the rest being self-fertilizing hermaphrodites, releasing eggs which contain clones of the parent fish. Though they can be mop-spawned, they apparently can also lay eggs on muddy banks, which will then hatch in a few days if regularly misted.
Final Thoughts
The rivulines are interesting, if somewhat mysterious fish. Not especially challenging to keep, they are somewhat harder to acquire than other killifish and thus there may be better options among the pupfish, topminnow, and even the livebearer families for recreating a North American biotope unless local specimens can be caught.
Sources
Fish Identification, Fishbase
FLMNH Ichthyology Department: Mangrove Rivilus, Florida Museum of Natural History
American Aquarium Fishes, Robert J. Goldstein
Mangroves, Mud and Miracles, D. Scott Taylor
North American Native Fishes for the Home Aquarium, David M. Schleser
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New Post has been published on Killifishs of the World
New Post has been published on http://killifish.aquarium-and-fish-care.com/rivulus-news/
Rivulus News
How to avoid the pitfalls of inbreeding
By having just the right amount of sex, the mangrove rivulus (Kryptolebias marmoratus) avoids this pitfall by boosting variability in the immune-system genes that confer pathogen resistance, according to a study published last week in Proceedings of …
Read more on Nature.com
Martin County Community Calendar, updated Jan. 9
Scott Taylor, “Specialized Habitat and Adaptations of the Mangrove Rivulus.” 6:30 p.m. April 7. Gabby Barbarite, “Vibrio Bacteria in the Indian River Lagoon.” 6:30 p.m. April 21. CPR for Healthcare Provider: 6 p.m. Jan. 14. Markey Square Shopping …
Read more on TCPalm
Martin County Community Calendar, updated Jan. 6
Max Planck Florida Institute for Neuroscience Public Lecture Series: A Guided Tour of the Cerebral Cortex.” Max Planck Florida Institute for Neuroscience, One Max Planck Way, Jupiter. 5:30 p.m. Reception. 6:30 p.m. lecture. RSVP: 561-972-9027; …
Read more on TCPalm
New Post has been published on Killifishs of the World
New Post has been published on http://killifish.aquarium-and-fish-care.com/rivulus-xiphidius-crique-boulanger/
Rivulus xiphidius "Crique Boulanger"
One pare of Rivulus xiphidius “Crique Boulanger” from French Guiana exhibited at the Kilifish Show 05.09.2009 by the KFN (Netherlands).
Video Rating: 5 / 5
