Mangrove Killifish (or Mangrove rivulus)

Kryptolebias marmoratus

Profile / Morphology 2

The mangrove rivulus has a long, slender, dorsally flattened body and a rounded caudal fin. It is dark brown to green in coloration. The body may be mottled with small black dots and there may be a little orange coloration on body and fins.

Mangrove rivulus is one of a few known self-fertilizing hermaphrodites (both the eggs and the sperm are produced by one parent, and the young are genetically identical to the parent); although there are some populations that are non-hermaphroditic in Belize. It is the only vertebrate with this form of reproduction. Individuals are either male or hermaphroditic, females don't seem to exist. The proportion of males depends on the environmental temperature. Below 20°C, the majority of individuals are males, above 25°C all are hermaphrodites. There is large morphological variation among populations from different areas of their range.
They are known to be very tolerant to exposure to air for periods up to 30 days or more (Litwiller et al. 2006), and they can be found in habitats such as tree logs to take advantage of this tolerance. When living out of water their gills have altered physiology that allows them to retain water and nutrients, while they excrete nitrogen waste through their skin. These changes revert when the fish return to living in water.

Diet 3

Diet includes terrestrial and aquatic invertebrates (such as mosquito larvae, polychaete worms, and copepods), and the mangrove rivulus is known for its cannibalistic tendencies (e.g., eating its own eggs when in captivity). They forage infrequently, but usually during mangrove forest flooding.

Average lifespan in the wild 3

1-2 years

Size / Weight 3

This is a small, slender, elongated fish averaging 1 inch (2.5 cm) in length, but they can reach 3 inches (7.6 cm) total. They weigh no more than an ounce or two (57 grams).

Habitat 3

Mangrove rivulus have been collected in microhabitats - small localized habits within a larger ecosystem – of mangrove tree forests including: crab burrows, stagnant pools, sloughs or ditches (often intermittently dry) and some fossorial (burrow) niches (inside or under logs, debris, leaf litter, etc.). On the east coast of Florida, the preferred microhabitat is in the land crab (Cardisoma sp.) burrows. In south Florida, and on the west coast, the preference is for stagnant pools and old mosquito control ditches in mangrove forests. In mangrove ecosystems, this rivulus has been collected in salinities ranging from 0 to 68 ppt, and in the laboratory juveniles remained viable at 70 to 80 ppt (Taylor 1999). These are levels well above the typical salinity of the ocean (35 ppt). Mangrove rivulus have also been reported from cave systems and solution holes adjacent to mangroves in the Bahamas.
Mangrove rivulus are tolerant of poor water quality and pollution and are extremely tolerant of salinity and temperature variations. They are euryhaline.

Range 3

The mangrove rivulus can be found from south-central Florida south through the West Indies to coastal areas of South America. It can also be found throughout the waters of Cuba, the Bahamas, Jamaica, and the Yucatan Peninsula.

Reproductive / Life span 3

Mangrove rivulus are internal fertilizers that lay eggs. Males have a modified anal fin that helps inseminate females. Eggs hatch in two to four weeks and only require dampness, not standing water.

Mangrove rivulus can reach maturity in six months in productive waters and only live a year or two.

Relatives 3

Mangrove rivulus are most closely related to guppies, killifishes, pupfishes, pearlfishes and other live-bearing fishes.

Found in the following Estuarine Reserves 3

Jobos Bay (PR)

Water quality factors needed for survival 3

•Water Temperature: 18- 24°C but can survive 5 to 38°C
•Turbidity: low to moderate
•Water Flow: low
•Salinity: 0 to 80 ppt
•Dissolved Oxygen: can tolerate low oxygen levels

Threats 3

•Habitat alteration and fragmentation
•Mosquito control

No direct estimates of abundance are available for mangrove rivulus, partly because of the diverse habitats used by this fish and the difficulty of sampling for fish in some of these vegetated and soft-sediment habitats. Nevertheless, habitat loss can be used as a proxy for likely abundance changes. This species is extremely vulnerable to habitat modification and fragmentation, environmental alteration, and human development/encroachment.

Much of the suitable habitat for this species has been isolated and fragmented as a result of the destruction of mangroves through removal and practices such as mangrove “trimming” and impounding of high marshes for mosquito control. Although it is obvious that the population size of this species has been dramatically reduced, it is difficult to evaluate the distribution and status because of its natural rarity and mysterious tendencies. Because of its preference to live in the burrows of land crab (Cardisoma) burrows, the mangrove rivulus may have less habitat available as the numbers of Cardisoma decrease for other reasons. More information needs to be gathered regarding the relationship between these two species.

Conservation notes 3

Importance to Humans and Estuaries
Mangrove rivulus may have occurred in high numbers and been a significant source of nutrients in estuaries. Study of their unique reproductive mode and air breathing physiology help scientists better understand the evolution and physiology of these unusual traits.

How to Help Protect This Species
This species lives in and uses estuaries and freshwater areas but are susceptible to water pollution and damage to and alteration of stream channels and riparian zones. Therefore, efforts to protect the species include:
•Minimize runoff of neighborhood pollutants, fertilizer, and sediment into local streams are helpful to this species, and other estuary dwelling species.

•Join a stream or watershed advocacy group in your area to protect your local estuary ecosystems.
•Support research into the unique ecology and physiology of the species and more surveys of their abundance.

Sources and Credits

  1. (c) Cardet co6cs, some rights reserved (CC BY-SA),
  2. Adapted by GTMResearchReserve from a work by (c) Wikipedia, some rights reserved (CC BY-SA),
  3. (c) GTMResearchReserve, some rights reserved (CC BY-SA)

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