Zooplankton of the Great Lakes


................Triops spp.
................Lepidurus spp.

General Information

*Not found in Great Lakes

Triops translates in Latin to three eyes and longicaudatus refers to the elongated abdomen and associated structures. Triops longicaudatus inhabits freshwater, ephemeral ponds (i.e. vernal pools) ranging from the southern regions of western Canada, through the United States and into Central and South America. Two genera (Triops [formerly Apus] and Lepidurus) constitute nine to twelve species within the Notostraca taxa. Triops is distinguished from Lepidurus by the absence of an anal plate. Fossil records indicate that these crustaceans evolved over 350 million years ago during the Devonian period and have remained relatively unchanged in external morphology. The persistence of these taxa during several geological extinctions may be related to the ability to remain in embryonic stasis for several decades. The specifics of such ontogeny are discussed below.

Anatomy and Morphology

Tadpole shrimp can range in size from 10-40mm. The head and anterior portion of the trunk is covered by a large shieldlike carapace. The anterior dorsal portion of the carapace is equipped with a relatively unique optical arrangement. Triops are named due to their eye morphology. Laterally, two prominent compound eyes are situated near the anterior portion of the carapace. A small ocellus is located medially to these two compound eyes. Posterior to the eyes is a nuchal or dorsal organ that may be used in chemoreception. The posterior abdomen somites are fused in annuli that are apparently formed by the fusion of two to six segments. The oral region of the cephalon possesses specific appendage types including antennules, antennae, maxillae, and mandibles that are covered by a labrum and used for food handling. The anterior portion of the trunk possesses several pair of phyllopod thoracic appendages. The 1st thoracic appendage is enlarged, protrudes from underneath the carapace, and is primarily used for movement. The proximal portion of the abdomen possesses 35 to 70 pair of fine, hair-like abdominal appendages that beat in rhythmic fashion to assist in movement and food channeling. The distal portion of the abdomen terminates into a prominent telson and subsequently branches into two large caudal rami.

Reproduction and Ontogeny

Triops can be either dioecious or functional hermaphrodites, as well as parthenogenic. Such plasticity in reproductive strategies, as well as the ability of the eggs to withstand desiccation, had been a significant factor in the evolutionary success of these crustaceans. The fertilized eggs or "cysts/resting eggs" are carried by the female in brood pouches for several hours before being deposited in the water . At this point the cysts settle to the bottom substrate and may remain dormant for decades until rehydrated. In some cases, a period of desiccation is required for successful hatching. Once rehydrated, the cysts split open and the embryo is released. The embryo will develop into a metanauplius within several hours and begin filter feeding. The first three pairs of appendages (antennules, antennae, and mandibles) are typically well developed in the metanaupli and segmentation is already apparent on the abdomen. It is at this stage that Triops can be considered planktonic. Additional moltings will occur rapidly and within a few days (dependent on environment) a small tadpole shrimp will migrate to the benthos and continue feeding and eventually reproduce.


The absence or presence of temporary water sources encompasses the ecology and life history of Triops. Without such environments, Triops will remain in stasis and their fitness will remain as such. When flash floods or heavy seasonal rains occur, the cysts are reanimated and the majority of these crustaceans' life cycles must occur within the time allotted by the ephemeral ponds. However, since this taxa is basically ubiquitous, several of these ephemeral environments may remain for several weeks and allow Triops to thrive. Triops are facultative detritus feeders, scavengers, or predators. Although they are able to exploit several niches, they are commonly found in the benthos of ponds and pools. During feeding, the appendages are used to funnel or convey food down a ventral groove between the paired appendages to the mandibles and subsequent mouth. Due to such plasticity and success in feeding behavior, Triops longicaudatus is known as the rice tadpole shrimp and is considered a pest species in several areas, particularly rice fields. Tadpole shrimp chew off the roots and leaves of seedling rice plants and subsequently destroy large crops. Also, their benthic digging activity muddy the water and decrease the amount of light available to the plants. Conversely, the vernal pool tadpole shrimp (Lepidurus packardi) is only found in certain areas of California and is considered an endangered species. Although this taxa of crustacean has been present for millions of years, the conservation and management of them today will be dependent upon our understanding of their biology and ecological role.

Triops longicaudatus

Adult Triops longicaudatus.

Adult Triops (ventral view)

Notostroca Anatomy (from McLaughlin, 1980)

Triops cyst/resting egg

Triops metanaupli at 1 day (planktonic stage)

Triops metanauplii at 2 days showing more similarity to adult morphology

Triops at 3 days with development of carapace and general adult form

Triops subadult at 6 days. Carapace and exoskeleton thickening and deposited with calcium carbonate

Adult Triops

Preferred habiat (ephemeral pool) of Triops longicaudatus. The selected reproductive adaptations of the Notostraca are heavily dependent on the preservation of such habitat types.

Adult Triops in hand illustrating the large size of this crustacean.


McLaughlin, P.A. 1980. Comparative Morphology of Recent Crustacea. W.H. Freeman and Company, San Fransisco, California.

Schram, F.R. 1986. Crustacea. Oxford University Press, Inc., New York.