EXPERT TOPIC CRAYFISH
INTRODUCTION CRAYFISH
EXPERT TOPIC
“Crayfish or crawfish?” The term is interchangeable, and its use most often reflects regional preferences, although the name crayfish is more popular than crawfish. Other names include: crawdads, crawldads, freshwater lobsters, mountain lobsters, mudbugs or yabbies’. By whatever name they are called, crayfish are fresh water crustaceans resembling small lobsters, to which they are related taxonomically. Crayfish are the only large decapod crustaceans (decapods are animals with ten legs) to occur in freshwater environments. There are more than 640 species in three different families: Astacidae, Cambaridae and Parastacidae. The highest diversity occurs in North America (more than 330 species in nine genera). There are seven species in two genera in Europe while the Japanese species is endemic to the region. The Madagascan species and Australian species are endemic to those regions, and it would be important to know that there are more than 100 species distributed in Australia. Crayfish breathe through feather-like gills. Some species are found in brooks and streams where there is running fresh water, while others thrive in swamps, ditches and paddy fields. Most crayfish cannot tolerate polluted water, although some species such as Procambrus clarkia, are hardier. Crayfish feed on animals and plants, either living or decomposing and detritus. Crayfish tend to copulate in autumn, although copulation may occur throughout the year in some species. Male crayfish immobilise the female by grabbing her large chelae on walking
by Vaughn Entwistle, Features Editor, International Aquafeed leg 1 with his large chelae and rolling her onto her back. Sperm, in the form of a spermatophore or sperm plug, are transferred via a modified pair of pleopods (called gonopods) located on the first abdominal segment. Females may carry a spermatophore for several months before oviposition occurs the following spring. When releasing eggs, females secrete a sticky substance called glair that is used to attach the eggs to her pleopods. Once a female has released all of her eggs and the glair has hardened she is said to be ‘in berry’. The number of eggs a female can carry while in berry is a function of body size, egg size, and species. Females will carry eggs for several weeks; the duration of this period depends on the species and is also influenced by water temperature. Crayfish exhibit direct development; there are no free-living larval stages and the young crayfish that hatch from the eggs look like small adult crayfish. When the eggs hatch the young crayfish initially remain attached to the female. The young will undergo three molts during this period. It is at this stage that the young leave the female and become free-living. They are approximately one cm long (total length). Young crayfish are highly vulnerable to a wide range of predators and tend to aggregate in habitats in which they are the safest, e.g., shallow water habitats. During their first summer they molt several times and increase in size considerably. By the end of their first summer they may have quadrupled in size. With their increased size they have outgrown many of their potential aquatic predators with the exception of large fish and adult crayfish. The lifespan of crayfish varies from species to species. Most species found in surface waters live two–four years. However, some species that live at higher latitudes may live seven–10 years.
34 | June 2018 - International Aquafeed