ABSTRACT
Bay lobsters of the genus Thenus are acommonand valuable by-catch of the shrimp trawl fisheries of northern Australia. Until recently, this was considered a monospecific genus, but two species are now recognized from Australia, Thenus orientalis (Lund, 1793) and Thenus indicus Leach, 1816, and additional species are likely to be described from other regions. The economic importance of theAustralian species was at odds with the meager biological information available until detailed studies were made in the 1980s and 1990s, which generated important information for fisheries management, and revealed the specialized nature of these lobsters. Thenus spp. inhabit the soft, sedimentary mud and sand of the continental shelf, particularly in inter-reef areas along the tropical coastline of Australia. Their morphology and behavior share much in common with other scyllarids, but also have unique features which reflect successful adaptation to their environment. Most notable are their ability to swim, often long distances, and the capacity to bury into the sediment. Thenus indicus is generally the smaller of the two species (maximum size ∼65 mm carapace length, CL) and inhabits shallow, inshore waters, 10 to 30 m deep in areas characterized by fine sand and silt. Thenus orientalis grows to a larger size (maximum size ∼95 mm CL) and inhabits waters of 30 to 60 m depth, where sediments are characterized by medium to coarse sands. For both species, sexual dimorphism is very subtle. Mating is believed to involve a brief encounter, and the spermatophoric mass is likely to be very short lived. Mean fecundity is 12,455 eggs for T. indicus and 32,230 for T. orientalis. Reproductive seasonality is marked by two spawning peaks corresponding to spring and midsummer. Growth is quite rapid: for both species aCLof 40mm(∼40 g total weight) is reached within the first 12 months, and approximately 80% of maximum size is reached by two years of age. Maximum age for T. indicus is between two and four years and between four and eight years for T. orientalis. Both species are nocturnally active with clear peaks in activity at dusk and just prior to dawn. During daylight hours, Thenus spp. bury themselves in the sediment with only the eyes and antennules exposed. During periods of activity, Thenus spp. occasionally leap from the sediment into a swimming mode effected by contractions of the abdomen and controlled by movement of the antennae. Such locomotion swimming may be sustained for periods up to 40 min, enabling lobsters to move distances of several hundreds of meters. The fishery for Thenus in Australia is managed and regulations include a total ban on the taking of berried females and a minimum size of 75mm CL. CPUE has been stable over the past two decades, although total catch has diminished with reductions in fishing effort. For Queensland, a maximum catch of 755 metric tons was recorded in 1997, and more recently, the annual catch has been around 400 metric tons.
