Fine structure of the spermatozoon in three species of Cambaridae (Arthropoda: Crustacea: Decapoda)Cambarus robustus,Orconectes propinquusandOrconectes rusticus: a comparative biometrical study

The ultrastructure of spermatozoa in three species of cambarid crayfish,Cambarus robustus,Orconectes propinquus, andOrconectes rusticus, were studied and compared with eight previously studied species from different crayfish families using morphological features and biometrical data. The ultrastructure of spermatozoa show a generally conserved pattern including an acrosome and nucleus in the anterior and posterior parts of the cell, respectively, radial arms that wrap around the nucleus, and the whole cell is enclosed by an extracellular capsule. The most outstanding morphological feature in spermatozoa of three studied cambarid crayfish is the crest-like protrusions in the anterior part of the acrosome that can be used as one of the features for distinguishing the members of this family. Results of biometrical data reveal that acrosome size in the representatives of Parastacidae are the smallest, while representatives of Astacidae show the biggest acrosome. The acrosome size in species belonging to Cambaridae occupy an intermediate position between the two other families of freshwater crayfish. In conclusion, a combination of morphological features and biometrical data of spermatozoa can help distinguishing different species of the freshwater crayfish.

Estimates of egg deposition and effects of lake trout (Salvelinus namaycush) egg predators in three regions of the Great Lakes

Excessive mortality from spawning through the first year of life is likely responsible for recruitment failures in many lake trout (Salvelinus namaycush) populations. We evaluated relationships between interstitial predator species and lake trout egg deposition in three regions of the Great Lakes. The introduced crayfish Orconectes rusticus dominated the nearshore zone in northern Lake Michigan, whereas other sites in Lake Michigan and Parry Sound were dominated by native Orconectes propinquus. In Lake Champlain, sculpin (Cottus spp.) were the most common interstitial predator, yet densities were comparable with those in Lake Michigan. Predator densities in Lake Michigan (mean ± standard error = 15.5 ± 0.4 predators·m–2) were twice those in Lake Champlain or Parry Sound (6 ± 1 predators·m–2). Densities of eggs were higher in Lake Champlain (2994.1 ± 398.3 eggs·m–2) and Parry Sound (454.7 ± 36.3) than in Lake Michigan (7.5 ± 1.3; Kruskal-Wallis, P ≤ 0.05). Ratios of eggs to predator provide simple measures of the effectiveness of lake trout rehabilitation efforts. The average ratio calculated from 10 successful spawning locations is 190 eggs·predator–1·m–2. Ratios may help establish target egg densities for restoration efforts and the concentration of spawning adults required to allow survival to the fry stage.

Impact of crayfish predation on exotic zebra mussels and native invertebrates in a lake-outlet stream

To test the hypothesis that Orconectes spp. crayfishes could be a significant source of zebra mussel (Dreissena polymorpha) mortality in streams, we conducted a laboratory experiment and a field experiment. In the laboratory experiment, maximum zebra mussel size consumed increased with increasing crayfish size for Orconectes rusticus, Orconectes propinquus, and Orconectes virilis. Slopes differed among crayfish species but not between O. rusticus sexes. To quantify crayfish impacts on seminatural zebra mussel populations, we placed 16 enclosures (0.2 m2) lined with artificial cobble substrata in a midwestern lake-outlet stream (mean width 17 m). Zebra mussels were stocked in each cage (20000 individuals/m2) 14 days before male O. rusticus (20 individuals/m2) were stocked in each of eight enclosures. Densities of all sizes of zebra mussels were reduced in enclosures, with a total density reduction of 31% over the 28 days of the experiment, relative to the exclosures. Crayfish also reduced gastropod densities by 54%, but did not significantly affect other invertebrates. Laboratory and field results suggest that predation by Orconectes crayfishes may significantly reduce zebra mussel populations in streams.