Rapid estimation of in situ growth rates of Caridina nilotica (Crustacea: Decapoda) inLake Victoria: Description and pilot application of a simple, field‐compatible technique

Abstract

A simple rapid approach to estimating in situ growth rates of Caridina nilotica (Roux), a small shrimp that plays a pivotal role in Lake Victoria's food web, is described. The approach, potentially applicable to many arthropods, is based on moulting intervals (MI) and per moult size increments (PMI) determined during brief experimental confinements. Physiological justification of its reliability as a measure of in situ rates is given. Caridina moults at night. Feral animals collected shortly (≤2 h) before dusk were sorted into one of five arbitrary size classes and held around 27°C overnight without food. MI, the inverse proportion of a batch moulting overnight, increased from 2 d in small shrimps (carapace length [CL] ≤1.8 mm) to >9 d in larger animals (CL ≥4.3 mm)‐a value comparable to egg development time (10.3 d) at the corresponding temperature. PMI was measured from differences in CL of postecdysal shrimps (CLi‐1) and corresponding cast exuviae (CLi). In absolute terms, PMI, surprisingly, was constant (0.284 ± 0.027 mm moult−1) over the size range of shrimps tested, although relative growth (PMI as a percentage of CLi) declined with size. Growth trajectories modeled with regressions fitted to the data (MI = 1.573 x CL0.999 and CLi‐1 = 0.284 + 0.977 x CLi) show that C. nilotica grows significantly faster (by at least 20%) in L. Victoria than previously estimated (Ignatow et al.), with corresponding implications to other evaluations of lake productivity. Prospective refinements and future uses for this simple technique are outlined.

Abundance and population structure of the Atlantic horseshoe crab Limulus polyphemus in Pleasant Bay, Cape Cod

“…Percent growth was highest among juvenile horseshoe crabs under 40 mm, and diminished as crabs became larger (Fig. 8), as is common in many animals (Mauchline 1976, Miller & Watson 1976, Fogarty 1995, Valiela 1995, Hart 2001. The magnitude of this decrease is comparable to that observed for Cape Cod horseshoe crabs reared in the laboratory (R. S. Jackson unpubl.)…”

Stable isotope analyses and demographic responses counter prospects of planktivory by Caridina (Decapoda: Atyidae) in Lake Victoria

“…Juvenile and larval stage of brown shrimp Penaeus aztecus and krill Euphausia superba, with high growth rates, have rapid isotopic turnover; turnover in adults is correspondingly slower (Fry and Arnold 1982;Frazer et al 1997). However, growth rates observed for Caridina in Lake Victoria are rapid; moulting intervals range roughly between 2 days for small juveniles and 10 days for relatively large adults; shrimps reach maturity around 45 days after hatching (Hart 2001). The Caridina used in the field enclosures were generally large (sub-adult and adult), probably minimizing turnover rates, and resulting in correspondingly slow changes in stable isotope values during in situ enclosure confinements over 19 days.…”

“…For ANOVA, moult intervals were size-normalized to provide a relative moult interval (MI = IMI/CL, days mm 1 ). This direct normalization was justified by the observed linear increase in IMI with CL (Hart 2001). The experiment was run in a cool, shady laboratory, subjected to natural thermal and light variations, in which water temperature varied between roughly 25C and 27C (see Hart 2001).…”

“…This direct normalization was justified by the observed linear increase in IMI with CL (Hart 2001). The experiment was run in a cool, shady laboratory, subjected to natural thermal and light variations, in which water temperature varied between roughly 25C and 27C (see Hart 2001). Statistical analyses were undertaken with Minitab Version 12.1 (ANOVA), and Statistica 5.1 (ANCOVA).…”

Size and temperature-dependent variations in intermolt duration and size increment at molt of Northern Shrimp, Pandalus borealis

“…Temperature or seasonal fluctuations in temperature directly influence IP but temperature effect on MI is less predictable (Comeau and Savoie 2001;Hartnoll 2001). In most studies, MI was weakly or not influenced by temperature (Hartnoll 1982;Iguchi and Ikeda 1995;Hart 2001;Sudo 2003). However, decreases and increases in MI with increasing temperatures have been observed in different species (Paglianti and Gherardi 2004;Kulmiye and Mavuti 2005).…”