Environmental cues or conspecific attraction as causes for egg mass aggregation in hydrobiosid caddisflies

Abstract

The distribution of egg masses throughout the environment can strongly influence the population dynamics of aquatic insects. Many species lay eggs in aggregations and most eggs will subsequently hatch from only a few locations-knowing how and why these aggregations arise is therefore needed to understand the population dynamics of these species. The hydrobiosid caddisfly Ulmerochorema rubiconum lays eggs in large aggregations on the undersides of emergent rocks in streams. Our aim was to test whether females oviposit in response to conspecific egg masses or the environmental characteristics of rocks by manipulating the number and age of egg masses initially present on rocks and monitoring the accumulation of new masses. Our first experiment used rocks that had recently been used for oviposition and initial egg masses encompassed a range of ages; our second used rocks with no history of oviposition and initial masses of uniform age. Females did not respond to conspecific eggs of any age, as removing initial egg masses had no impact on the number of new masses laid in either experiment. There was a strong positive relationship between the number of initial and new masses, regardless of whether initial egg masses were removed, at the scale of whole rocks and for rock microhabitats. The results suggest that females select oviposition sites based primarily on physical characteristics of emergent rocks. We demonstrate for the first time that the spatial arrangement of egg masses may be consistent and predictable through time, with females showing a high degree of fidelity for particular rocks and microhabitats.

A test of the preference–performance hypothesis with stream insects: selective oviposition affects the hatching success of caddisfly eggs

“…There are three potential explanations for the observation that Apsilochorema egg masses only occur in slow flows: (i) females are incapable of ovipositing in fast flows and may perish in the attempt; (ii) females oviposit indiscriminately in all flow conditions, but only those egg masses in slow flows survive to be surveyed; or (iii) females oviposit preferentially on emergent rocks in slow flows. The second explanation seems unlikely because egg masses typically required 24–48 h to shear completely, yet in no study of Apsilochorema oviposition (including this one) have partially sheared egg masses been observed in fast flows (Reich, , ; Lancaster et al ., ; Reich & Downes, ,b, ; Reich et al ., ; Bovill, ). Unfortunately, we cannot disentangle whether Apsilochorema oviposits in slow flows via choice (explanation iii) or necessity (explanation i).…”

Egg masses of some stream‐dwelling caddisflies (Trichoptera: Hydrobiosidae) from Victoria, Australia

“…All had egg masses with a thick layer of spumaline and round white eggs arranged somewhat irregularly within the mass. dimension 8-10 and 5 mm, respectively) and often occurred in large aggregations with 10 to >100 egg masses per rock (see also Lancaster Reich & Downes 2003a;Reich et al 2011). 5a) were the largest in this group (max.…”

“…Oviposition substrates are discrete objects (emergent rocks), which can be sampled quantitatively and, e.g. Strong aggregative behaviour was shown by U. rubiconum and U. seona resulting in many egg masses on individual rocks (Lancaster et al 2003;Reich et al 2011). Tests of hypotheses involving ecologically similar species (e.g.…”

“…Traditionally, freshwater ecological research has focussed on the aquatic juvenile stages and predominantly late instar larvae, but estimating the factors that drive population dynamics requires examination of multiple life stages, including the often-neglected stages of eggs, pupae and adults. The success of such studies, however, depends on understanding the environmental factors that influence oviposition behaviours, and hence the spatial and temporal distribution of eggs within water bodies (Hoffmann & Resh 2003;Lancaster et al 2003;Reich & Downes 2003a, 2003bEncalada & Peckarsky 2007;Lancaster et al 2010b;Reich et al 2011;Storey et al 2016), and on understanding the factors that can influence successful hatching of eggs and recruitment of juveniles into the population (Siva-Jothy et al 1995;Purcell et al 2008;Bovill et al 2013;Bovill et al 2015;. Pupae are rarely considered by freshwater ecologists but high, density-dependent mortality of pupae can regulate some populations (Marchant & Hehir 1999).…”

“…There are over 15 genera and over 60 species of Australian Hydrobiosidae, occurring primarily in eastern Australia, with the highest species richness in south-eastern Australia and Tasmania (Dean 1997). Subsequent ecological research on these same genera has provided further insight into the consequences of oviposition behaviour for the spatial arrangement of egg masses (Lancaster et al 2003;Reich et al 2011), and into the effects of flow and egg predation on hatching success (Bovill et al 2013;Bovill et al 2015). an 'emergent rock'), climb down the side of the rock and lay their eggs in a single, plaque-shaped mass covered in spumaline and attached to the underside of the rock.…”