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Fresh, green leaves are increasing as resources in aquatic ecosystems due to more frequent severe spring and summer storms, but research on allochthonous resources typically uses senescent leaves. We examined macroinvertebrate colonization of green leaves of three native deciduous trees (red maple, red oak, and tulip poplar) over two weeks within both a stream and fishless pond. Leaf colonization varied depending on the taxa of leaves and colonizers, submersion time, and the ecosystem examined. Within the stream, the densities of isopods (Lirceus sp.) and snails (mostly the invasive Potamopyrgus antipodarum) did not vary significantly across leaf species. In contrast, mayflies (Tricorythodes sp.) in the stream colonized tulip poplar in greater numbers than red oak leaves, while higher densities of planarians (order Tricladida) occurred within red oak leaves. The numbers of mayflies and snails decreased significantly by the second week, but the densities of isopods and planaria within stream leaf packs were consistent. In contrast, within the pond, significantly more isopods (Caecidotea communis) were collected after the first than after the second week of submersion and in tulip poplar leaves. Clams (Sphaeriidae) in the pond, on the other hand, were more prevalent in the second week but did not discriminate among leaf species. While the number of leeches (mostly Helobdella stagnalis) was consistent across weeks, significantly fewer leeches resided within tulip poplar leaves than within red oak leaves. Our results suggest that there are no consistent colonization-preference rankings of species of fresh leaves across freshwater benthic macroinvertebrate taxa. Even within a functional feeding group (e.g., the two types of isopods and snails, all detritivore-herbivores), there were differences in colonization patterns. Therefore, increased allochthonous inputs of fresh leaf litter due to severe spring- and summer-time storms are likely to promote the populations of various taxa to different extents.

While some streams have dense populations of aquatic detritivorous isopods, research on the colonization of leaf packs typically focuses on aquatic insects. To determine whether shifts in dominant local forest species might impact isopod populations, we placed leaf packs of red/sugar maple, American beech, and red oak on the substratum of riffles and pools in Cedar Creek (Allentown, PA, USA) in April 2019. We retrieved the packs after one week, re-deployed them, and re-collected them after two weeks of submersion, enumerating the number of isopods (Lirceus sp.) upon each retrieval. Surprisingly, neither the species of leaf nor the stream microhabitat significantly affected the number of isopods. However, the duration of leaf conditioning was important; significantly more isopods inhabited leaves after two weeks of submersion than after only one week. Maple and oak leaves displayed significantly more skeletonization after two weeks than the beech leaves, which remained intact. However, the similar numbers of isopods across leaf species suggest either the presence of acceptable, consumable microbial communities on all three species of leaves or that a tradeoff exists between the value of food and the importance of refuge provided by intact leaves.

Marine hermit crabs inhabiting different species of shells in the rocky intertidal may receive varied benefits and costs. Thick shell walls, sculpture, and certain silhouettes may increase protection against crushing predators and waves, but also the energy cost for bearing such a shell. To determine the relative energy demands on Pagurus granosimanus supporting one of three different species of shells commonly inhabited in nature, we measured the consumption of food by free-roaming, size-matched individuals after four days of food deprivation in the laboratory. Surprisingly, while all hermit crabs consumed a significant amount of food, that amount did not differ significantly across the types of shells inhabited, despite the significantly heavier weight of one type of shell (>150% of each of the other two species), and markedly different shell silhouettes. Whether our results were due to the inherent abilities of arthropods to support proportionally immense weights (physiology), reduced movements by animals in heavier shells (plastic behavior), or the way the hermit crabs bear the burden of the shell requires further investigation. Regardless, our results suggest that individuals of P. granosimanus that inhabit heavier shells that likely provide increased protection do not experience corresponding elevated food consumption requirements over the short term, even if those shells are not centered over the body mass of the hermit crab. Pagurus granosimanus crabs can bear the burden.