Upper thermal threshold of Lepidurus arcticus (Branchiopoda, Notostraca) in lakes on the southern outreach of its distribution range

The Arctic tadpole shrimp Lepidurus arcticus has a circumpolar distribution and the Scandes (Fennoscandian Mountains) marks its southernmost limit in Europe. Within this area, 391 natural and 88 regulated lakes with L. arcticus have been identified, of which 87% are above the treeline. The lakes hosting L. arcticus decrease in altitude from south to north, which results from its temperature preferences. The majority of the locations are at a lower lake air temperature than 11°C which is equivalent to a water temperature near 14°C. This is assumed to be near the upper thermal threshold for L. arcticus. In lakes that exceed this average summer water temperature (1 July – 15 September), sustainable populations seem to be rare. In warmer lakes, life cycle mismatches are assumed to explain the absence of L. arcticus, most likely by affecting the embryo and juvenile stages. The distribution appears to be dichotomous, with one large northern area north of 65°N and one separated southern “island”. Only two locations of L. arcticus are known for the area between latitudes 62.88 and 64.39°N. In this part of the Scandes, the lakes are likely too warm to host L. arcticus as most of them are situated below 700 m a.s.l. This may also be the case in the northernmost region, north of 70°N, where only 11 populations are recorded. Most of the lakes in this area typically occurs below 400 m a.s.l. L. arcticus populations are sensitive to fish predation, and dense fish populations may be another stressor limiting its distribution. In contrast to water bodies in the High Arctic where L. arcticus only exists in shallow, fishless ponds, in the Scandes they co-exist with fish in 97% of the findings. Global warming has already modified the environment of the Scandes, and populations of L. arcticus are at threat in many of the small and shallow water bodies at low altitudes.

Thwarting predators? A three-dimensional perspective of morphological alterations in the freshwater crustacean Daphnia

Predation is a major selective agent, so that many taxa evolved phenotypically plastic defensive mechanisms. Among them are many species of the microcrustacean genus Daphnia, which respond to an increased predation risk by developing inducible morphological alterations. Some of these features are obvious and easily recognized, e.g., crests in D. longicephala, while others are rather hidden, such as the bulkier shape of D. magna induced by the presence of the tadpole shrimp Triops. In this study we investigated the extraordinary diversity of morphological adaptations in the presence of predators with different foraging strategies in six predator-prey systems. For the first time we were able to analyze the unexposed and predator-exposed morphs comprehensively using three-dimensional scanning and reconstruction. We show that morphological changes are manifold in appearance between species and predators, and go beyond what has been known from previous 2D analyses. This further demonstrates the enormous trait flexibility of Daphnia. Interestingly, we found that among this variety some species share morphological strategies to counter a predator, while others use a different strategy against the same predator. Based on these intra- and interspecific comparisons, we discuss the mechanisms by which the respective defense might operate. These data therefore contribute to a deeper understanding of the inducible defenses’ morphology as well as their diversified modes of operation in Daphnia, being a cornerstone for subsequent investigations, including the determination of costs associated with morphological change.

Phenotypic plasticity of senescence in Daphnia under predation impact: no ageing acceleration when the perceived risk decreases with age

Recognising the nature of the predation risk, and responding to it accurately, is crucial to fitness. Yet, even the most accurate adaptive responses to predation risk usually entail costs, both immediate and lifelong. Rooting in life-history theory, we hypothesize that an animal can perceive the nuances of prey size and age selectivity by the predator and modulate its life history accordingly. We test the prediction that—contrary to the faster or earlier senescence under predation risk that increases with prey size and age—under predation risk that decreases with prey size and age either no senescence acceleration or even its deceleration is to be observed. We use two species of indeterminate growers, small crustaceans of the genus Daphnia , Daphnia Pulex and Daphnia magna , as the model prey, and their respective gape-limited invertebrate predators, a dipteran, midge larva Chaoborus flavicans , and a notostracan, tadpole shrimp Triops cancriformis . We analyse age-specific survival, mortality and fertility rates, and find no senescence acceleration, as predicted. With this study, we complete the picture of the expected non-consumptive phenotypic effects of perceived predation pressure of different age-dependence patterns.

Effect of Rice Winter Cultural Management Practices on the Size of the Hatching Population of Triops longicaudatus (Notostraca: Triopsidae) in California Rice Fields

The tadpole shrimp [Triops longicaudatus (Leconte)] has emerged as a significant pest of rice grown in California in recent decades. The change in T. longicaudatus’ pest status has coincided with changes in cultural management of residual rice straw postharvest. Policy changes have reduced the postharvest burning of fields from nearly 95% to less than 10%, promoting increased use of winter flooding as a means of accelerating straw decomposition. Field and laboratory trials were conducted from 2015 to 2017 at the Rice Experiment Station in Biggs, CA and in greenhouses at the University of California (UC) Davis to evaluate the effects of burning, flooding, and a fallow control on T. longicaudatus population dynamics. Experiments demonstrated that burning of rice straw failed to suppress densities of hatching T. longicaudatus and actually had the reverse effect, causing a 51% increase in numbers hatching, perhaps as a result of burning triggering termination of multiyear T. longicaudatus egg dormancy. Winter flooding had no measurable effect on T. longicaudatus hatch. Thus, these changes in winter cultural practices do not appear to be responsible for the emergence of T. longicaudatus as a major rice pest.

Environmental conditions limit the distribution of Lepidurus arcticus (Branchiopoda, Notostraca) in lakes on the Hardangervidda mountain plateau, Southern Norway

The Arctic tadpole shrimp Lepidurus arcticus has a circumpolar distribution where the Hardangervidda mountain plateau in Norway marks its southernmost limit. Within this area, we searched for L. arcticus in 238 lakes in 27 catchments. On Hardangervidda, the distribution pattern of L. arcticus is highly skewed. In the 16 catchments located in the central and eastern parts, L. arcticus was recorded in 70% of all the lakes studied (n=191). The remaining 11 catchments located in western areas, are almost free of lakes with L. arcticus (n=47). The most striking difference between these two areas is the significantly higher level of snow deposition in the western areas. This delays the ice break-up, which results in lower water temperatures and a shorter growing season. The water of lakes in western areas (N=36) is also more dilute than those in the central and eastern areas (N=201), with mean calcium concentrations of 0.81±0.48 and 1.62±1.12 mg L-1, respectively. In the lakes in the central and eastern areas hosting L. arcticus (N=95), the mean value was slightly higher (1.67±1.14 mg L-1). The combination of low water temperature, a short growing season and dilute water low in calcium may explain the near total absence of L. arcticus in the western part of Hardangervidda. All lakes contain brown trout Salmo trutta, and as L. arcticus is heavily sought for as food, the analyses of fish stomachs are the most reliable method of detecting the species. However, this prey-predator relationship may severely reduce the population of L. arcticus, and their presence may also be a function of the proximity of species refugia. This is evident in the context of fish predation, but also of water quality. Hence, in the central and eastern parts of the plateau, where L. arcticus is common, their occurrence increased significantly with lake size, being found in 54% of the lakes <1.0 km2, as opposed to 97% in the bigger lakes. Furthermore, L. arcticus is most frequently found in lakes at altitudes between 1100 and 1299 m a.s.l. We conclude that environmental constraints limit the distribution of L. arcticus on Hardangervidda. The projected increase in temperature towards the end of this century may exterminate L. arcticus from the lower parts of Hardangervidda, especially in the most shallow lakes. Many of the lakes have water quality with pH <6.0 and calcium concentration <1.0 mg L-1. In such lakes L. arcticus is living on the edge of its survival, and the projected increase in precipitation may dilute the waters even further, pushing L. arcticus nearer to its extinction threshold.

A record of Notostraca on Socotra Island and the importance of local conservation of the habitat

The first record of Triops Schrank, 1803 (Crustacea: Branchiopoda: Notostraca: Triopsidae) from Socotra Island is presented herein. Besides Madagascar and the current note, the genus is unknown from Indian Ocean islands. A brief morphological exploration indicates that the Socotran Triops cf. granarius (Lucas, 1864) (trachyaspis form) belongs to an African clade within this widespread Old World species complex and not to T. granarius s.str. from East Asia. Detailed morphological and molecular revision of the poorly studied Middle Eastern and Northern African populations in comparison to East Asian populations are needed to understand this group and the phylogenetic position and status of the Socotran tadpole shrimp. Found in a single locality (the archeological site Eriosh) in the rapidly changing coastal plains on Socotra, together with other large branchiopods (endemic Anostraca and unidentified Spinicaudata), the insular Triops population and the regionally neglected temporary lake habitat deserve a special protection status from a biodiversity conservation perspective. Triops cf. granarius is hereby suggested as a local flagship species for the conservation of temporary pool habitats on the island.