Degrees of honesty: cleaning by the redlip cleaner wrasse Labroides rubrolabiatus

Cleaning symbioses among coral reef fishes are highly variable. Cleanerfishes vary in how much they cooperate with (i.e. remove only ectoparasites) or cheat (i.e. bite healthy tissue, scales or mucus) on their fish clients. As a result, clients use various strategies to enforce cooperation by cleaners (e.g. punishment or partner choice), and cleaners use tactile stimulation to manipulate cheated client behaviour. We provide the first detailed observations of cleaning behaviour of the redlip cleaner wrasse Labroides rubrolabiatus and ask where interactions with this cleanerfish lie on the continuum of cleanerfish honesty, client control, and cleanerfish manipulation. Ninety per cent of redlip cleaner wrasses took jolt-inducing cheating bites from their clients, but they did so at a very low rate (~ 2 jolts per 100 s inspection). Retaliatory chases by clients were uncommon. Three-quarters (30 of 40) of cleaner wrasses used tactile stimulation on their clients, but rarely did so to reconcile with cheated clients. Instead, the majority (70%) of tactile stimulation events targeted a passing client that then stopped for inspection. The relationship between redlip cleaner wrasses and their clients appears to be less conflictual than those documented in other Labroides cleanerfishes. Future studies should test whether this low level of conflict is consistent across space and time and is underpinned by a preference for ectoparasites over other client-gleaned items. As an active cleaner that appears to take few cheating bites from their clients, L. rubrolabiatus has the potential to be as important a driver of fish health and community structure on coral reefs as its better-known relatives.

     

Hypoxia impairs visual acuity in snapper (Pagrus auratus)

We investigated the effect of environmental hypoxia on vision in snapper (Pagrus auratus). Juvenile snapper inhabit estuarine environments where oxygen conditions fluctuate on a seasonal basis. Optomotor experiments demonstrated that visual acuity is impaired by environmental hypoxia, but not until levels approach the critical oxygen tension (P _crit) of this species (around 25 % air-saturated seawater). In 100, 80, and 60 % air-saturated seawater, a positive optomotor response was present at a minimum separable angle (M _SA) of 1°. In 40 % air-saturated seawater, vision was partially impaired with positive responses at M _SAs of 2° and above. However, in 25 % air-saturated seawater, visual acuity was seriously impaired, with positive responses only present at M _SAs of 6° and above. Snapper were found to possess a choroid rete, facilitating the maintenance of high ocular oxygen partial pressures (PO₂) during normoxia and moderate hypoxia (PO₂, between 269 and 290 mmHg). However, at 40 and 25 % water oxygen saturation, ocular PO₂ was reduced to below 175 mmHg, which is perhaps linked to impairment of visual acuity in these conditions. The ability to preserve visual function during moderate hypoxia is beneficial for the maintenance of a visual lifestyle in the fluctuating oxygen environments of estuaries.     

Observations of below-ground characteristics of young redwood trees (Sequoia sempervirens) from two sites in New Zealand – implications for erosion control

Aims

Radiata pine (Pinus radiata D. Don) plantations are widely used to control erosion in New Zealand. However, other species with similar growth but longer rotation lengths and ability to coppice may offer future alternatives to radiata pine. Comparing performance of alternative species to radiata thus becomes important if policy is to be developed to promote them.

Methods

The below-ground characteristics (roots) of young redwood (Sequoia sempervirens (D. Don) Endl.) trees from two established plantations in New Zealand were examined and compared with those of radiata pine, and selected poplar and New Zealand native species.

Results

Roots with diameters less than 10 mm comprised over 99 % of total root length in 3-yr-old trees and 98 % of total root length in 4-yr-old trees. For roots greater than 2 mm in diameter, total root length of young redwood trees was greater than that of young radiata pine, poplar and the best performing New Zealand native plant. Total root length at a given root collar diameter for young (1–4 year old) redwood trees was significantly greater than for radiata pine trees. Roots of redwoods were finer and more numerous than for radiata but the below-ground biomass for a given root collar diameter showed no statistical difference between the two species.

Conclusions

Redwood, because of its comparable growth rate and the production of many fine lateral roots, has the potential to become a keystone erosion-control species in New Zealand, especially on steep lands where there is an increased risk of post-harvest landsliding associated with moderate to severe rainstorm events.     

Broad Salinity Tolerance as a Refuge from Predation in the Harmful Raphidophyte Alga Heterosigma akashiwo (Raphidophyceae)

The ability of harmful algal species to form dense, nearly monospecific blooms remains an ecological and evolutionary puzzle. We hypothesized that predation interacts with estuarine salinity gradients to promote blooms of Heterosigma akashiwo (Y. Hada) Y. Hada ex Y. Hara et M. Chihara, a cosmopolitan toxic raphidophyte. Specifically, H. akashiwo's broad salinity tolerance appears to provide a refuge from predation that enhances the net growth of H. akashiwo populations through several mechanisms. (1) Contrasting salinity tolerance of predators and prey. Estuarine H. akashiwo isolates from the west coast of North America grew rapidly at salinities as low as six, and distributed throughout experimental salinity gradients to salinities as low as three. In contrast, survival of most protistan predator species was restricted to salinities >15. (2) H. akashiwo physiological and behavioral plasticity. Acclimation to low salinity enhanced H. akashiwo's ability to accumulate and grow in low salinity waters. In addition, the presence of a ciliate predator altered H. akashiwo swimming behavior, promoting accumulation in low‐salinity surface layers inhospitable to the ciliate. (3) Negative effects of low salinity on predation processes. Ciliate predation rates decreased sharply at salinities <25 and, for one species, H. akashiwo toxicity increased at low salinities. Taken together, these behaviors and responses imply that blooms can readily initiate in low salinity waters where H. akashiwo would experience decreased predation pressure while maintaining near‐maximal growth rates. The salinity structure of a typical estuary would provide this HAB species a unique refuge from predation. Broad salinity tolerance in raphidophytes may have evolved in part as a response to selective pressures associated with predation.