Sailfish generate foraging opportunities for seabirds in multi-species predator aggregations.

While various marine predators form associations, the most commonly studied are those between subsurface predators and seabirds, with gulls, shearwaters or terns frequently co-occurring with dolphins, billfish or tuna. However, the mechanisms underlying these associations remain poorly understood. Three hypotheses have been proposed to explain the prevalence of these associations: (1) subsurface predators herd prey to the surface and make prey accessible to birds, (2) subsurface predators damage prey close to the surface and thereby provide food scraps to birds, and (3) attacks of underwater predators lower the cohesion of prey groups and thereby their collective defences making the prey easier to be captured by birds. Using drone footage, we investigated the interaction between Indo-Pacific sailfish (Istiophorus platypterus) and terns (Onychoprion sp.) preying on schooling fish off the eastern coast of the Malaysian peninsula. Through spatio-temporal analysis of the hunting behaviour of the two predatory species and direct measures of prey cohesion we showed that terns attacked when school cohesion was low, and that this decrease in cohesion was frequently caused by sailfish attacks. Therefore, we propose that sailfish created a by-product benefit for the bird species, lending support to the hypothesis that lowering cohesion can facilitate associations between subsurface predators and seabirds.

California sea lions interfere with striped marlin hunting behaviour in multi-species predator aggregations.

The open ocean offers a suite of ecological conditions promoting the occurrence of multi-species predator aggregations. These mixed predator aggregations typically hunt large groups of relatively small and highly cohesive prey. However, the mechanisms and functions of these mixed predator aggregations are largely unknown. Even basic knowledge of whether the predator species' interactions are mutualistic, commensal or parasitic is typically missing. Moreover, recordings of attack and capture rates of marine multi-species predator aggregations, which are critical in understanding how and why these interactions have evolved, are almost completely non-existent owing to logistical challenges. Using underwater video, we quantified the attack and capture rates of two high-trophic level marine predators, California sea lions (Zalophus californianus) and striped marlin (Kajikia audax) attacking schools of fishes in the Southern California Current System, offshore the Baja California Peninsula. Recording over 5000 individual attacks across 13 fish schools, which varied in species, size and predator composition, we found that sea lions kleptoparasitized striped marlin hunts and reduced the frequency of marlin attacks and captures via interference competition. We discuss our results in the context of the phenotypic differences between the predator species and implications for a better understanding of multi-species predator aggregations. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Mechanisms of prey division in striped marlin, a marine group hunting predator.

Many terrestrial group-hunters cooperate to kill prey but then compete for their share with dominance being a strong predictor of prey division. In contrast, little is known about prey division in group-hunting marine predators that predominately attack small, evasive prey (e.g. fish schools). We identified individual striped marlin (Kajikia audax) hunting in groups. Groups surrounded prey but individuals took turns attacking. We found that competition for prey access led to an unequal division of prey among the predators, with 50% of the most frequently attacking marlin capturing 70-80% of the fish. Neither aggression, body size nor variation in hunting efficiency explained this skewed prey division. We did find that newly arrived groups of marlin gained on average more access to the prey. This raises the possibility that newly arrived marlin were hungrier and more motivated to feed. However, this result does not necessarily explain the unequal prey division among the predators because the skew in prey captures was found at the level of these groups. Dynamic prey division is probably widespread but under-reported in marine group-hunters and the inability of individuals to monopolize prey until satiation likely reduces the importance of social hierarchies for prey division.

[Duplication cyst of the stomach antrum as the cause of an acute gastric emptying disorder in an adult]. / Duplikationszyste des Magenantrums als Ursache einer akuten Magenentleerungsstörung beim Erwachsenen.

HISTORY AND CLINICAL FINDINGS: A 35-year old woman without previous history of gastrointestinal complaints developed increasing upper abdominal pain with nausea and postprandial vomiting over 2 days during which she had been unable to take any food. She had no fever; her general physical condition was slightly reduced. While her abdomen felt soft on palpation, she had epigastric pain on pressure but no resistance to palpation. INVESTIGATIONS: Blood picture and routine laboratory tests were unremarkable. Plain film of the abdomen demonstrated marked gastric retention. Abdominal ultrasound showed an invagination in the gastric antrum suggesting a layering phenomenon. Gastroscopy revealed a submucosal tumour-like lesion with central ulceration which obstructed gastric exit. TREATMENT AND COURSE: After conclusion of the diagnostic tests on the first hospital day partial gastric resection (Billroth I) was performed as, in the absence of any surrounding reaction, a leiomyoma had been suspected. The further course was without complication and she was discharged on the 13th hospital day. The surgical specimen revealed a duplication cyst of the gastric antrum. CONCLUSION: (Endo)sonography, computed tomography and endoscopy make it possible to diagnose duplication cyst, as long as it is included in the differential diagnosis of acute abdomen in an adult. Emergency resection of the cyst is indicated because of the acute nature of the symptoms. But even in less urgent cases or as an incidental finding it is the treatment of choice.