Mueller's pearlside

Photophores are glandular organs that, in M. muelleri, are made up of several parts. The photogenic chamber, made up of small, spherical light-producing cells, is split into a subspherical tank and conic projector, embedded inside a reflector made of guanine crystals. Ventral to the photogenic chamber is a cellular lens that is itself covered on the ventral surface by a gelatinous dioptric layer.[4]

Maurolicus muelleri has large eyes with a retina uniquely adapted to the animal's mesopelagic habitat. M. muelleri inhabits surface waters only during twilight hours, requiring acute mesopic vision which in most vertebrates is achieved through combining dim-light rod cells and bright-light cone cells. Fish in the genus Maurolicus have developed a unique photoreceptor where a cone opsin and phototransduction cascade is found in cells transmuted into a rod-like morphology. These rod-like cone receptors are tuned to the blue-shifted mesopic light conditions dominant in M. muelleri's habitat and are likely a more efficient method of mesopic vision than would be feasible with two improperly-functioning photoreceptor types.[5]

Mueller's pearlside is a zooplanktivore, with exact diet composition varying geographically and seasonally. For example, Copepods are the main constituent of their diet in the Sea of Japan[6] and in waters surrounding Korea,[7] with the euphausiid species Euphausia pacifica of secondary importance near Japan.[6] Euphausiids and copepods are the dominant prey items year-round off near the eastern continental slope of Tasmania.[8] In Masfjorden, Norway, copepods are most important in the autumn,[9] while earlier in the year Cladocerans are most important.[10] Amphipods and pteropods have also been reported from stomach contents.[6]

Maurolicus muelleri inhabits a tertiary trophic position[6] and, as such, provides a trophic link between zooplankton and larger predators. A wide range of fish species prey on M. muelleri, including commercially-important species such as albacores, skipjack tuna, hake, and blue whiting.[11][12][13][14] They are also predated on by several cephalopods, including the squids Illex coindetii and Todaropsis eblanae[15] and the octopus Enteroctopus magnificus[16], and marine mammals including common dolphins, sei whales, Bryde's whales, and fin whales.[11][17][18][19]

Due to its trophic position, M. muelleri plays a role as an intermediate or paratenic host to a variety of parasitic taxa, with very few parasites reaching adulthood while infecting the pearlside. For example, in a study that examined 1329 individual Maurolicus muelleri specimens, 3720 parasites were found with only 5 individual adult parasites.[20] Endoparasites recorded from Maurolicus muelleri include the trematodes Derogenes varicose, Brachyphallus crenatus, and Lecithaster confusus, cestodes including Bothriocephalus sp. and Scolex pleuronectis, the nematodes Hysterothylacium aduncum and Anisakis simplex[20][21]. One of the only species to reach adulthood parasitising M. muelleri is the ectoparasitic copepod Sarcotretes scopeli.[20] A "fungoid mass", tentatively identified as being from the protist genus Ichthyophonus, has been identified in pearlsides caught near Australia.[22]

As with many mesopelagic species, Maurolicus muelleri undergoes diel vertical migration (DVM), however this behaviour is more complex and varied in M. muelleri than that descriptor usually entails. The specific nature of this migration can vary seasonally, between years, an across geography, as well as across the ontogeny of individual fishes.[23]

The vertical migration of M. muelleri has been best studied in Masfjorden, where a fifteen-month acoustic survey was undertaken. In Masfjorden, M. muelleri formed distinct scattering layers, with the deepest layer composed of adults and a shallow layer composed of post-larvae.[23] The depth of these scattering layers is thought to be a result of M. muelleri having a "light comfort zone", inhabiting depths where light levels are neither too bright nor too dark. Individual fish may move between scattering layers, indicating that the comfort zones are broader than suggested in the typical isolume hypothesis.[24] Here, M. muelleri displayed consistent, typical DVM patterns (i.e. remaining at depth during the day and ascending to the surface at night) during summer months only, with individuals feeding at dawn and dusk.[23]

Their behaviour, however, varied in the autumn and winter. In years where their Calanus copepod prey, which overwinter at depth, were abundant, adults in the winter delayed their vertical migration until approximately three hours before dawn due potentially to a reduced need to feed at the surface, with some individuals remaining at depth for the entire night, feeding entirely on deep-overwintering prey. Towards the end of the winter, adults underwent interrupted ascents, migrating to depths that were greater than reached during typical DVM as they preyed on deep-wintering prey partway through their seasonal ascent to the surface. Some individuals undertook a reversed DVM during winter, diving to slightly greater depths during daylight hours, to feed on deep-overwintering prey in optimal light conditions.[23] In the Benguela system and in the Gulf of Oman, DVM is known to occur, with fish ascending to within 10m of the surface in response to the first light of dawn before diving into deep waters.[25]

In latitudes where summer nights are short and bright, such as in the Arctic Circle, M. muelleri may school in shallow waters at night to reduce the threat of predation.[26] Maurolicus muelleri in scattering layers can detect predators at distances of several metres during the day, and respond by diving as far as 50m below their original depths at speeds of 15–20 cm/s.[24] While most individuals reside in scattering layers to reduce predation risk, certain "bold individuals" will make forays into shallower waters above scattering layers, presumably in order to feed in move favourable light levels. It is unknown whether these bold individuals are atypical or whether a change in individual state (e.g. hunger) prompts these forays.[24]