Larval distribution of the ocean sunfishes Ranzania laevis and Masturus lanceolatus (Tetraodontiformes: Molidae) in the Sargasso Sea subtropical convergence zone

Abstract

Sunfishes or Molidae are a rarely encountered family within the teleost order Tetraodontiformes and most details of their reproductive biology including times and places of spawning and their larval ecology are rather unclear. Spawning of two species of Molidae was suggested in the Sargasso Sea before, yet comprehensive data on larval distribution from this area or elsewhere have never been published. Here we report on the abundance and size distribution of 383 sharptail mola (Masturus lanceolatus) and slender sunfish (Ranzania laevis) larvae, present novel information on their larval growth and development and test correlations with prevailing hydrographic data. Only 18 mostly larger Masturus larvae were caught evenly distributed over the study area and with no obvious hydrographic preferences. We conclude that there was no active spawning of M. lanceolatus in the area during the time of the cruise. In contrast, Ranzania larvae were caught primarily inside and south of a thermal frontal zone with increasing abundances toward warmer surface layers in the southeast of the study area. Due to the consistent presence of young Ranzania, it can be assumed that spawning activity was ongoing throughout the month of April, 2015. Our findings confirm the Sargasso Sea as a spawning area for R. laevis.

Spatiotemporal segregation of ocean sunfish species (Molidae) in the eastern North Pacific

“…4D) closely corresponds to the long-term average latitude (30°−32°N) of the subtropical convergence zone north of Hawai'i (Roden 1980, Dinniman & Rienecker 1999. This is consistent with the distribution of slender mola larvae inside or south of a thermal front within the Sargasso Sea subtropical convergence zone at nearly the same latitude in the North Atlantic (Hellenbrecht et al 2019). Furthermore, the region from ~15°−28°N and to the west of 150°W, where slender mola catch is concentrated ( Fig.…”

“…These <1 yr old fish (according to growth models by Liu et al 2009) would have been hatched ear-lier in the season, possibly coinciding with the model-predicted summertime peak of adult sharptail mola in the area. Furthermore, there is evidence of segregated spawning activity by sharptail and slender mola in the Sargasso Sea (Hellenbrecht et al 2019). Thus, the temporally disjoint presence of adult sharptail and slender mola near the Hawaiian Islands (this study, Fig.…”

“…While knowledge of individual molid species has increased in recent years, little is known of the environmental niche separation and differences in the reproductive or movement ecology of molids in sympatry. Limited evidence suggests that molid species exhibit distinct distributions in relation to sea surface temperature (SST) and latitude in the western South Pacific (Nyegaard et al 2018b), segregated spawning activity in the western North Atlantic (Hellenbrecht et al 2019), and different patterns in seasonal abundance in the Mediterranean Sea (Dulčić et al 2007). Comparisons of molids in allopatry have noted potential differences in their environmental preferences or behavior but were confounded by the disparate regions or oceans in which the underlying studies were conducted (e.g.…”

Distribution and diel vertical migration of mesopelagic fishes in the Southern Sargasso Sea — observations through hydroacoustics and stratified catches

“…In this study, Cyclothone represents only a minor fraction of the trawl samples, which most likely is due to the large mesh sizes in the codend of the trawl net employed (1.5 cm), as it already was the case in the eastern tropical North Atlantic (Fock and Czudaj 2018;Czudaj et al submitted). Large mesh sizes may also be the reason why Ranzania laevis, a small species of the sunfish family Molidae, was not present in any of the hauls, even though spawning activity and increased larval and post-larval abundance during the March and April in the area were recently described by Hellenbrecht et al (2019). It is noteworthy though that nine post-larval (Molacanthus stage) specimens of Masturus lanceolatus, another fairly unexplored species of Molidae, were caught in this study.…”

“…Subtropical Gyre and surrounded by ocean currents, which can trap the water in the core of the Sargasso Sea for decades (Maximenko et al 2012). This large marine ecoregion is influenced by various hydrographical features, including complex patterns of thermal fronts, mesoscale eddies, advective transport of water masses and seasonal convective overturn, which all have an influence on the biota and lead to a pronounced spatio-temporal variability in planktonic productivity (McGillicuddy et al 1998;Hansell and Carlson 2001;Palter et al 2005;Eden et al 2009;Hellenbrecht et al 2019). Based on the prevailing low nutrient levels, the Sargasso Sea is generally classified as oligotrophic.…”