Reproduction of the long-spined sea urchin Diadema setosum in the Gulf of Aqaba - implications for the use of gonad-indexes

Yossi Loya,Andreas Kroh,Omri Bronstein

doi:10.1038/srep29569

Yossi Loya, Andreas Kroh + Show 1 more

Open Access

https://doi.org/10.1038/srep29569

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Abstract

As global warming and climate-change proceeds ever more rapidly, organisms depending on seasonal cues to synchronize reproduction face an unclear future. Reproduction in Diadema setosum in the Gulf of Aqaba (Red Sea) is seasonal, with mature individuals occurring from July to October. Gonad indexes (GI), in contrast, indicate that spawning occurs from August through December and suggests two main spawning events. Histological analysis, however, indicate that the second peak of GI values cannot be related to spawning, but rather correspond to recovering individuals. In Diadema, examination of GI values alone may thus lead to erroneous conclusions. GI was moderately-strong positively correlated with sea-surface temperatures, but not with chlorophyll-a concentrations or photoperiod. Spawning coincides with the onset of the annual chlorophyll-a increase, however, which might be advantageous for nutrition of the developing larvae. First significant GI increase coincides with the shortening of day-length, which may act as a cue for D. setosum gametogenesis. Gametogenesis is highly synchronised between sexes, although the mature phase of females exceeds that of males. The non-complete overlap may represent sampling bias or represent an adaptive strategy for enhancing fertilisation success. Skewed sex ratios (♀:♂ 1:0.59, n = 360) in the Gulf of Aqaba may be related to pollution.

Highlights

Sea urchins from the genus Diadema are some of the most widespread, abundant and ecologically important echinoids in tropical regions[1]
It is of special research interest since it differs in its environmental parameters from both the Red Sea Main Basin (RSMB) and the Gulf of Suez (GOS) and has been suggested to have acted as a refugium for the Red Sea fauna during Pleistocene sea-level low-stands and salinity crises[22]
The sex ratios of Diadema setosum from the Gulf of Aqaba (GOA) deviated significantly from a ratio of 1:1 indicating that females are more abundant than males (96 males, 164 females; Chi-square test, x2 = 17.785, df = 1, p < 0.0001), in contrast to the equal ratio recorded in Zanzibar (199 males, 215 females; Chisquare test, x2 = 0.62, df = 1, p = 0.432)

Summary

Introduction

Sea urchins from the genus Diadema are some of the most widespread, abundant and ecologically important echinoids in tropical regions[1]. Stage III (growing): With the onset of vitellogenesis oocytes grow in size and become decreasingly basophilic Both early and late vitellogenetic oocytes may be present along the ovarian wall and gradually migrate to the ovarian lumen as they mature (indicated by arrow) (e). All stages of germ cells are evident in the male germinal epithelium and continuously increase in number as new spermatogonia develop basally while spermatocytes migrate to the testicular lumen, where they accumulate as mature spermatozoa, forming visible columns of darkly stained cells (f). Latitudinal range of Diadema setosum facilitates an examination of the ‘equatorial model’, one of the most prevalent paradigms in marine invertebrate reproduction This model predicts that for species with a broad latitudinal distribution, continuous reproduction is expected in the tropics, while a restricted breeding season is expected at higher latitudes[19]. While the reproductive cycle of D. setosum is well studied for the GOS10,12 and the RSMB28, no data are so far available for the GOA

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