Bronstein O et al. (2016), Reproduction of the long-spined sea urchin Diad...

ECB-ART-44809

Sci Rep 2016 Jul 12;6:29569. doi: 10.1038/srep29569.

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

Bronstein O , Kroh A , Loya Y .

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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.

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Genes referenced: gnao1 LOC100887844 LOC100893907 LOC590297

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	Figure 1. Distribution map and spawning periodicities of Diadema setosum.Locations of known spawning times are indicated by dots. Radial plots indicate estimated spawning periods (black portions) for each location. The plots are constructed of 12 equally sized slices representing the monthly annual cycle from January to December (clockwise). In sites of continuous year-round spawning, peak spawning months are indicated by asterisk (if such distinction was mentioned in the literature). Distribution estimates were based on: Pearse1257; Clark and Rowe58; Marsh and Marshall59; Rowe and Gates60; Shin61; Lessios et al.4; James62; Sastry63; Yokes and Galil64; Nader and El Indary65 as well as data available from the OBIS website (http://www.iobis.org/home). Lowercase letters indicate the underlying publication of spawning periodicities for the particular location (only publications specifically referring to spawning periodicities were considered for the construction of this map). a: Kenya - Muthiga18; b: Al-Ghardaqa - Pearse10; c: Wadi-el-Dom (Gulf of Suez)- Pearse10; d: Port Tawtiq (Gulf of Suez) - Fox46; e: Eilat (Gulf of Aqaba)- this study; f: Kuwait - Alsaffar and Lone17; g: Sichang (Thailand)- Kobayashi45 (report of spawning in a two month study during February and March) ; h: Seto (Japan)- Onoda66, Kobayashi and Nakamura67; i: Misaki (Japan)- Yoshida30; j: Philippines - Tuason and Gomez44; k: Singapore - Hori et al.31; l: Low Isles (Great Barrier Reef) - Stephenson68; m: Rabaul (New Britain Island) - Pearse12 (report of spawning by field observations on 24.02.1965 based on pers. comm. from J. J. Gonor); n: Fiji - Coppard and Campbell14. The map was created based on the Wikimedia Commons public domain map file BlankMap-World6.svg (https://commons.wikimedia.org/wiki/File:BlankMap-World6.svg) and manually edited using CorelDRAW x4.
	Figure 2. Size and sex relationships in populations of Diadema setosum from the Red Sea (Eilat) and Western Indian Ocean (Zanzibar).Linear regression models of diameter and weight (log transformed) in two populations of D. setosum: (a) Eilat, (b) Zanzibar, and (c) the pooled data from both locations. Females are denoted by red circles; males by green triangles. Regression lines with 95% confidence interval are fitted for each plot and the corresponding equations and respective r2 and p-values are provided. The number of samples used, for both females (nf) and males (nm), is given below the corresponding equations.
	Figure 3. The reproductive stages of Diadema setosum from the Gulf of Aqaba.Histological photomicrographs of ovaries (a,c,e,g) and testes (b,d,f,h). Cross-sections through acini representing reproductive stages I–IV. Stage I (spent): Gonads are largely devoid of contents showing ova-free lumen in females (a), and spermatozoan-free lumen in males (b) and may contain unspawned ova and spermatozoa undergoing lysis. A thin layer of NPs is present along the ascinal walls in both sexes and may form a pale meshwork across the ascinus. Strongly basophilic previtellogenetic oocytes or primary spermatocytes, staining dark purple with Hematoxylin and eosin, may be present along the ascinal wall. Stage II (recovering): NPs proliferate throughout the gonads from the ascinal wall to the centre, gradually filling the lumen of ovaries (c) and testis (d). Limited groups of primary spermatocytes and clusters of previtellogenetic oocytes start appearing in the testicular and ovarian germinal epithelia, respectively, and may occasionally project centrally. 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). NP deplete and progressively occupy less space in both males and females. Stage IV (mature): By the end of this stage the NP layer in both ovaries and testes is largely exhausted. Ovaries are packed with mature ova, while oocytes at different maturation stages may still be evident in the germinal epithelium (g). The testicular lumen is densely packed with spermatozoa. Occasionally some ova and spermatozoa may be evident in the coelom (h). Scale bars represent 100 μm. Ge germinal epithelium; C coelom; Po previtellogenetic oocyte; EVo early vitellogenetic oocyte; LVo late vitellogenetic oocyte; NP nutritive phagocytes; Ov ova; L lumen; Sc spermatocytes; Sz spermatozoa; Ps primary spermatocytes.
	Figure 4. The annual gametogenetic cycle of Diadema setosum from Eilat (Gulf of Aqaba).The relative frequencies (%) of gonad developmental stages in monthly samples of (a) females and (b) males as defined by histological cross-sections. Frequencies are based on histological analysis of 20 specimen per month studied from January 2010 to February 2011. Colours indicate reproductive stages I–IV (corresponding to stages: Spent, Recovering, Growing and Mature, respectively) as defined in the text (see Fig. 3 for detail). ND corresponds to no data for that sampling month. Radial schematic plots provide a graphical representation of the transition and overlap of the different reproductive stages based on a monthly annual cycle.
	Figure 5. Temporal patterns of environmental gradients during the reproductive cycle Diadema setosum.Monthly gonad index (GIW) calculated as wet gonad weights/total wet body weight × 100 of Diadema setosum from the GOA and monthly gradients of selected environmental variables. Boxes represent monthly GIW; centre black lines show the medians; box limits indicate the 25th and 75th percentiles; the 95% confidence interval of each median is represented by the notches and is defined as +/−1.58 × IQR/sqrt(n) (with n representing the monthly number of samples); whiskers extend to minimum and maximum values with open circles representing outliers; width of the boxes is proportional to the square root of the sample size. Indexes calculated based on 20 specimens per month. No data are available for January 2011. The colour of the boxplots corresponds to the dominant reproductive stage of that month (see Fig. 4 for details). Grey zone illustrates photoperiod. Red line illustrates daily measured sea surface temperatures (°C) and green line illustrates daily measured chlorophyll-a concentrations (μg/l) fitted as a smooth curve (solid lines) and standard errors (shaded margins). The smooth was calculated by local polynominal regressions.
	Figure 6. Temporal variation in ova and oocyte diameters.Diameters (μm) of ova (red boxes) and oocytes (gold boxes) from female Diadema setosum from the GOA. Measurements conducted from January 2010 through February 2011. Boxes represent monthly average oocyte diameters; centre black lines show the medians; box limits indicate the 25th and 75th percentiles; the 95% confidence interval of each median is represented by the notches and is defined as ±1.58 × IQR/sqrt(n) (with n representing the number of samples as indicated under the boxes); whiskers extend to minimum and maximum values with open circles representing outliers; width of the boxes is proportional to the square root of the sample size.

References [+] :

Chow, DNA barcoding and morphological analyses revealed validity of Diadema clarki Ikeda, 1939 (Echinodermata, Echinoidea, Diadematidae). 2016, Pubmed, Echinobase