Journal of Environmental Biology

Comparative fertilization and morphological studies of the recently speciated tropical sea urchins (Echinometra spp.) on the coral reefs of Okinawa and Hawaii

M. Aminur Rahman^1,3*, Y. Arakaki², Sang-Go Lee¹ and Fatimah Md. Yusoff³

¹World Fisheries University Pilot Programme, Pukyong National University (PKNU), 45 Yongso-ro, Nam-gu, Busan 48513, Korea

²Department of Tourism, Faculty of International Studies, Meio University, Nago, Okinawa-905-8585, Japan

³Laboratory of Marine Biotechnology, Institute of Bioscience, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

*Corresponding Author E-mail: aminur1963@gmail.com

Key words

Echinometra

Fertilization

Gamete incompatibility

Morphology

Sea urchins

Speciation

Publication Data

Paper received : 20.02.2017

Revised received : 15.05.2017

Re-revised received : 25.06.2017

Accepted : 28.12.2017

Abstract

Aim: The Indo-Pacific genus of tropical sea urchin Echinometra is composed of at least five reproductively isolated species. Among them, two sympatric species, the brownish Echinometra sp. C (Ec) and the black Echinometra oblonga (Eo) inhabit Okinawan reef margins and the allopatric black Echinometra sp. E (Ee) commonly occurs on Hawaiian reef flats. To investigate the mechanism(s) for reproductive isolation and speciation, a detailed cross-fertilization and morphological studies were conducted among the sympatric Ec and Eo and allopatric Ee. ????

Methodology: Gametes were obtained from three species of sexually matured Echinometra (Ec, Eo and Ee) after inducing spawning by injecting of 0.5 M KCl into the coelomic cavity. Cross-fertilization among them was conducted using all possible combinations of ova and sperm at room temperatures (27-28?C). Fertilization rate at different sperm concentrations was estimated at 1.25-1.5 hrs after gamete mixing by counting the number of embryos reaching 2-4 cells among the first 100 eggs observed. The morphological characteristics for describing the differences were recorded or measured from Ec Eo and Ee were: color patterns of oral and aboral spines and test, gamete sizes, spicules in the tubefeet and gonads, pedicellaria valve length, Aristotle`s lantern, test sizes, spine lengths and pore pairs. ?????

Results: Cross-fertilization rates with limited sperm concentration were highest between sympatric Ec and Eo, lower between allopatric Ec and Ee, and lowest between allopatric Eo and Ee. The lower fertilization success between these allopatric Hawaiian and Okinawan sea urchins indicates that gamete recognition molecules are diverging due to genetic drift, and not by reinforcement. Such a system might eventually lead to complete gametic incompatibility.? Morphological characteristics of gametes, spines, pedicellaria valve length, Aristotle`s lantern, and gonad and tubefoot spicules of Ee differed more from those of Eo than Ec, while other characteristics such as color patterns, test sizes and pore pair ratios of Ee were closer to Eo.

Interpretation: The closer affinity of majority of the characteristics of Ee to those of Ec, as well as the relative gamete compatibilities indicate that they evolved into distinct species after the separation of E. oblonga.

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