Tankovich A.E.*, Kalachev A.V. 2026. Serotonin-positive elements are absent from nervous system in adult rudiment of sea urchins // Invert. Zool. Vol.23. No.2: 325–337 [in English].
Laboratory of Embryology, A.V. Zhirmunsky National Scientific Center of Marine Biology, Far Eastern Branch, Russian Academy of Sciences, Palchevskogo Str. 17, Vladivostok 690041 Russia.
* Corresponding author
Alina Tankovich: alytankovich@gmail.com https://orcid.org/0000-0002-0892-0149
Alexander Kalachev: akalachev@imb.dvo.ru https://orcid.org/0000-0003-2392-3636
doi: 10.15298/invertzool.23.2.08
ABSTRACT: The literature data on the presence of serotoninergic neurons in the nervous system within the developing adult rudiment in the larvae of sea urchins are controversial. Studies of the sea urchins with planktotrophic larvae showed that the adult rudiment in competent larvae lacks any serotoninergic neurons. However, it is unclear whether any serotoninergic neurons are present within the developing adult rudiment at early stages of the rudiment development. Besides, in a sea urchin species with lecithotrophic larvae, serotoninergic neurons were observed in the adult rudiment at early stages of its development, but these neurons disappeared later. The present work aims to verify whether serotoninergic neurons are absent from the adult rudiment over the full course of its development. The study was conducted on planktotrophic larvae of three sea urchin species, Mesocentrotus nudus (A. Agassiz, 1864), Strongylocentrotus intermedius (A. Agassiz, 1864) (regular sea urchins from the order Camarodonta), and Scaphechinus mirabilis A. Agassiz, 1864 (an irregular sea urchin from the order Echinolampadacea). The results have shown that the developing nervous system in the adult rudiment lacks any serotonin-positive elements throughout its development. Furthermore, no serotonin-positive processes projecting from the larval nervous system into the adult rudiment have been found from the very first stages of the rudiment development. Our results contribute to the existing data on regular sea urchins. In addition, we, for the first time, provide evidence of the absence of serotonin-positive elements in the developing adult rudiment of an irregular sea urchin species, S. mirabilis. However, a study of 4-day-old juveniles of M. nudus showed that serotonin-positive processes are present in radial nerves and circumoral nerve ring, in the center of the adhesive disk of primary and secondary podia as well as the base of adult spines. The distribution of serotonin-positive elements in 4-day-old juveniles of S. mirabilis followed that of M. nudus, except for the circumoral nerve ring, where no serotonin-positive elements were observed. We suggest that because serotonin-positive elements are absent from the developing adult rudiment in the larvae, it is possible that the development of these elements in the nervous system of sea urchins begins after larval settlement and metamorphosis.
KEY WORDS: echinoderms, sea urchins, larval development, nervous system, serotonin.