Larval development of the pedunculate barnacles Octolasmis angulata Aurivillius 1894 and Octolasmis cor Aurivillius 1892 (Cirripedia: Thoracica: Poecilasmatidae) from the gills of the mud crab, Scylla tranquebarica Fabricius, 1798

Highlights

• The larvae of Octolasmis angulata and Octolasmis cor are morphologically similar.

• Larvae of O. angulata and O. cor shared a number of homologous structures with Lepadidae and some groups of balanomorphans.

• Numerous peculiarities were observed on the naupliar structures of O. angulata and O. cor.

• The setal classification system is changed to better represent setal homologies.

• The cypris larvae retained and modified the naupliar structures to adapt into their life-style.

Abstract

Detailed studies of larval development of Octolasmis angulata and Octolasmis cor are pivotal in understanding the larval morphological evolution as well as enhancing the functional ecology. Six planktotrophic naupliar stages and one non-feeding cyprid stage are documented in details for the first time for the two species of Octolasmis. Morphologically, the larvae of O. angulata and O. cor are similar in body size, setation patterns on the naupliar appendages, labrum, dorsal setae-pores, frontal horns, cyprid carapace, fronto-lateral gland pores, and lattice organs. Numbers of peculiarities were observed on the gnathobases of the antennae and mandible throughout the naupliar life-cycle. The setation pattern on the naupliar appendages are classified based on the segmentation on the naupliar appendages. The nauplius VI of both species undergoes a conspicuous change before metamorphosis into cyprid stage. The cyprid structures begin to form and modify beneath the naupliar body towards the end of stage VI. This study emphasises the importance of the pedunculate barnacle larval developmental studies not only to comprehend the larval morphological evolution but also to fill in the gaps in understanding the modification of the naupliar structures to adapt into the cyprid life-style.

Introduction

The spectacular diversity of the type of larvae produced and larval developmental forms exhibited by barnacles has aroused the interest of understanding the pattern in their life history evolution. In the past few decades, the larval development of barnacles has been studied to scrutinize the comparative character sets for taxonomic studies and larval identification. Studies of early ontogeny and ultrastructural of barnacle larvae have been suggested to be pivotal in understanding the monophyly of barnacles since it mainly depends on the distinctive apomorphies of the larval morphology (Høeg, 1992, Glenner et al., 1995). Moreover, studies of barnacle larval development may provide invaluable information on the functional morphology in feeding and swimming behaviours (Walley, 1969, Anderson, 1994, Walossek et al., 1996) that probably assist in the functional ecological studies. However, there is still lack of information on the larval development of barnacles, particularly the pedunculate barnacles as compared to the acorn barnacles (Anderson, 1994). To date, the larval development of pedunculate barnacles has only been described for 13 species from eight families (Batham, 1945a, Batham, 1945b, Anderson, 1965, Kaufmann, 1965, Lewis, 1975, Lang, 1976, Lang, 1979, Dalley, 1984, Dineen, 1987, Moyse, 1987, Molares et al., 1994, Jeffries et al., 1995, Lee et al., 2000, Yan et al., 2005).

The pedunculate barnacles in the order Pedunculata are very unique in their adult appearances possessing strong and flexible muscular stalks which attach to the substratum with their capitulum covering the brood chamber and feeding appendages (Anderson, 1994). Buckeridge and Newman (2006) have revised the order Pedunculata and the pedunculate barnacles are now divided into four new orders: the Cyprilepadiformes, Ibliformes, Lepadiformes and Scalpelliformes based on morphological (type and number of the capitular plate and the types of stalk) and molecular data. They attach to different substrata such as buoy, man-made materials flotsam in the tropical and sub-tropical oceanic waters, with a few species that live on the biotic surfaces. The larvae of these pedunculate barnacles are different in morphology, for instance, naupliar larvae of the pedunculate barnacles from the order Ibliformes (Batham, 1945b, Anderson, 1965, Yan et al., 2005) and Scalpelliformes (Batham, 1945a, Kaufmann, 1965, Lewis, 1975, Dineen, 1987, Molares et al., 1994, Lee et al., 2000) are very similar to the acorn barnacle species from the order Sessilia in having short body length, frontal horns, dorsal thoracic spine, thoraco-abdominal process and naupliar appendages, whereas the naupliar larvae from the order Lepadiformes (Lang, 1976, Lang, 1979, Dalley, 1984, Moyse, 1987, Jeffries et al., 1995) differ from the aforementioned group in having longer body length, frontal horns, naupliar appendages, possessing an elongated dorsal thoracic spine and thoraco-abdominal process.

Members of Octolasmis, Gray 1825 are pedunculate barnacles belonging to the family Poecilasmatidae within the order of Lepadiformes. Hitherto, a total of 43 species of Octolasmis have been found and they are distributed throughout the Pacific, Atlantic Ocean and Indian Ocean (Jones et al., 2000, Chan, 2014, Voris et al., 2014). Of these 43 species, 26 species of Octolasmis have been recorded in the South China Sea and they are found attached on the gills of decapods, on the body of certain marine seasnakes and on the stem of corals (Jones et al., 2000). An interesting fact about Octolasmis species is that they are found as epibionts in the gill chamber of decapods where they complete their entire life-cycle in the branchial chamber of the host (Blomsterberg et al., 2004). However, the larvae of the Octolasmis species have received very little attention as compared to the other free-living barnacles.

To date, the larvae of Octolasmis cor from Phuket Island, Thailand have been briefly described by Jeffries et al. (1995), and the larvae Octolasmis muelleri and Octolasmis forestii have been reported from South Carolina, Southeastern United States and Belize, Central America by Lang, 1976, Lang, 1979. The descriptions of the larvae of O. cor and O. forestii are incomplete as the morphological information of the antennules, antennae and mandibles and setation formulae are not given (Lang, 1979, Jeffries et al., 1995), whereas there is paucity of information on the ultrastructure of the larval morphology of O. muelleri.

In the present paper, we provide comprehensive descriptions and comparisons of the naupliar and cyprid stages of Octolasmis angulata and O. cor with focus on the ontogeny of the naupliar appendages (antennules, antennae and mandible) and cyprid lattice organ. The functional morphology of different larval structures of O. angulata and O. cor is discussed, and the information of the setation sequence patterns of the appendages and morphology of the lattice organ are used to understand the systematic and phylogenetic position of these cirripeds.