CARAPIDAE
Eco-morphology in Carapidae (pearlfish)
Carapidae fish are known as pearlfish. The origin of this name would be the discovery of dead carapid fish, paralysed and completely covered in mother of pearl in the inner face of the valves of the shell of certain oysters. Although different species are free-living, the fish belonging to the genus Onuxodon, Carapus and Encheliophis are capable of penetrating and residing inside different invertebrates such as sea-cucumbers, sea-stars, bivalve molluscs and ascideans.
.
The study concerns the fish adaptations in relation with the symbiosis. Topics are mainly the functional morphology, the ecomorphology, the phylogeny, the ontogeny, the evolutive process and the ethology. This research involves also the sound production, the otolith study, the respiratory abilities and the mucus composition.
Diet
The Diet of different Carapini species living with sea cucumbers and starfish was studied using three complementary approaches:
- Stomach content analysis, reflecting the most recent meal.
- Morphofunctional characteristics of feeding (buccal apparatus) and processing (pharyngeal jaws, digestive tract) structures provide indications of an organisms ability to utilize food resources.
- Use of stable isotope ratios of carbon and nitrogen provides complementary data in integrating measure of the dietary components over a much longer period of time than gut content analysis.Carapus species are commensal, using their host as a shelter and leaving it to hunt small prey, such as annelids, crustaceans (shrimp, decapod, amphipod) and small fish. They can also be cannibalistic, feeding on other carapids within their host.
Encheliophis species are parasitic, feeding on the internal tissues of their host, mainly the gonads and the respiratory tree.
Metamorphosis
The Carapini life cycle may be divided into four stages:
(1) the vexillifer larva stage corresponding to the dispersal pelagic period and characterised by a complex specialisation of the dorsal fin called vexillum.
(2) the tenuis stage, marked by the loss of the vexillum and by a considerable lengthening of the body, ends when the fish settles. Tenuis larvae undergo a reduction in body length of about 60% while shifting habitat from pelagic life to the lagoon settlement, where fish find their holothurian hosts for the first time.
The metamorphosis implies a progressively increased decalcification of all vertebral centra along a postero-anterior gradient, the complete disappearance of the posterior vertebrae and associated structures, and an important shortening of vertebral centra, from the anterior to the last remaining ones. The further development of the vertebrae began with ossification of the neural and haemal arches before that of the vertebral body.
Different larvae of C. homei were caught when settling on the reef and kept in different experimental conditions for at least 7 days and up to 21 days: darkness or natural light conditions, presence of sea cucumber or not, and food deprivation or not. Whatever the nutritional condition, a period of darkness seems sufficient to initiate Metamorphosis. Twenty-one days in natural light conditions delayed metamorphosis, whereas the whole metamorphosis process is the fastest (15 days) for larvae living in sea cucumbers.
(3) the juvenile stage, reached after the reduction in length, which gives it an adult-like morphology.
(4) the adult stage – the way of reproduction is actually not known.
Otolith
The Carapini otoliths have a pattern common to teleost fish: they grow by the alternate deposition of matrixdominant (D-unit) and mineral-dominant (L-unit) layers according to a specific process leading to aspecies-specific form. However, sagittal sections revealed a three-dimensional asymmetry with a nucleus close to the proximal surface. The comparison of commensal and parasitic species revealed structural differences that could be related to their life styles.
The sagittae encompass three main ontogenetic zones. The first zone develops during the pelagic life, which ends when tenuis larvae enter the lagoon (blue arrow). The second zone, characterised by the presence of low-contrast and wider increments, corresponds to the metamorphosis of Carapidae. It ends with a post-metamorphic mark (red arrow) beyond which the development of a third zone corresponds to the juvenile and adult periods.
Previous experiment on delayed metamorphosis showed that the formation of a transition zone varied depending on the experimental conditions. Larvae maintained in darkness had an otolith transition zone with more increments (around 80), albeit wider than those (more or less 21) of individuals kept under natural lighting. The presence of a transition zone in delayed-metamorphosis larvae suggests that these otolith changes record the endogenously-induced onset of metamorphosis, whereas body transformations seem to be modulated by the environmental conditions of settlement.
The pelagic larval duration (PLD) corresponds to the period between hatching and settlement on the reef (zone 1). It is estimated on the basis of the increment number in otoliths. Moreover, back-calculations using settlement dates and PLD also permit to encircle the reproduction period. Larval duration variations were observed between four Carapini species (Carapus boraborensis, C. homei , C. mourlani and Encheliophis gracilis), but also within species. The latter involves that the larval developmental state is different at the time of settlement. Differences in settlement periods could be factor limiting competition or improving fitness, the four species being found in the same water, at the same depth, and in certain cases in the same host. This plasticity mainly results from the ability of larvae to delay their settlement on the reef. It is interesting to note that some Carapini larvae show longer larval duration than their conspecifics and that back-calculation shows that their spawning period does not correspond to those of other larvae at the same site. This is consistent with the possibility of larval drift from one reef to another.
t
Sounds
Carapus homei
Copyright © 2002, Laboratoire de Morphologie Fonctionnelle et Evolutive
Webdesign : Yves-Eric Corbisier