Background
Costasiella ocellifera “is a small (~13 mm) species of sea slug, a shell-less marine opisthobranch gastropod mollusk in the family Costasiellidae.” Costasiella ocellifera, and other members of the Costasiellidae family are often mistakenly classified as nudibranchs because they superficially resemble other species of that clade, but they are actually a part of the Sacoglossa superorder of sea slugs, also known as the “sap-sucking sea slugs” or the "solar-powered sea slugs." The Brazilian species, Costasiella liliana, is a synonym of C. ocellifera.
Etymology
The name 'ocellifera'' comes from the Latin word for eye, ''ocellus'' and the Latin word "ifer" which means "to carry" and is in reference to the blue eyespot found on the top of Costasiella ocellifera’s head.
Taxonomy
Description
The body of C. ocellifera is a white, somewhat translucent color and speckled with small black dots. A majority of the upper surface of the body is covered in pointed cerata. Similar to the body, the cerata is also transparent, however, they contain digestive gland ducts, so it appears green because of the chloroplasts the sea slug has digested. The tips of the cerata are often white, but some can also have an orange-yellow colored ring towards the top of the cerata as well. The cerata is also speckled with small black, white and blue iridescent spots. The top of the head is an orange-yellow color, and it has two large horn-like rhinophores. The rhinophores are semi-transparent, but towards the tip they appear black due to large clusters of black dots. They have two distinct black eyes in between the rhinophores. The area around the eyes is almost always completely white and is sometimes described as resembling an eye-mask. Directly behind the eyes and rhinophores is a blue eyespot that is distinguishing characteristic of this species. These slugs, like all other sacoglossan slugs, only have a radula with one row of teeth that allow the slugs to skillfully punctures the cell wall of the algae, Avrainvillea.
Distribution/habitat
This species of Costasiella has only been found within the tropical/subtropical Atlantic Ocean region, but within that region it has been found in several places, including, the Bahamas, Belize, Bermuda, Dominican Republic, Florida, Honduras, Jamaica, and some areas of Mexico.
Costasiella ocellifera inhabits the subtidal photic zone and is almost always found on the plant species Avrainvillea nigricans. In fact, the distribution of A. nigricans affects the distribution of C. ocellifera.
Characteristics
Theses slugs do not have protective shells, so in order to protect themselves from predators, C. ocellifera was able to develop a chemical defense system by using secondary metabolites that come from the algae they feed on, Avrainvillea. Using TLC chromatography, scientists were able to identify the metabolite as avrainvilleol. This chemical defense is so effective that in a laboratory test, several slugs were placed into a tank filled with predatory fish (bluehead wrasses). It was observed that whenever a sea slug was eaten by a fish it was almost immediately spit back out, unharmed.
In addition to the chemical defense, these sea slugs have also evolved an effective camouflage defense because when they digest the algae chloroplasts their cerata turn green in color which helps them blend in with their surroundings.
Costasiella ocellifera also shows long-term retention of functional kleptoplasty, which means that they have the ability to ingest functioning chloroplasts from algae, and survive using photosynthesis for up to 65 days when starved.
Kleptoplasty/feeding
Life cycle/reproduction
This species of sea slug, along with all other sacoglossan species, are simultaneous hermaphrodites. Meaning, they possess the reproductive organs of both sexes, and produce both types of gametes. They reproduce through internal fertilization, and then lay their eggs in a unique spiral pattern on the plants that they eat, Avrainvillea nigricans. This species is unique because they are poecilogony, which means they have two different types of larval development. In several studies it was shown that C. ocellifera can have both lecithotrophic (aplanktonic) and planktotrophic larval development. Lecithotrophic larvae feed from the egg reserves, such as the yolk sacs. Planktotrophic larvae, on the other hand, feed on plankton in the water column. For approximately a month after hatching, the planktotrophic larvae are free-swimming, while the lecithotrophic larvae go through their metamorphoses process while in their eggs.
Phylogeny
Results of a study based on the analysis of almost 3,000 base pairs of DNA from four genetic loci which suggest the following evolutionary relationships of the Costasiella species:
