Guide to Opisthobranchs Aplysiida

Aplysiida

Aplysiids (formerly Anaspideans) are a fairly morphologically homogeneous group and contains the larger species within opisthobranchs, the genus Aplysia, commonly known as sea hares. They are herbivores and feed on different species of algae and seagrasses. About 80 species are known worldwide.

Aplysia punctata by Enric Madrenas by Enric Madrenas

The aplysiids are well cephalized, their head showing a pair of anterior labial tentacles, a pair of rolled rhinophores and a pair of eyes. The rest of the head is attached to the body by a more or less extensible neck. The visceral mass is bulky and has a pair of mantle extensions or parapods that are usually folded over it for protection. The shell is very small, fragile and covered by a fold of the mantle, some species lack of any shell. In Akeridae the shell is external and poorly calcified, and the animal can not completely withdraw into it. The mantle cavity is a groove located on the right side of the body, between the mantle and the base of the parapod, it is wide open to the outside and contains the gill, which is of the folded type. The anus leads to the back of the pallial groove, and it is usually surrounded by a fold of the mantle forming the anal siphon. The pallial gland (or purple gland) and the hypobranchial gland (or opaline gland) are located inside the mantle cavity, and they are capable of secreting toxic substances used in the defense of the animal. The gastrointestinal tract is characterized by having a gizzard full of hard chitinous plates. The aplysiids are hermaphrodite like most of the opisthobranchs, and their reproductive system is characterized by having a the genital opening separated from the penis, and between them there is a ciliated groove along the sperm transferred in the intercourse must move.

Biology
The Aplysiida are a group of little biodiversity, but some of the species are very important in shallow infralittoral algal communities where they graze on various photophilic seaweeds and also because of their great size. Sea hares have been known for more than 2,000 years, mainly by the poisonous properties of their secretions and, for example, back in the first century AD, Pliny the Elder called Lepus marinus (sea hare) to specimens of the genus Aplysia by the similarity of the animal’s rhinophores with the ears of terrestrial hares. European sea hare species like Aplysia fasciata, A. punctata and A. depilans can reach over 20 cm in length, and the Californian Pacific species Aplysia vaccaria can exceed 70 cm in length, with a weight over 10 kg. They usually have nocturnal habits so they usually remain inactive during the day. Some species may bury themselves in the sand for protection. The great development of the mantle parapodia allows certain species like Akera bullata and Aplysia fasciata to swim. All aplysiids are herbivores and feed on different species of seaweeds (Laurencia, Dictyota, Sphaereococus, Ulva, Enteromorpha, Polysiphonia in European waters) and seagrass, and they can sequester their pigments resulting in a near perfect crypsis on the substrate, for example Petalifera petalifera or Phyllaplysia depressa are almost undistinguishable over leaves of Posidonia oceanica. Sea hares can expel certain secretions from the purple gland and opaline gland when disturbed, producing a purplish dye in the first case and milky white dye in the second. This is a clearly defensive mechanism with the mission to repel most of the possible predators. Certain secondary metabolites have been isolated from the tissues of some species, these substances have clear ichthyotoxic activity, and could be related to the chemical defense of the animal. Like other opisthobranchs, the aplysiids are hermaphroditic (they are both male and female) so mating is reciprocal. For reproduction, sea hares tend to congregate in shallow bottoms and can practice a chained intercourse with the participation of several individuals. The first of the chain acts as a female to receive sperm from the following, the last of the chain acts as male, producing sperm for the animal located in front of it, while all other individuals in the chain give sperm to the animal in front of them, and receive sperm from the animal behind. The spawn consists on a long transparent mucilaginous cord filled with numerous (over 100,000 cited for Aplysia punctata) yellow or orange colored eggs. The eggs hatch larvae veligers that feed on plankton. Some species of sea hares are used for food in China and Hawaii and Aplysia californica is commonly used in neurophysiology experiments because of the presence of giant axons in the nervous system.

Distribution
Most Aplysiida species live in temperate or warm waters, and very few species are found in cold water, with a special case as Aplysia punctata that can live along the Greenland coast. Some species such as Aplysia parvula and A. dactylomela have an anfiatlantic distribution, and Aplysia argus, a very similar species has been reported all around the Indo-Pacific. Other species such as Aplysia juliana and Dolabrifera dolabrifera are circumtropical, living both in the Indo-Pacific and tropical Atlantic waters. The cosmopolitan species Bursatella leachii has endured a Lessepsian migration since the early XXth century and can be found around the Mediterranean Sea; B. leachi has also been reported in the western Mediterranean basin, including the Catalan coast.

Classification
The order of the Aplysiida (formerly known as Anaspidea or Aplysiomorpha) consists of two superfamilies, the Akeroidea and the Aplysioidea, each one containing a single family. The Akeroidea comprises only 5 species of the genus Akera. They are primitive opisthobranchs that were originally included among Cephalaspidea. They have a slightly calcified outer shell, and a long body modified for digging in soft sediments. The cephalic region is small and have no oral tentacles nor rhinophores. Some species such as A. bullata can swim with their parapodia. The Aplysioidea, commonly known as “sea hares”, consist of 12 genus, among which genus Aplysia has over 30 different species. Measures are highly variable, from less than 1 cm to over 70 cm. They generally have a large flattened head, with a pair of chemoreceptive rhinophores and a pair of tentacles on each side of the mouth. The shell, when present, is reduced to a poorly calcified and inner shield covering the back of the visceral mass, including the gills and the heart.

The updated taxonomy of the Aplysiida, according to Bouchet el al (2017) and accepted by WoRMS is:

Aplysiida

Akeroidea Mazzarelli, 1891
- Akeridae Mazzarelli, 1891
  - Akera O. F. Müller, 1776
Aplysioidea Lamarck, 1809
- Aplysiidae Lamarck, 1809
  - Aplysia Linnaeus, 1767
  - Bursatella Blainville, 1817
  - Dolabella Lamarck, 1801
  - Dolabrifera J. E. Gray, 1847
  - Notarchus Cuvier, 1816
  - Petalifera J. E. Gray, 1847
  - Phycophila A. Adams, 1861
  - Phyllaplysia P. Fischer, 1872
  - Stylocheilus A. Gould, 1852
  - Syphonota H. Adams & A. Adams, 1854