Bobtail squid (order Sepiolida) are a group of cephalopods closely related to cuttlefish. Bobtail squid tend to have a rounder mantle than cuttlefish and have no cuttlebone. They have eight suckered arms and two tentacles and are generally quite small (typical male mantle length being between 1 and 8 cm). Sepiolids live in shallow coastal waters of the Pacific Ocean and some parts of the Indian Ocean as well as in shallow waters on the west coast of the Cape Peninsula off South Africa. Like cuttlefish, they can swim by either using the fins on their mantle or by jet propulsion. They are also known as dumpling squid (owing to their rounded mantle) or stubby squid.
Bobtail squid have a symbiotic relationship with bioluminescent bacteria (Vibrio fischeri), which inhabit a special light organ in the squid's mantle. The bacteria are fed a sugar and amino acid solution by the squid and in return hide the squid's silhouette when viewed from below by matching the amount of light hitting the top of the mantle. The organ contains filters which may alter the wavelength of luminescence closer to that of downwelling moonlight and starlight; a lens with biochemical similarities to the squid's eye to diffuse the bacterial luminescence; and a reflector which directs the light ventrally.
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Amphipods
Amphipoda is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. The name amphipoda means "different-footed", and refers to the different forms of appendages, unlike isopods, where all the legs are alike. Of the 7,000 species, 5,500 are classified into one suborder, Gammaridea. The remainder are divided into two or three further suborders. Amphipods range in size from 1 to 340 millimetres (0.039 to 13 in) and are mostly detritivores or scavengers. They live in almost all aquatic environments; 750 species live in caves and the order also includes terrestrial animals and sandhoppers such as Talitrus saltator.
Amphipods are typically less than 10 millimetres (0.39 in) long, but the largest recorded living amphipods were 28 centimetres (11 in) long, and were photographed at a depth of 5,300 metres (17,400 ft) in the Pacific Ocean. Samples from the Atlantic Ocean with a reconstructed length of 34 centimetres (13 in) have been assigned to the same species, Alicella gigantea. The smallest known amphipods are less than 1 millimetre (0.04 in) long. The size of amphipods is limited by the availability of dissolved oxygen, such that the amphipods in Lake Titicaca at an altitude of 3,800 metres (12,500 ft) can only grow up to 22 millimetres (0.87 in), compared to lengths of 90 millimetres (3.5 in) in Lake Baikal at 455 metres (1,500 ft).
Amphipods are found in almost all aquatic environments, from fresh water to water with twice the salinity of sea water. They are almost always an important component of aquatic ecosystems. Most species in the suborder Gammaridea are epibenthic, although they are often collected in plankton samples. Members of the Hyperiidea are all planktonic and marine. Many are symbionts of gelatinous animals, including salps medusae, siphonophores, colonial radiolarians and ctenophores, and most hyperiids are associated with gelatinous animals during some part of their life cycle.
Compared to other crustacean groups, such as the Isopoda, Rhizocephala or Copepoda, relatively few amphipods are parasitic on other animals. The most notable example of parasitic amphipods are the whale lice (family Cyamidae); unlike other amphipods, these are dorso-ventrally flattened, and have large, strong claws, with which they attach themselves to baleen whales. They are the only parasitic crustaceans which cannot swim during any part of their life cycle.
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Pipefish
Pipefishes or pipe-fishes (Syngnathinae) are a subfamily of small fishes, which, together with the seahorses, form the family Syngnathidae.
Pipefish look like straight-bodied seahorses with tiny mouths. The name is derived from the peculiar form of their snout, which is like a long tube, ending in a narrow and small mouth which opens upwards and is toothless. The body and tail are long, thin, and snake-like. They have a highly modified skeleton formed into armored plating. This dermal skeleton has several longitudinal ridges, so that a vertical section through the body looks angular, not round or oval as in the majority of other fishes.
A dorsal fin is always present, and is the principal (in some species, the only) organ of locomotion. The ventral fins are constantly absent, and the other fins may or may not be developed. The gill openings are extremely small and placed near the upper posterior angle of the gill cover.
Many are very weak swimmers in open water, moving slowly by means of rapid movements of the dorsal fin. Some species of pipefish have tails that are prehensile, as in seahorses. The majority of pipefishes have some form of a caudal fin (unlike seahorses), which can be used for locomotion. See fish anatomy for fin descriptions. There are species of pipefish with more developed caudal fins, such as the group collectively known as flagtail pipefish, which are quite strong swimmers.
Pipefishes, like their seahorse relatives, leave most of the parenting duties to the male. Courtship tends to be elaborately choreographed displays between the males and females. Pair bonding varies wildly between different species of pipefish. While some are monogamous or seasonally monogamous, others are not.
