Friend or Foe Friend or FoeTRISTAN LOUGHER B.Sc. graduated from Manchester University in 1992 with a Part 7 – The Crustaceans degree in Zoology. He has worked at Cheshire Waterlife for five years. DIGITAL IMAGES BY THE AUTHOR any moons ago I was sitting in a study was to identify all the species of Phylum Arthropoda drab lecture theatre in the invertebrate life found there. These Crustaceans are part of a larger assemblage University of Manchester’s included several planktonic crustaceans M of animals called Arthropods. Arthropod School of Biological Sciences waiting for called copepods whose classification to means "joint footed" and alludes to the fact the next lecture in the "Invertebrate species level can centre on many features of that all arthropods have jointed appendages Diversity" series as part of my degree their anatomy but to cut a long story short I used primarily for locomotion. Like the course in Zoology. The lecturer then found myself counting the hairs on the legs Annelid polychaetes we looked at in the appeared at the door, uttered the words of these copepods in order to correctly last article, arthropods are segmented but "Crustaceans – too many legs" and then identify them! When you consider that they have one further major adaptation and left! After the initial confusion at the these animals are sometimes only a couple that is the exoskeleton or cuticle. This is a actions of the distinguished Doctor John of millimetres long you can appreciate the usually hard exterior to the animal’s body Dalingwater we eventually established that difficulty of this task (and they’ve got loads which affords muscle attachment and the reason for this peculiar behaviour was of legs!) protection. Because this external layer is due to the incredible array of forms within Consequently, in this instalment of the essentially dead it must be shed in order for the crustacean assemblage which renders Friend or Foe series I am not going to the animal to grow. In crustaceans and all their classification an absolute nightmare! mention species names unless I am arthropod groups this is done through a This was brought home to me some absolutely certain they are correct. Indeed, process called moulting or ecdysis. In many years later when I was working as part of a in some cases my knowledge is sufficiently groups of arthropod the body form changes research team studying Britain’s deepest sparse for me to attempt to classify them with each successive moulting which makes lake, Loch Morar in the Northwest beyond even their Order assignation. their correct identification that bit more Highlands of Scotland. Part of the project’s difficult. Each individual stage between moultings is termed an Instar. 2 1 3 Life cycle of a typical shrimp 1. Early nauplius 2. Later nauplius 6 3. First protozoea 4. First mysis 5. First post-larva 6. Adult shrimp 5 4 18 • MARINE WORLD – OCTOBER/NOVEMBER 2002 Friend or Foe Sub-phylum Chelicerata on their body segments plus gills for respiration under water. The Chelicerates are a group which Chelicera contains only one sub-class that the marine aquarist is likely to come across by Class Cirripedia Body accident in a reef aquarium, and a single No sooner have I declared that I will species that they may purchase. This Sub- concentrate on large crustaceans in phylum’s name comes from the specialised this article I immediately start with a feeding structures termed chelicerae which Leg group of animals that are usually can be alikened to the pincers of crabs. smaller than the 10mm maxiumum The main characteristic that separates this size! This is for good reason as the group from other arthropods is their lack Cirripedia represent a group of of antennae. crustaceans that are significantly (one million described species) than the Chelicerates include the extinct Trilobites, different from the rest of the crustacea. Crustacea. Taxonomists believe that spiders, mites, scorpions and little known Many temperate species do not reach more crustaceans are amongst the most primitive groups such as sun spiders and whip than 5mm in diameter but it is common to extant arthropods due to the vast majority scorpions. have accidental imports reaching our of species retaining the ancestral marine It is important to recognise that these minimum size. We know them as barnacles existence. Perhaps one of the reasons the are distinct groups from the crustaceans and they represent a group of crustaceans crustaceans have had continued success for with which they share a common ancestor that have given up the ability to move many millions of years is the fact that but, for example, the extinct trilobites did freely. Instead, they remain in a fixed insects have not conquered this not evolve into anything else, they simply position with the body surrounded by a environment. Most of the insects died out. number of calcium carbonate plates. This encountered on rocky shores in the U.K. are means that a barnacle must rely on food to primitive species that are not truly aquatic reach it. Thus we find them associated with Subclass Xiphosaura and will die if submerged for long periods. areas with a strong flow and/or much Common Name : Horseshoe crabs An aquatic existence opens many niches for suspended food particles such as phyto or Latin Name : Limulus polyphemus crustaceans. For example, they can achieve zooplankton. By using barnacles as a guide The Horseshoe crabs are often referred to larger sizes due to the buoyancy provided we can deduce the type of area a specimen as "living fossils" due to their ancient by water. came from. ancestry and primitive looks. There are four Because of the space limitations I must Barnacles have specialised feet – the known species belonging to 3 genera. adhere to this article I will include some of name cirripede means "hair foot" and they Limulus polyphemus is the American the larger crustacean species that can be use the fan-like feet to collect food. Sessile species, the other three are Asian. encountered by aquarists. Subsequent barnacle are called Balanomorphs and it is Horseshoe crabs are generally kept by articles will continue this theme and then these that are most prevalent in our aquaria aquarists requiring them for their novelty will focus on some of the smaller and more and can be recognised by their distinctive value or indeed in a sand-shifting role. unusual species of crustacean that one may shape. Some species are found in Their spade-shaped carapace acts like a experience. association with hard corals such as Porites plough as it shuffles through the sand. This The defining feature of crustaceans is sp. and Euphyllia spp. It is likely that the is a fascinating creature but as marine that they all possess two pairs of antennae corals have overgrown specimens in these aquarists will only acquire this species by on the head. Insects have a single pair, cases but due to the repeated feeding purchasing it there is no need for further spiders and their allies have none. Most actions of the barnacle they are not capable description here. species also have a number of appendages Sub-class Pycnogonida Common Name : Sea Spiders This group of chelicerates is given its common name due to the fact that most species have 8 walking legs much like spiders. However, they are a strange group that most aquarists may overlook or never experience in their aquarium as they are liable to be short lived and usually very small (<10mm). I have found several dead specimens in association with Live Rock but am yet to see a living one in an aquarium situation. They can be found feeding on bryozoans, colonial anemones and other encrusting organisms but are easily removed if so desired. Sub-phylum : Crustacea Crustaceans are the second largest sub- division of the Arthropod Phylum with Two 15mm diameter barnacles slightly obscured by Disc anemones. This picture was taken around 38,000 described species. Only the just after the lights came on for the first time. Only a few minutes later the barnacles were insects have a greater number of species smothered thus any attempts at feeding had to occur when the lights were off. MARINE WORLD – OCTOBER/NOVEMBER 2002 • 19 Friend or Foe of smothering the barnacle completely. that severe as the The head and thoracic region of a mantis shrimp. This works to the barnacle’s advantage as it chances of the parasite This is a "smasher". Note the very large compound is obscured from attack by many of its becoming fertilised are eyes and club-like second appendage which are a enemies plus the stinging cells of the corals small but they can be pale purple colour in this species. provide an excellent deterrent for creeping avoided completely by predators such as whelks which enjoy careful scrutiny of any nothing better than drilling a hole in the crab purchases barnacle and sucking out the contents. particularly porcelain Have a look at Plume rock (Porites sp.) as crabs (Petrolisthes along with the abundant tubeworms it is spp.). not uncommon to have barnacles present. Due to their feeding habits barnacles Class are unlikely to survive for long in marine aquaria unless they are supplied with Malacostraca sufficient particulate food such as Plancto Malacostracans make or Marine Snow. Most species of barnacle up the majority of are entirely harmless but it is worth species within the mentioning a group which, although Crustacea with over unlikely to bother the aquarist greatly, may 22,500 species one thing straight. If you hear a sound like present a significant problem to the other currently described by science.